INFORMATION TO USERS
T h is r e p r o d u c t i o n was m a d e f ro m a c o p y o f a d o c u m e n t s e n t t o u s f o r m ic rofilm in g.
Wh i l e t h e m o s t a d v a n c e d t e c h n o l o g y h a s b e e n u s e d t o p h o t o g r a p h a n d r e p r o d u c e
this

docum ent,

the q u alit y

o f the r e p ro d u c tio n

is h e a v i l y d e p e n d e n t

upon

the

quality o f the material su b m itte d .
The

following e x plana tion

o f t e c h n i q u e s is p r o v i d e d t o h e l p c l a r i f y m a r k i n g s o r

n o t a t i o n s w h i c h m a y a p p e a r o n t hi s r e p r o d u c t i o n .
1. T h e sign o r “ t a r g e t ” f o r p a ge s a p p a r e n t l y l a c k i n g f r o m t h e d o c u m e n t
p h o t o g r a p h e d is “ Mi s si ng P a g e ( s ) ” . I f it w a s ' p o s s i b l e t o o b t a i n t h e m i s s i n g
p a g e ( s ) o r s e c t i o n , t h e y ar e s p l i c e d i n t o t h e f i l m a l o n g w i t h a d j a c e n t p ag e s . T h i s
m a y h a ve n e c e s s i t a t e d c u t t i n g t h r o u g h an i m a g e a n d d u p l i c a t i n g a d j a c e n t p a g e s
to assure c o m p l e t e c o n t i n u i t y .
2. W h e n a n i m a g e o n t h e film is o b l i t e r a t e d w i t h a r o u n d b l a c k m a r k , it is an
indication o f eith e r blurred co py because o f m o v e m e n t during e x posure ,
duplicate co p y , o r c o p y rig h te d materials th a t sh o u ld n o t have been filmed. F o r
b l u r r e d pag e s, a g o o d i m a g e o f t h e p ag e c a n b e f o u n d in t h e a d j a c e n t f r a m e . If
c o p y r i g h t e d m a t e r i a l s w e r e d e l e t e d , a t a r g e t n o t e will a p p e a r l is ti n g t h e p a g e s in
the adjac ent frame.
3. W h e n a m a p , d r a w i n g o r c h a r t , e t c . , is p a r t o f t h e m a t e r i a l b e i n g p h o t o g r a p h e d ,
a definite

m ethod

of

“ sectioning”

the

material

has

been

followed.

It

is

c u s t o m a r y t o b egi n f i l m i n g at t h e u p p e r l e ft h a n d c o r n e r o f a large s h e e t a n d t o
c o n t i n u e f r o m left t o r i g h t in e q u a l s e c t i o n s w i t h s m a l l o v e r l a p s . I f n e c e s s a r y ,
s e c t i o n i n g is c o n t i n u e d aga i n - b e g i n n i n g b e l o w t h e f i r s t r o w a n d c o n t i n u i n g o n
until c o m p l e t e .
4. F o r

illustrations

that

cannot

be

satisfactorily

reproduced

by

xerographic

m e a n s , p h o t o g r a p h i c p r i n t s c a n b e p u r c h a s e d a t a d d i t i o n a l c o st a n d i n s e r t e d
i n t o y o u r x e r o g r a p h i c c o p y . T h e s e p r i n t s are a v a i l a b l e u p o n r e q u e s t f r o m t h e
Dis se rt at i o n s C u s t o m e r Services D e p a r t m e n t .
5. S o m e p a g e s in a n y d o c u m e n t m a y h ave i n d i s t i n c t p r i n t . In all ca s e s t h e b e s t
a v ai l a bl e c o p y h a s b e e n f i l m e d .

University
Microfilms
International
300 N. Z eeb Road
Ann Arbor, Ml 48106

8513925

M ille r, T h o m as E d w ard

COMPETITION AND COMPLEX INTERACTIONS AMONG SPECIES:
COMMUNITY STRUCTURE IN AN EARLY OLD-FIELD PLANT COM M UNITY

Michigan State University

University
Microfilms
International

3 0 0 N. Z eeb R oad, Ann Arbor, Ml 48106

Ph.D.

1985

COMPETITION AND COMPLEX INTERACTIONS AMONG SPECIES:
COMMUNITY STRUCTURE IN AN EARLY
OLD-FIELD PLANT COMMUNITY

By

Thomas E dw a rd M i l l e r

A DISSERTATION

Subm itted to M ichigan S ta te U n i v e r s i t y
in p a r t i a l f u l f i l l m e n t o f the re q u ir e m e n ts
fo r the degree of

DOCTOR OF PHILOSOPHY

W. K.

K ellogg B io lo g ic a l S t a t i o n
and
D epartm ent o f Zoology
1985

'Sura h i e e t p o t a m c e r v i s i a m *

ii

ACKNOWLEDGEMENTS

I

am v e r y

support

a nd

g ratefu l

advice

to

w hile

I

sincerely

thank

the

other

Goldberg,

Donald

H all,

a nd

and

advise

throughout

and J u d i t h

the

the

at

Scott

G leeson,

C id-B enevento,
Da v i d

advice

of

S tate

Mat hew
John

been

m em b e r s
Earl

of

also

Dungan,

advisor,
I

also

com m ittee,

their

for
wish

Drs.

discussion,

her
to

Deborah
criticism

my s t u d i e s .

Drs.

in p ro v id in g

i d e a s a nd d i s c u s s i o n .

to

this

In

Craig

provided

p articu lar,

Amelia

I

wish

A lice

Jessie

the

Gross

came

by t h e

fr om

Ecology
to

Winn,

H efferlin ,

appreciated

P etersen,

K atherine

dissertatio n

Osenberg,

H art,

greatly

C hris

my

for

U niversity.

Terese

my

M ichigan.

friendships

L eibold,

I

of

contribution
and

W erner,

in

Werner

invaluable

H art,

W ilson.
Mike

have

environm ent,

M ichigan

P atricia

course

largest

in teractio n s,

group

a nd

the

S o u le were a l s o

P ossibly

Dr.

thank
Carmen

Da v i d

P eart,

e n c o u r a g e m e n t a nd

Wing,

and o t h e r s

at

the

U n iv e r s ity of Arizona.
I

also

wish

encouragement
Don

H all,

comfort

at

along

Robert

com panionship,

cooperation

way.

M iller,

tim es

Invaluable

particu larly

the

im portant

support sev eral

C harlotte

to

Deborah

and J im

tim es.

Dr .

is

provided

Adams,

Steve

and

assistance

not

Goldberg,

Brown

people

Cooper

for

Kay G r o s s ,

provided

W illiam

and
by

Weis,
of

possible

to

field

advice,
also

aid

Judy

an d

Soule,

support,

an d

came t h r o u g h w i t h

assistance,

Pam C a r l t o n ,
A rthur
Jim

Kellogg A g r i c u l t u r a l E x p erim e n ta l
It

several

i n many d i f f e r e n t w a y s .

technical
were

thank

W eist,

Bronson

S tation

thank Robert

iii

John

and

G orentz,

and

Kathy

Harold

is g ra te fu lly
a nd

as

P atricia

w ell
Lori

as
H all,

W eist.

W ebster

of

The
the

acknowledged.
M iller

enough f o r

th eir

genetic

shall

always

and f o r b e i n g
F inancial
N ational
of

and
be

environm ental

g rateful

to

A lice

on

Winn f o r

this

her

study.

hedonism

F inally,
and h e r

I

calm,

there.
support

for

Science Foundation

A g ricu ltu ral

Foundation

influences

this

r e s e a r c h was p r o v i d e d

(DEB 7 9 - 2 3 9 4 5 ) ,

(84-CRCR-1-1396),

a nd

f o r F a m i l y E m p l o y m e n t (DJH 1 ) .

iv

the U nited
the

Donald

by g r a n t s
S tates
and

from the

Department
Peggy

H all

ABSTRACT
COMPETITION AND COMPLEX INTERACTIONS AMONG SPECIES:
COMMUNITY STRUCTURE IN AN EARLY
OLD-FIELD PLANT COMMUNITY
By
Thomas Edward M i l l e r

Experim ental
first-y ear

old

determ ine

the

interspecific)
determ ining

studies
field

and

in

range

densities.
among

greatest

com petitive
response

followed

the

in

There
five

=

h i e r a r c h y was e s t a b l i s h e d
s t u d y and i n a l l

abundance

species
of

com petitors
there
the

was a g e n e r a l

species.

com petitive
(biom ass/m ).

a

approaching

on

a

a
to

interactions

in

of

across

species.

had
the

the
least

Ambrosia

> Plantago

Chenopodium

a

com petitive

a nd d e m o n s t r a t e d

other

=

com binations

artem isiifo lia

repens

repens

was

lanceolata
album.

>

This

in b o th

years

sp ecies m ixtures.

the
zero.

no d i f f e r e n c e s
single

in

(intraspecific,

hierarchy

response

focal

to

com petitive
The

per-am ount eq u iv ale n ce of

T here were
effects

of

non-linear
w ith

species
conducted

by m ids ummer and was c o n s i s t e n t

com petitors,

gradually

species

by A g r o p y r o n

different

had

species

Ambrosia

presence

T rifolium

direct

consistent

on o t h e r

the

were

higher-order)

fivea

Michigan

plant

The f i v e s p e c i e s w e r e g r o w n i n a l l

species.

hierarchy

cam pestre

was

effect
to

and

among f i v e

of

(indirect,

four-,

the

com petitive

M os t

im portance

complex

tw o-,

ab ility

o f the

southw estern

t h e community s t r u c t u r e .

one-,

Lepidium

in teractio n s

relativ e

possible
of

of

increases
effect

of

also

suggest

resu lts

com petitive

among a s s o c i a t e
species

at

in

any

effects
species

p articu lar

the

adding
that
among
in

the

yield

A method
an d

higher-order

another

in

the

detrim ental
or

wa s

zero,

developed
components
full

effects

species;

com petitive

species

the

were

The

Thus,

the

asym m etric
Ambrosia,

of s t i l l
plant

of

com petitive

for

is

restricted

by s tr o n g

interm ediate

com petitive

intra-

interspecific

a nd

direct,
of

one

effects

the

the

is

effects.

small
The

subordinate

th e dominant s p e c i e s

growth

of

Agropyron,
The

the

by

dominant

effects.
is

hierarchical,

and h i g h e r - o r d e r e f f e c t s .

species,

The s p e c i e s o f

restricted

com petitively

P l a n t a g o , L e p i d i u m , T r i f o l i u m , and C h e n o p o d i u m , a r e

by i n t e r s p e c i f i c

always

on a s u b o r d i n a t e

structured

in trasp ecific

effects.

of

on

in th e community.

The

ab ility ,

were

were v e r y

direct

species

in d irect,
species

com petitively

the y i e l d

studied

effects.

d irect

a dominant

species

community

effect

against

im portant

other

separate

The i n d i r e c t e f f e c t s

acting

effect

the

total

was a l w a y s a f u n c t i o n o f b o t h

and t h e y i e l d

species,

of

large.

fa cilitativ e,

higher-order
the

predict

community.

a nd o f t e n q u i t e

and

to

by

both

subordinate
restricted

TABLE OF CONTENTS
Page
LIST OF TABLES .................................................................................................................................

vi

LIST OF FIGURES ............................................................................................................................... v i i
CHAPTER 1:
INTRODUCTION ........................................................................................................
P r e v i o u s S t u d i e s on Complex E f f e c t s ..................................................................
C o m p e t i t i o n and Complex I n t e r a c t i o n s i n P l a n t C o m m u n i t i e s . . .
T h e s i s O r g a n i z a t i o n .......................................................................................................

1
9
15
20

CHAPTER 2:
EXPERIMENTAL METHODS ...................................................................................
F i e l d S i t e ..............................................................................................................................
P l a n t C o m m u n i t y ...............
G e n e r a l A p p r o a c h ..........................................................................
F i e l d E x p e r i m e n t a l D e s i g n ...............
F i e l d D e s i g n 1982
..............
F i e l d D e s i g n 1983... ...............................................................................................

22
22
22
28
29
30
34

CHAPTER 3 :
COMPETITIVE EFFECTS ANDRESPONSES
OFSPECIES ........................
M e t h o d s ................................................................................................................
R e s u l t s .........................................................................................................
Y e a r 1 ..........................................................
Y e a r 2 ............................................................................................................................
D i s c u s s i o n ..............................................................................................................................
D iffe re n c e s in the C om petitiveE f f e c t s
andR e s p o n s e s . . .
V a r i a t i o n W ithin a Season
....................................................................
V a r i a t i o n B e t w e e n TwoS u c c e s s i v e S e a s o n s ...........................................

36
37
40
40
43
48
48
51
52

CHAPTER 4 :
THE DYNAMICS OF PLANTPOPULATIONINTERACTIONS ........................
I n t r a s p e c i f i c E f f e c t s ..................................................................................................
I n t e r s p e c i f i c E f f e c t s ..................................................................................................
T o t a l C o m p e t i t i v e E f f e c t ...........................................................................................
C o n c l u s i o n s ............................................................................................................................

55
57
62
71
80

CHAPTER 5:
ANALYSIS OF MULTISPECIES INTERACTIONS .......................................
Summary o f M e t h o d s
........................................................................
A n a l y s i s of th e F i e l d Data
................................................................................
A Mod el o f P l a n t I n t e r f e r e n c e ..............................................................................
E v a l u a t i o n o f t h e Model .............................................................................................
D i s c u s s i o n ...............................................................
G e n e r a l I m p l i c a t i o n s .....................................................................................................

83
85
86
91
98
108
114

CHAPTER 6 :

117

GENERAL CONCLUSIONS AND SUMMARY ......................................

LITERATURE CITED ............................................................................................................................

v

120

LIST OF TABLES
Table
2.1

2.2

2.3

Page
S p e c i e s a b u n d a n c e s i n B a i l e y Ya rd f o r t h e y e a r s
1 9 8 1 - 1 9 8 3 ....................................................................................................................

23

E x p e r i m e n t a l d e s i g n f o r e x p e r i m e n t s o f 1982 a n d 1983
s h o w i n g t h e s p e c i e s p r e s e n t i n e a c h t r e a t m e n t .......................

31

E q u a tio n s used to e s t i m a t e the biom ass of i n d i v i d u a l s
o f e a c h s p e c i e s u s i n g m o r p h o l o g i c a l ' m e a s u r e s ..........................

33

3.

1
P e r c e n t s u r v i v a l o f f o c a l i n d i v i d u a l s when g r ow n i n
com bination w ith d i f f e r e n t a s s o c ia te s p e c ie s in
Y e a r 2 ............................................................................................................................ 44

3.2

P r e c i p i t a t i o n by m onth a t t h e K e ll o g g B i o l o g i c a l
S t a t i o n f o r t h e y e a r s 1982 a n d 1983
............................................

54

The a m o u n t o f v a r i a n c e e x p l a i n e d
by t h r e e e q u a t i o n s
d e s c r ib in g p l a n t perform ance in m onocultures as a
f u n c t i o n o f d e n s i t y ..........................................................................................

60

L i n e a r c o r r e l a t i o n s b e t w e e n mean w e i g h t o f f o c a l
s p e c i e s and t h e y i e l d o f d i f f e r e n t a s s o c i a t e s p e c i e s
i n t w o - s p e c i e s m i x t u r e s ................................................................................

67

The v a r i a n c e e x p l a i n e d by t h r e e d i f f e r e n t e q u a t i o n s
d e s c r i b i n g t h e mean g r o w t h o f i n d i v i d u a l s o f f o c a l
s p e c ie s as a fu n c tio n of the a s s o c ia te sp e c ie s y ie ld

70

4.1

4.2

4.3

...

4.4

C o e f f i c i e n t s o f d e t e r m i n a t i o n f o r fo u r d i f f e r e n t models
o f t w o - s p e c i e s m i x t u r e s ................................................................................ 75

4.5

P a r t i a l c o r r e l a t i o n c o e f f i c i e n t s from m u l t i p l e
r e g r e s s i o n a n a l y s i s o f t w o - s p e c i e s m i x t u r e s ............................. 77

5.

1
P e r c e n t s u r v i v a l of f o c a l i n d i v i d u a l s f o llo w in g the
r e m o v a l o f d i f f e r e n t a s s o c i a t e s p e c i e s i n Y e a r 2 ................ 87

5.2

D e f i n i t i o n s o f t e r m s f o r a model o f m u l t i s p e c i e s
i n t e r a c t i o n s ............................................................................................................

5.3

The e s t i m a t e d d i r e c t a n d i n d i r e c t e f f e c t s b e t w e e n
s p e c i e s i n Y e a r 1 a n d Y e a r 2 ................................................................

vi

95

104

LIST OF FIGURES
Figure
1.1

3. 1

3.2

4. 1

4.2

5.1

5.2

5.3

5.4

5.5

Page
The d i f f e r e n t t y p e s o f s p e c i e s i n t e r a c t i o n s t h a t
can p o t e n t i a l l y r e s t r i c t th e growth of a s p e c ie s

6 ,

The mean g r o w t h o f i n d i v i d u a l s o f e a c h s p e c i e s i n
t h e mi ds ummer and a u t u m n o f Y e a r 1 when g r o w n i n
m o n o c u l t u r e s a nd a l l p o s s i b l e t w o - s p e c i e s m i x t u r e s

..........

42

The mean g r o w t h o f i n d i v i d u a l s o f e a c h s p e c i e s i n
t h e mi ds ummer and a u t u m n o f Y e a r 2 when g r o w n i n
m o n o c u l t u r e s and a l l p o s s i b l e t w o - s p e c i e s m i x t u r e s

..........

46

I n t r a s p e c i f i c e f f e c t s o f d e n s i t y on t h e g r o w t h o f
i n d i v i d u a l s f o r f i v e s p e c i e s i n Y e a r 2 .......................

59

I n t e r s p e c i f i c e f f e c t s of the y ie ld of v a rio u s a s s o c ia te
s p e c i e s on t h e g r o w t h o f i n d i v i d u a l s o f f i v e f o c a l
s p e c i e s i n Y e a r 2 .........................................

66

The me an g r o w t h o f i n d i v i d u a l s o f e a c h s p e c i e s i n t h e
f u l l community and i n a l l p o s s i b l e s i n g l e s p e c i e s
r e m o v a l s i n Y ear 1 and Y e ar 2
.........................................................

90

The g e n e r a l r e l a t i o n s h i p b e t w e e n t h e mean g r o w t h o f
i n d i v i d u a l s o f t h e f o c a l s p e c i e s and t h e t o t a l y i e l d
o f a s s o c i a t e s p e c i e s ..............................

93

The e f f e c t s o f t h e t o t a l y i e l d o f a s s o c i a t e s p e c i e s
on t h e g r o w t h o f i n d i v i d u a l s o f t h e f i v e f o c a l s p e c i e s
i n Y e a r 1 .......

100

The e f f e c t s o f t h e t o t a l y i e l d o f a s s o c i a t e s p e c i e s
on t h e g ro w th o f i n d i v i d u a l s o f t h e f i v e f o c a l s p e c i e s
i n Y e a r 2 .....................................................................

102

The r e l a t i o n s h i p b e t w e e n t h e d i r e c t a n d t o t a l e f f e c t s
o f a s s o c i a t e s p e c i e s on f o c a l s p e c i e s f o r e a c h
a s s o c i a t e - f o c a l s p e c i e s p a i r i n Y e a r 1 a n d Y e a r 2 ...........

107

vi i

Chapter 1

INTRODUCTION

Each

sp ecies

different

forces,

community

and

ecology is
species

in

a

m ultispecies

through

w ith

the

interactions

ab iotic

to u n d e r s ta n d

community

this

w ith

is
other

environm ent.

complex s e t o f

in

the

community;

that

is,

"structure"

of

the

spatial

and

tem poral

abundance of

pattern

of

resource

allo catio n

community.

to

goal

in

the

of

in teractio n s
what

structure

species
among

species

The

understand

Communi ty

a f f e c t e d by

is

many

in

the

community

affecting

each

determ ines

the

the

pattern

of

a community a s w e l l as th e
species

(Cody

and

Diamond

in teractio n s

between

1975).
A ttem pting
different
define
of

to

species

and

how a l l

between

the

species

various

and

th eir

resources

affect

t h e community s t r u c t u r e h a s p ro v e n to be v e r y d i f f i c u l t .

th is,

poorly

the

field

focused

more k i n d l y

of

community

(MacFadyen

put,

the

difficu lty

for

investigating

have

understand

is

1975,

d ifferent

"still

in

caused

by t h e

fact

com m unities

There
ecology:
community

dynamics
studies
have
the

Smith

a field

a t t e m p t e d many d i f f e r e n t

community

ecology

and

a nd

19 75,

its

(Inger

been

perceived

M cIntosh

1976,

infancy" (Pianka

that
and

often

structure.

has

there is
C olw ell

d isp arate

This

has

no

Because
as

being

1 9 8 0a )

1983).

or

or,

Much o f

standard protocol

1977).

In vestigators

ways o f

understanding

caused

confusion

when

a r e compared.
been

level

structure

two

of

prim ary

organization

( reductionism

causes
at
vs.

of

which

d iv isio n

in

to

understanding

holism )

begin
and

the

community

choice

of

aggregate
The

variables

currency

nonoverlapping
ecology.

or

"currencies"

question a c tu a lly

separates

fields,

ecology

ecosystem

Ecosystem ecology views

organization

a nd

ecosystem

Odum 1 9 8 3 ) .

(see

h o listic
level.
has

1976,

addressing

A common t h e m e
emergent
as

being
1976,

system s

com m unities

can

cause-effect
operational

through

ch aracteristics,

O 'N e ill

studies,

general,

the

"sum o f

Patten
ecology,

be

as

basis

for

at

the

parts"

one

1966,

that

complex

processing

approaches

and

com m unities.

of

However,

only as

filters

ignores

the

severely

flow

been

energy

because

the

use of

includes

this

very
a nd

the

and

approach views

P opulation-based

approach

approaches

to

patterns

of

and

reductionist

species

number

when

abundance

view

1983).

individual
must

pass,

organism s
largely

species.
questions

This
or

level.

structure

h o listic

infer

the

through

it

use

populations

o f s t u d y a nd a g a i n ,

to

The

sophisticated

addressing

The

that

interlocking

understanding

component

community

approaches.

an d

of ecosystem

nutrients

t h e o r g a n i s m a l an d p o p u l a t i o n

and i n d i v i d u a l o r g a n i s m s a s t h e c u r r e n c i e s
holist

of

Innis

1966).

for

associated

ch aracteristics

ecosystem

d r a w i n g on k n o w l e d g e a t

both

useful

th ro u g h which en e rg y o r m a t e r i a l s

diversity

lim its

have

1975,

of

Odum

trophic

investigate

al.

1982,

com puter s i m u l a t i o n m o d elin g o f complex s y ste m s (W att
Ecosystem

or

a

t h e community

offshoot

system s

the

stresses

cannot

(Lane e t

trophic

through

ultra-red u ctio n ist

Patten

system s ecology

community

their

community

A more r e c e n t
an

relativ ely

ecology

work i s

their

highly

pathways (Watt

terms o f

ecosystem

this

two

m aterials

that

takes

viewed

in

and

1976).

into

population-based

or

questions

running

ecology

a nd

ofenergy

In

( e n e r g y o r whole o rg a n is m s ) .

com m unities

t h e mo ve me nt

approach,

com m unities

to use

there are

approaches

causality

about

use
the

3

forces

structuring

abundance

relationships

statistical
1948,

com m unities.

d istrib u tio n s

MacArthur

norm al,

1957).

of

H utchinson

1977).

successful

on

m igration

to

describing

the

(see

The

island

what

These

but

useful

often

between p a t t e r n s
requ ire
ecology,

an

approach

often

emergent

p roperties

f r om t h e

in terrelatio n sh ip s

1980a,

1980b,

studies

still

or

reductionist

Brown
fail

ch aracteristics

1 9 81 ,

too

approach
may

be

are

been

patterns

by

assumes

that

complex

to

Sim berloff

has

G ilbert

been

species

useful

in

a

1980).

been

in

structuring

nonexperim ental,
correlations

th e mechanisms o f t e n

However,

like

com m unities

understand

1982).

sp ecies

of

finding

Tests of

numbers of
of

and

generally

s o m ew ha t

biogeography
the

im portant

approach.

(see

rate

rates

has

hypotheses

community

island

expected

(log
the

by

ecosystem

have

Because

address q u estio n s about

hidden

"building-up"

between i n d i v i d u a l s o r p o p u l a t i o n s

to d i r e c t l y

of

(M cIntosh

th is,

such

t h e a b u n d a n c e and

of in d iv id u a ls or sp ec ie s.

P opulation-based
c o m m u n i ti e s must

are

is

P reston

about

has

Brown 1 9 7 8 ,

an d p r o b a b l e m e c h a n i s m s .

experim ental
this

1 97 0 ,

approaches

developing

that

extinction

mechanisms

h o listic
for

the

h o listic

underlying

each

investigating

S i m b e r l o f f and W i l s o n

com m unities.
are

for

1943,

relatio n sh ip

theory p re d ic ts

on

species

framework

in

patterns

based

al.

this

approach

This

et

p articu lar

im p lications

operating

1967).

and

of

rank - sp e c ie s

resem ble

F isher

forms

h o listic

1963,

species

to

strong

community

population-based

suggesting

1932,

have

"islands"

conceptual

diversity

(Motomura

Another

local

noted

in teractio n s

( M a c A r t h u r a nd W i l s o n
species

been

etc.)

species

in

example,

The d i f f e r e n t

broken~stick,

mechanisms

useful

have

For

red u ctio n ist

approaches

i n some way b e a sum o f

their

assume

parts

that

whole

a nd s o t h a t

complex

4

dynamics

can

be

the contained
approach

is

species.

that

inter-

general

thorough

a nd

the

in teractio n s.

It

operating
but

that

yield

a

of

approach

(Haeckel

t o be i m p o s s i b l e

very

are

an i n d i v i d u a l
population

often

These

forces

may

also

w ill

generally

dissertation
forces,
define
these

I

including
as

sim ple

sim ple

literatu re
higher-order
sim ple

may

be

effects.

only

have

The

been

most

in teractio n s.

I

only

of

each

question::;
such as

or

1 9 75 ,

in

numerous

plexus

of

there

populations
between

in

a

species
are

many

in

a mixed

these

forces.

some e c o l o g i s t s

forces acting
of

two

or

abiotic;

discuss

b io tic

effects
given

and

caused
many

commonly

t h e s u c c e s s of
d i r e c t l y on t h e

or

more

however,
forces.

by

the

this

The d i r e c t

interactions

differen t

forces.
in

facilitatio n ,

called

define

feel

Brown 1 9 8 1 ) .

in teractio n

w ill

the

operating

that

that

predation,

been

that

p o ten tially lim it

b io tic

have

address

this

to be t h e m a j o r o b s t a c l e o f t h i s

that

an

com petition,

effects

but

involve

is

in teractio n s

s p e c i e s may e i t h e r b e s i n g l e
they

to

tangled

an o b s t a c l e

forces

or

individu als

of i n d i v id u a ls ,

individuals

1891),

types of

used

not

to overcome (Lane e t a l .

The d i f f e r e n t

the

approach

apparent

between

there

th e dynamics of

The a d v a n t a g e o f

sim ultaneously

This c o m p lex ity has been long p e r c e iv e d
type

be

of

etc.

b e c om e

d irectly

are

can

forces

can

has

study

red u ctio n ist

intraspecific
community

of

ch aracteristics

b ehavior,
of

knowledge

influences.

approach

the b i o l o g i c a l

difficu lty

community,

a

cu rren cies

g en etics,

m ultispecies

forces

the

this

th a t deal w ith

The

w ith

p o p u l a t i o n s a nd a b i o t i c

So,

evolution,

p redicted

names

indirect

I

between
in

effects

interactions

w ill

the
or

between

f o r c e s a s complex e f f e c t s .

There

are

four

d istin ct

wa ys

in

which

an

asso ciate

species

can

affect

the

include

growth of

sim ple

or

intraspecific
defined

focal

complex

effect

or

subject

effects

(see

attem pt

on a f o c a l

Figure

1.1).

should

type

be made

of

term

s p e c i e s a nd
to

effect,

it

ascribe

because

species,

and w h e t h e r

by a u n i q u e m a t h e m a t i c a l

associate

each

a

or

Each
that

based

of

on w h e t h e r

not

they

these

quantifies

i n c l u d e an

effects

the

or expo n en tial

anyb i o l o g i c a l

cause

d e s c r i b e d by v e r y s i m i l a r m a t h e m a t i c a l

biological

c a n be

effect

may a p o we r

different

they

of

an

t e r m . No

o r mechanism

mechanisms

to

may

be

term s.

Simple Effe ct s

1.

Sim ple

T ntraspecific

performance
is

of

of

a

as

Simple
in

species

1 (e.g .

Interspecific

is

a

asso ciate,

term
sp ecies

focal species

effect

-

individuals

1.1a).

that

defines

on

the

M athem atically
the

1 (N^) > a n d ° n l y

effect

species

of

th is
the

1 , on t h e

c N ^ , w h e r e c a nd p a r e c o n s t a n t s ) .

the

separate
species

defining

the

2 (N^),
1 (e.g .

of

(Figure

term

an m i x t u r e on a f o c a l

th is

the

a

focal

success of sp ecies
2.

any

conspecifics

incorporated

abundance

-

effects

(Figure
effect

of asso c iate

1.1b).

of

the

and o n l y s p e c i e s

species

M athem atically,
abundance

2 , on

the

of

an

success of

cN^)«

Co mp le x E f f e c t s .
3.

C om pl ex

Intraspecific

asso ciate
on

sp ecies

itself.

differen t

-

the

on t h e

Complex
"pathw ays":

intraspecific
the

the abundance of the

effect

in

1. 1c )

or

the

associate

resu lt

intraspecific

affect

com bination

net

species

can

abundance

of a focal
can

be

the a s s o c ia te

species

w ith a sim ple

the

effect

effects

abundance of
focal

of

due

of

an

species
to

two

species

can

(a sim ple i n t e r s p e c i f i c

intraspecific
affect

the

effect;

Figure

way i n w h i c h

the

6

© — CD— i

B
\
□

__

\

©

- □

1

indirect

©

©

SIMPLE

INTERSPECIFIC

©

INTRASPECIFIC

COMPLEX

higher-order
©

©
- »

□

©

© -I

□

F igure 1.1.
The d i f f e r e n t t y p e s o f s p e c i e s i n t e r a c t i o n s t h a t
can p o t e n t i a l l y r e s t r i c t th e grow th of a s p e c i e s .
The c i r c l e s
re p re s e n t the s p e c ie s p roducting the e f f e c t ( a s s o c ia te s p e c ie s );
the squares re p re s e n t the sp ec ie s re c e iv in g the net e f f e c t
( f o c a l s p e c i e s ) . The a r r o w s i n d i c a t e t h e d i r e c t i o n o f t h e
e f f e c t , p o i n t i n g from t h e a s s o c i a t e s p e c i e s t o t h e f o c a l
species

7

focal

species

abundance of

affects
the

itself

focal

w ithout

species

(Figure

e f f e c t s have been g e n e r a l l y d is c u s s e d

l.le ).

affectin g

N either

previously,

of

the
these

b u t b o t h c a n be

r e p r e s e n t e d m a t h e m a t i c a l l y by a term w h ich i n c l u d e s

the abundance

of

focal

the

(e.g .
4.

d irectly

associate

species

and

Interspecific

asso ciate

sp ecies

asso ciate

species

effects

can

-

the

on
on

come

a

the

the

abundance of

second

m agnitude

of

associate

sp ecies

this

been

has

asso ciate

the

sp ecies

A lternatively,
affect

associate

on t h e

referred

the

species

the

effect

an

the

of
focal

in teractio n

on t h e s u c c e s s o f s p e c i e s

the

facilitatio n
of

have

sim ple
been

complex e f f e c t s

of

studied,
in

a

( N^ )

effect

(Figure

of

second

pathways,
can a f f e c t

species

first

asso ciate

this

has

(Figure

1. I f ) .

the

second

G enerally

of

the

first

(N^)»

( b)

species

between

species:

sp ecies

the

1 .Id).

effect

in teractio n

an

interspecific

t wo d i f f e r e n t
sp ecies

of

the

been

(N^)

second
referred

E i t h e r way,

the

by a t e r r a w h i c h i n c l u d e s
(e.g .

effect

of

cCN^N^)^

1 , N^)

effects
w ell

of

focal

can be m a t h e m a t i c a l l y r e p r e s e n t e d

abundance of both a s s o c i a t e

While

the

1) .

abundance

effect

Indirect

the

species

(N2 ) > t h e r e b y c h a n g i n g

sp ecies

as
on

mechanism

to a s a h i g h e r - o r d e r

least

associate

focal
to

the

C om pl ex

associate

abundance of

and

of

interspecific

(^3)

the

at

first

sim ple

resu lt

species.

through

The a b u n d a n c e o f

im portance

of

interspecific

focal

about

the

net

the

(a)

the

abundance

t h e e f f e c t o f c ( N 1N2 ) P on t h e s u c c e s s o f s p e c i e s

C om pl ex

can

the

com petition,
very

community

little

is

structure.

predation,

and

known

the

about

One a p p r o a c h

to

understanding

complex

effects

has

loops re p re s e n tin g c a u s e -e f fe c t
sp ecies w ith
by

the

itself

combined

effect

representation,
effects

two

should

be

common a r e

both

the e f f e c t

of

the

loop

first

loop.

That

(see

or

species

the

be

there

species

C.

between

sp ecies,

there

to tal

n um be r

known

w ill

Secondly,

if

in teractio n s

th at,

of

it

determ ine

be a s t r o n g

w hile

there

c a n be

a very

the

only

large

represented
Using

general,

a

in

B and

the e f f e c t

im portance

be

complex

A on s p e c i e s

effect

this

a species

seems t h a t

indirect

can

is

having

species

C ) , then

to

in

as

or connecting a

loops.

t wo l o o p s

effect

B on s p e c i e s

must

sim ple

connected

suggest

F irst,

(e.g .

view

A complex e f f e c t

more

arguments

im portant.

strong

is,

1975).

two

sim ple

to

pathways between s p e c i e s

Levins
of

been

the

of

second

of

species

A on

single

direct

loop

number o f

potential

com plex

loops.
The

of

fu nction of

t h e number o f

the

loops

complex

that

the

reasons,
complex
1983,

sum o f
it

effects

The

1 98 1 ,

purpose

system .

A we e dy p l a n t

Lhe

of

had

com munity.

quite

suggested

by

and

also

in

the

shown

loops,

common

exponential

Even i f

In

several

dissertation

complex

in teractio n ,

an

on a v e r a g e

it

would

part

for

these

ecologists

that

(Lawlor

1979,

seem

Abrams

1984).

this

all

is

strong.

effects

c o m m u n i t y was s e l e c t e d

in teractio n s

studies

of

loops

as the d i r e c t

im portant

consideration

dynamics o f

sim ple

pilot

im portance

the

may be

Bender e t a l .

a nd

rapid

in

been

both

complex

strong

effect

may b e

general

under

as

recently

presence

species

sp ecies

not

th eir

has

Schaffer

are

p o ssib le

grow

(2) it

a

for

relativ ely

to

investigate

natural

rapidly,

the

can

be

sim ple

quite

the

m ultispecies

study because

was known f r o m p i l o t

community
that

in

is

(1)

the

leading

to

studies

strong,

in teractio n s

a nd

that
(3)

between

9

s p e c i e s were a l l ..c o m p e titiv e , w hich r e s t r i c t s
to

in teractio n s

suggest

that

investigate

between

this

com m unity

the e f f e c t s

S p ecifically ,

of

th is

to

old-field

p l a n t community,

m ixtures,

and

the

species

natural

community.

be

a

g o od

consists

effect

of

( 2 ) quantify

the

ch aracteristics

system

the

interactio n s
These

of

a

in

which

field

com petition

the p o t e n t i a l

in

to

presence

among

three

a

and

group

objectives

are

a

first-year

and t w o - s p e c i e s

relativ e

of

experim ent

sim ple c o m p etitiv e

community u s i n g m o n o c u l t u r e s

(3) .determ ine

complex

4 , and 5,

in

would

These

complex i n t e r a c t i o n s .

quantify

complex e f f e c t s

of t h e complex e f f e c t s

effects.

d issertation

designed

an d

(I)

com petitive

all

five

im portance

com petitors

presented

of

in

in c h a p t e r s

a
3,

respectively.

P r e v i o u s S t u d i e s on C o m p l e x E f f e c t s
Several
be v e r y

th eo retical

im portant

no n -lin earities
interspecific
pointed

out

b io lo g ical

in

in

s t u d i e s have su g g ested

com m unities.

intrasp ecific

higher-order
that

it

system s

Abrams

is

terms

(S m ith-G ill

a nd

to

G ill

s t u d i e s on i n d i r e c t e f f e c t s

com m unities

an d

higher-order

effects;

N evertheless,

given

indicated
sim ple

that

in tra-

indirect

linear,

Levine

these

w ill

re stric tiv e
interactio n s

and i n t e r s p e c i f i c

Several
Lo f i n d

1978,

has noted

that

there

other

Ayala e t

Lawlor

species
1979,

assum ptions,
may

be

f o r c e s and t h a t

very

that

also

authors

al.

1973).

Most

(i.e.

Schaffer

strong

in

equilibrium

effects

these

be

have

complex e f f e c t s

have assumed s t a b l e ,

additive

1976,

1983)

require

expect

th eo retical

com pletely

(1980a,

in teractio n s.

reasonable

t h a t c o m p l e x e f f e c t s may

no

1981).

studies
relativ e

have
to

s e r i o u s e r r o r s may o c c u r

10

from n o t
(see

on

co nsidering

also

Bender e t

Mo st

of

the

the

vole

found

of

the
al.

indirect

experim ental

predation

in

w o r k on c o m p l e x

controlling

is

the

f e e d s on c o m p e t i t i v e l y d o m i n a n t
prey

Harper
1 9 78 ,
and

that

1969,

Paine

Lubchenco
M or e no

predator

second

1983).

This

the

species

predator

in

in teractio n s

studied.
effects

effects

resu lt

there

is

because
acting
is

no

the

need

sim plest

again st

the

outcome

sequence.
we

h erbivory,

the

If
to

in

are
from

is

as

1980,

effect

by

the

prey

the

between

Jara

of

the

where a

preventing
species

K erfoot

being

1966,

Lubchenco

"vaulting"

1980,

effects

w ithout

com m unities,

generally

and

a
of

deMott

interm ediate
prey

has

indirect

these

facilitativ e
that

between

some

very

interactions

com petitive

two n e g a t i v e

the

sim ple
and

species

indirect

so

the

be

in

If

fa cilitativ e,

in teractio n s

in teractio n s

com plex

im plications

This

interactions

been
and

forces.

interestin g
should

have

intra-

com petitive

in teractio n s.

(com petitive)

the

predation

com petitive

in teractio n s

our view

Duggins

predator

Vandermeer

(Paine

1976,

indirect

from e x c l u d i n g

com m unities

of

change

1 9 78 ,

second

suggested

the p e r s is t e n c e of

1977).

this

sim ple

a

indirect

In p l a n t

interspecific
would

of

been

H olt

al.

p r e d a t o r which

Menge

Another e f f e c t

1970,

have a l s o

et

focused

One e f f e c t

excluded

1971,

interspecific

sp ecies

interspecific

C om pl ex

Dayton

prey.

(Dodson

("apparent com petition",

little

an

prey

a "keystone"

and t h u s a l l o w s

Estes

existence

dominant

P redators

1969,

is

of

has

div ersity .

be c o m p e t i t i v e l y

1978,

subordinate

allow s

com petitively

Vadas

and Menge

the

in communities

in teractio n s

species

effect

prey,

otherw ise

and

1984).

on

predator

the

might

processes

1984).

i n many c o m m u n i t i e s

other

higher-order

are

facilitatio n
acting
strong,

com m unities

in
then
of

11

com petitors are
There
attem pted

have
to

between

only

been

determ ine

com petitors

knowledge o f
the

solely an tag o n istic.

first

(1969).

the

His

work

sp ecies

growth

L otka-V olterra
obtained

to

m ixtures.
m ixtures,
complex

By
he

His

potential

for

complex

in terp retatio n

Pomerantz
that
the
is

1981,

the

im portant

of

his

effects

indication
in

tw o-species
final

interactio n s.
resu lts

from

determ ining

the

in

all

using

a

Probably
Vandermeer

possible

and

w ith

and

in

four

actual
that

the

resu lt

have

been

pairs

values
species

resu lts

did
of

not

to

linear

param eter

species

f r om

include

m ultispecies

unexpected.

The

i n t h e e x t i n c t i o n o f two o f t h e
m ixture

interactio n s

can

be d e s c r i b e d

m ixture
and

no

has

by

a

l ow

potential

for

T h e r e h a s b e e n some c o n t r o v e r s y o v e r
(B renchley

1 97 9 ,

to

p red ict

( Thomas and P o m e r a n t z

the

i.e .

d ifficu lties

the

The c o n c l u s i o n o f

data

by

equation

tw o-species

were n e c e s s a r y

the

level,

Vandermeer c u l t u r e d

equations

not

have

predicted

used

each

predict

intraspecific

fo u r-sp ecies com petition
some

of

resu lted

Vandermeer 1981).

only sim ple

He

which

interactions.

lo g istic

linear

the

trophic

be

of

and

p redictions

the

because

com plex i n t e r s p e c i f i c
the

alone

com petition
of

a

of experim ent.

c o n c lu sio n should

behavior

equations

these

studies

protozoans

example

eq uations.

that

can

w ith

the

s u f f i c i e n t to

four-species

linear

type

success

comparing

concluded

The

good

using

the

were

a

done

protozoans

rates

predict

terms

species.

of

w ithin

and i n t e r s p e c i f i c

this

com petition

in teractio n s.
actual

in

effects

work

provides

each

estim ate

the

of ex p e rim en tal

predation,

intra-

was

involved

four

complex

w ithout

controversies
of

if

sim ple

attem pt

a handful

that

sp ecies

this
the

Thomas

and

controversy is

final

1981), but

higher-order
abundances

1 9 81 ,

outcome o f
that

interactions
before

there
were

extinctions

12
occurred

(B renchley

A sim ilar

e x p e r i m e n t was p e r f o r m e d

D rosophila sp ec ie s
constructed
possible

1981).

to " s e a r c h

laboratory

com binations

also

higher-order

in teractio n s.

from

sim ilar

a

flaw

experim ents
So t h e
to

always

correct

complex

com petition-induced

the

complex
time

higher-order
calculated
using

depending

upon
as

the

species

the

which

assumed

that

(Pom erantz

he

also

1981).

is

They

species

direct

or

in

indirect

in terp retatio n

of

at

that

the

suffer

th ree-sp ecies

least
it

all

is

o ne

species.

not n ec essary

does not

exclude

the

p o ssib ility

im portant

in

determ ining

been

very

to

find

f r om
of

between

any

N eill

laboratory

algae.
pair

He
of

species

o f complex

evidence

system s.

species

other

final

abundances.

claimed

co efficients

the

(1974)

experim ents
was

able

species

present,

in teractio n s.

of

to

changed

which

W ilbur

he

(1972)

the d e n s i t i e s of th ree s p e c ie s of salam anders

on

that
the

the

various

presence

interpreted
both

in tra sp ecific
It

phenomena".

determ ine

species

of

using

to

m ultispecies

four

He f o u n d

However,
the

in

the e f f e c t

dependent

it

studies

id en tity

being

all

in teractio n s.

final

coefficient

enclosu res.
were

but

of

work:

t h e i r work i s

(1975)

three

a nd

extinction

may h a v e

of

ex p erim en tally m anipulated

effect

of

com petition

m ixtures

that

interpreted

the

experim ental

linear

various

pond

V anderm eer's

extinction,

interactio n s

dem onstrate

in

in

in

a nd

no e v i d e n c e

in teractio n s

interactions

early

two,

analysis

to e x t i n c t i o n or the

Two

one,

found

that

resulted

of

Their

in terp retatio n

include

that

to

al.

fo r emergent c o m p e titiv e

cultures
and

by R ich mon d e t

not

N eill

as

and

effects
p o ssible

and

measures of c o m p e titiv e
densities

evidence

Wilbur

used

were l i n e a r
to d is c e r n

of

for
an

w ith

still

higher-order
analysis

species

w hether

other

their

which
density
resu lts

13

were

caused

by

higher-order

effects

or both (see

N e ill's

analysis,

S eifert
analysis

p redicted

the

little

S eifert

(1976,

The

species

impact

S eifert

19 7 9)

interactions

(1979)

im portance

of

significance
regression

of

interactio n s

p ossible

cases.

densities

for

actu ally

found

models

were

any

type

complex

either

the

failed

to

However,
each

a

term
in

species

incorporate

was
both

were

to

tested
the

th e m a t r i c e s used

in

the

study,

test

to

in teractio n

in

the

This
the

not
of

at

from

very
and
the

testing

the

out

of

that

*-n
the

four

densities

the proposed

dynam ics,
or

im portant

t h *2

equilibrium

those

equilibrium

other

had

S eifert

predicted

sp ecies

that

suggested

(N^N9N^)

o ne

indicates

in

determ ine

by

term

d ifferent

explain

effects

studies

generally

second

significant

found

the abundances of

The

sim ple

quite

regression

equations

using

species

studies,

were

linear

yielded

higher-order

flow ers.

su fficien t

com m unities

very

single

in u n d is tu rb e d

not

In

a

The

intraspecific

i n s e c t com m unities

between

attem pted
of

in

variables.

abundances.

including

of

m ultiple

method

also

an aly sis.

nonlinear

four s p e c ie s ,

independent

on s p e c i e s

used

regression

as

com petitive

just

1981).

the abundance of each of

rem aining

that

Pomerantz

flow ers.

or

c om me nt s on s t a b i l i t y

to q u a n t i f y s p e c i e s

H eliconia

the

and

also

effects

because

the

factors

models
(e.g .

p r e d a t i o n o r complex e f f e c t s ) .
D avidson
species

(1980)

of h arv ester

quantified
ants.

sp ecies

She u s e d

the

in teractio n s
resource

among

overlaps of

several
s i z e s a nd

t y p e s o f s e e d s u s e d by t h e a n t s a s e s t i m a t e s o f c o m p e t i t i o n c o e f f i c i e n t s
(sim ple
m atrix
a

interspecific
for six

second

effects)

species of a n ts.

m atrix

who se

en tities

and

constructed

an

By u s i n g m a t r i x
gamma^

gave

estim ated

inversion,
a

measure

community

she o b ta in e d
of

the

total

14

(sim ple

i n t e r s p e c i f i c + complex i n t e r s p e c i f i c )

sp ecies

i (s e e Levine

D avidson used
negative
this

she

h ad

was a g o o d m e a s u r e
S eifert

a nd

Levine m a trix
the

form o f

difficu lt
able

to

using

a

poor

the

direct

1980b,

so

it

j

on

Davidson

use

of

method.

the

rem ains

various
in itial

was

very

is

the

type

species

about
it

of

community

or

the

Davidson

is
was

ants

m atrix

from

the

study

influenced

of

Culver

However,

Davidson

not d iscu ss

use

A lso,

differen t

strongly

unknown w h a t

(see

assum ptions

1979).

of

To

1983).

untested

and

reso u rce-o v er lap

effects

Aarrson

effects),

did

that

the

positive

sp ecies.

Lawlor

effects)

community s t r u c t u r e

U nfortunately,

species

the

(see

abundances

to tal

ant

com petitive

Because

(containing

this

and

the

method.

estim ated

suggests th a t

effects

p red ict

assum ption

Abrams

successful

p redict

m atrix

differences

between

direct

1976,

higher-order

m atrix

effects.

the

potential

criticize

(containing
inverted

make

S eifert

co rrectly
the

successfully

abundances

to

of

to

i n v e r s i o n m ethod makes c e r t a i n

the

to

in

of

L a w l o r 1979 f o r d i s c u s s i o n s o f t h i s m e t h o d ) .

t h e gamma m a t r i x

correlations

method,

1 9 73 ,

1976,

effect

also

by c o m p l e x

investigate

these

complex e f f e c t s

may

have been in v o lv e d .
Another experim ent
effects

is

method

for

provide

an

the

work o f

example

of hydra.

out

six

of

Case

Bender

and

the

the

possible

higher-order

species

are

involved

cases.
effects;
or

in

(1981).

presence

method

They found

complex

effects.

experim ental evidence

investig ating

species
of

providing

of

using

The a u t h o r s

higher-order

laboratory

it

may n o t

quantifying

only

detects

be u s e f u l
the

present

effects

m ixtures

sig n ific a n t higher-order
T h e i r method

for h ig h e r-o rd e r

in

im portance

effects
the

and

of

three

in

three

presence

determ ining
of

the

a

of

what

complex

15

C o m p e t i t i o n and Complex I n t e r a c t i o n s
A great
in

deal

certain

is

types

known a b o u t
of

ag ricu ltu ral

studies

known

the

about

com m unities,
in e i t h e r

requiring

it

these
are
that

plants

have

lig h t,

w ater,

affected
measure

by

i n m o n o c u l t u r e s and

from

a

However,
in teractio n

vast

nu mb er

of

alm ost

nothing

is

between

in teractio n s

known a b o u t

of

et

individual

in n a t u r a l

complex

interaction

level

of

uptake

plant

interactions

level.
to

1980).

of

for

need

of

a

we c a n

in trasp ecific,
experim ents

plants

in

know
(see

address

natural

soil

of

of

w ith

sim ple,

provide

p articu lar

about

the

actual

1982),
to

which

individual

questions
a nd

and

in teractio n

a

Tilman

other

two s p e c i e s

Fonteyn

studies

approach

(see

how i n d i v i d u a l

mechanism

also

nutrients

levels

1963,

more

among

a r e many s t u d i e s

com m unities

interspecific,
using

types

The

l e v e l , making

resource

(W illiam s

physiological
still

most

av ailab ility
to

nutrients.

t h a t measure

these

individual

in teractio n s

there

different

studies

resource

of

While

Even

between

more

strong

a p articu lar

we

each

the m o le c u la r

rates

av ailab ility

in teractio n s

Many c o m p e t i t i o n

are

three

C learly,

M eanwhile,

(e.g .

only

al.

in teractio n

resu lts

There

responses

needs,

number o f c r u c i a l
at

m oisture

evidence

require

resource

occurs

the

resource

response.

probably

the

a

sim ilar

quantify.

find

local

Foster

interactions.
with

is

resources

soil

I can

correlating

mechanisms
w ill

nothing

and a l i m i t e d

individual

circum stantial

species

of

example,

the

affect
1978,

to
for

by

H arper 1977),

only

mechanisms

very

these

resources;

Mahall

1977).

i s known a b o u t s p e c i e s

d ifficu lt

species

cultures

H arper

v irtu ally

and u s e o f

very

(see

of density

the greenhouse or f i e l d .

All

uptake

and

effects

tw o-species

actual

p la n ts, very l i t t l e

in P l a n t Communities

dealing

t h e mode o f

complex, e t c . ) .

have been

performed

16

and

it

is

obtained
been

tem pting

from t h e s e

very

by

in

effects
an d

these

tw o-species

and

in

often

perform ed

(see

im portance

in m u ltis p e c ie s

most

interactio n s)
com m unities.

of

the

very

complex

assem blages

environm ental

different

in

Trenbath

effects,
and,

complex

A l s o , most o n e -

artific ia l

are

have

com petitive

of

system s.

review s
of

for

types

include

under

that

studies

(intraspecific

not

done

in teractio n s

These

potential

sim plest

do

densities

the

system s.

the

the

com petitive

in m u l t i s p e c i e s

environm ents

To e x p l o r e

be

in

were

at

of

effects

studies

studies

natural

1977).
must

density

in teractio n s

tw o-species

found

dem onstrating

t h a t may b e o p e r a t i n g

conditions

knowledge

to m u ltis p e c ie s

exploring

interspecific

However,

extend

studies

useful

in teractio n s
an d

to

fr om

1974,

the

those
Harper

experim ents

ideally,

in n a tu r a l

com m unities.
C om petition
several

different

inform ation.
individual

con-

is

ways,

an

the

1970,

measure

com m unities

of

which

com plete

species

full

reveal

removal

allow s

Raynal

and

how much o f

experim ent
which

may

inform ation
of

involves
be

of

the

Bazzaz
the

a

total

experim ent

investigated
types

of

neighbors

about

an

growth

of

at

in ten sity

involves

the

all

total

of

the

in

densities

in trasp ecific
of

(e.g .

the

species.

species except

removal

com petitors,

removal

reduction

asso ciate

different
of

of

ind iv id u als

But

com petitive

th e removal of a l l

the

of

been

d ifferen t

com parison

effect

1975).

the d i f f e r e n t

m aintained

about

has

c o m m u n i t y w i t h g r o w t h when c o m p l e t e l y i s o l a t e d .

estim ate

c a u s e d by e a c h o f

type

each

the

focal

plant

a nd h e t e r o s p e c i f i c , o n t h e g r o w t h o f

Harper
not

a

in

provides

natural

F irst,
of

ind iv id u als
It

in

Putwain

and

e x p e r i m e n t s do
focal

species

A second type of
the
and

focal sp ecies,
thus

interactio n s.

a subset

both

of

the

provide
A third

species

in

17

the

community

individuals
Gross
to

are
in

species,

strongly

species.
or

do

complex

"press"

added

the use of
species

in

species

to

press

on e a c h

com m unities
removal

A bdul-Fatih

and

colonization

success

of

this

of

experim ental

1961, P i n d e r

the

species

focal

of

the

r e mo ve d

species

and

because

type of experim ent m easures

the

response of

the

removal

the

(or

a d d itio n ) of

by B e n d e r

total

rem aining

include

the

dominant

1979)

a nd

sp ecies

is

to

the

response
(e.g .

1977).

of

1982).

in teractio n

m easuring
sim ple

The r e s u l t s

between

to taleffects

interspecific

None o f
both the
im portant

the

each
does

pair
not

perform

a

series

above

com bination

natural
of

the

of
allow

a measure of
species
a

in

methods

alone

the

complex s p e c i e s

com m unities.
four

has

types

Each method
of

the

rem aining
al.

lim iting

of

S i l a n d e r and

total

of

use

experim ents

in ten sity

com m unity;

again,

effects

Bender e t a l .

potential

1 97 7 ,

interesting

1 9 81 ,
the

the

separation

and c o m p l e x c o m p o n e n t s ( s e e

s i m p l e and t h e v a r i o u s
in

provide

of dominant

factors

A more

(sim ple

Examples o f

Friedm an, e t
the

Press

o f t h e r e mov ed

role
of

single

effect

species.

determ ining

(W erner

technique

focal

a

(1984).

effects)

understanding

m easuring

al.

interspecific

an d i n t e r s p e c i f i c
the

et

rem oving e a c h s p e c i e s o f t h e community s i n g l y ( F o w l e r
Antonovics

1975,

response of

of the

Bazzaz

remaining

the

of

by

which

w ith

experim ents

experim ents

the

reveal

in teractio n s

provide a measure of

sp ecies

Harper

of

can a f f e c t

i n t e r s p e c i f i c + com plex i . n t r a or

and

response

These e x p e r i m e n t s a r e v e r y d i f f i c u l t

in teractin g

A fourth

as

Sagar

not

p o p u l a t i o n s to

defined

experim ents

subsequent

1982).

they

unidentified

individuals

the

populations (e .g .

because

most

focal

m easuring

H i l s a nd V a n k a t

in terp ret

changes
the

or

1 9 80 ,

group

a nd

into

1984).

to

investigate

in teractio n s

potentially

quantifies

in teractio n s

(sim ple

one

or

some

intra-

and

18

interspecific
provides
This

an d

complex

a measure

suggests

of

in tra-

either

that

a

of

study

and

the

interspecific)

complex

must

but

in teractio n s

incorporate

different

to q u a n t i f y

the

types of i n t e r a c t i o n s .

I h a v e found o n l y t h r e e
m u ltip artite
Harper

method

indiv id u ally .

several

e x p e r i m e n t a l m e t h o d s and u s e a c o m p a r i s o n o f t h e r e s u l t s
d ifferent

no

approacli

(1973)

to

p l a n t community s t u d i e s

understand

investigated

the

community

interactions

t h a t have used

structure.

between

this

H aizel

two weed

and

species,

I

w hite mustard
b arley .
authors
not

and w i l d

By

oats,

growing

determ ined

additive

a nd

the

that

com petitors
of

their

also
in

three
the

suggested

t h e t wo weed s p e c i e s .
produced

a nd

the

effects

species

effects
that

of

in

"synergistic"

(m ustard

and

effects

of

a

focal

species

the e f f e c t s

Fowler

(1982)

further
used

in teractio n s

between

investigated

in n a t u r a l

1981).

grew each

She

th ree-sp ecies
b i-cultures
species

in

to g e n e ra te
the

three

com m unities

She

species

oats

used

two d i f f e r e n t

sp ec ie s m ixtures.

than

t h e sum
They

sum o f

their

independent

resu lts

appear

t h e r e s e a r c h e r s were u n a b l e

to

sp ecies

(Fowler

alone

were

between

on b a r l e y ) .

Both o f t h e s e

studies

herbaceous

sp ecies

occurred

in d ir e c t or h ig h e r-o rd e r

greenhouse

five

m ixtures.

into

crop

p r o d u c e d m or e o f a r e d u c t i o n
the

however,

the

the

i n w h i c h t h e m i x t u r e o f two

wild

( b a r l e y a nd w i l d o a t s on m u s t a r d ) .

t o b e c a u s e d by c o m p l e x e f f e c t s ;
separate

than

on

effects

le s s of a re d u c tio n of a fo c al
actions

com binations,

two w e e d s

They f o u n d s i t u a t i o n s

independent

success

t h e s e w e e ds on t h e c r o p

various

the

found c o m b in a tio n s o f c o m p e t i t o r s t h a t
the

of

an d

in

the

she

all
data

the

first

com petitive

h ad

and A n t o n o v i c s

the

previously

1981,

p o ssible

from

p re d ic tio n s of
The

in teractio n s.

investigate
that

to

the

Fowler

pairs
mono-

and
and

success of each

prediction

used deW it

19-

com petition
p red ict
each

equations

the su c c e ss o f the

species

interaction
model

of

success

of

the m ix tu re

term.

The

the

provided

both

observed

the

predictions

Ism ail

(1983)

investigate

direct
in direct

from t h e

What
complex

the

are

(no

in

and

species

which

evidence
gave

for

some

ambiguous

which

sim ple

Fowler

complex
of

the

two

higher-order

that

p ro portional
predicted

the d e n s i t i e s of the
term ).

each

models

the

Both

sp ecies
90%

in

of

models

m ixtures,

the

The

author

yielded

very

im p lic a tio n s o f having

actual
did

not

d ifferen t

two a l t e r n a t e

resu lts.

at

two

interference
lanceolata

different

between

using

total

Lolium

pure

perenne,

a nd m i x e d

densities.

stands

The d a t a

f r om

a d e q u a t e l y b y de Wi t e q u a t i o n s , w h i c h i n c l u d e

higher-order
pathways

among

com petitors

1 98 1 ,

a

success.

the

term .

or

d eficiencies

in teractio n s

Bender

1 96 8 )

to

No

attem pt

separately

was

measure

made

to

higher-order

sim ple e f f e c t s .

the

among

used

approxim ately

of

previous

com petitors?

s t u d i e s , o n ly fo u r have s u f f i c i e n t l y
effects

Wi t

interspecific

by

success

investigated

species

im plicit

effects

m ixture

g l o m e r a t a , and P l a n t a g o
three

de

as a f u n c tio n of on ly

of

a l l m i x t u r e s was d e s c r i b e d
an

developed

focal

cases

an d

im plicit

explaining

of

sim ilar

Ba ue me r

p rediction

nor did she d is c u s s

models y i e l d

the

the

estim ate

variance

D actylis

sp ecies

of

an

second

predictions

investigate

of

focal

good

and

in teractio n

species
a

1 96 0,

fo c a l s p e c ie s as a fu n c tio n of the d e n s ity of

in

species

component

w ith

(deW it

Fowler
for m o f

resu lts.

(H aizel

1982).

Of

tested

Out

of

these

all

for

and H a r p e r

1973,

and

studies

that

the

the

these

complex e f f e c t s ,
None o f

studies

four
the
has

inv estigated

animal

and

plant

i n f l u e n c e o f complex
Davidson
studies,
fourth

1980,
three

C as e
found

( F o w l e r 1 98 2)

separately

measured

20

complex

indirect

five of

the

evidence

or

higher-order

studies

for

review ed

effects

earlier

effects).

im portance of

indirect

loops

increases

these

included

more

than

only

one

of

(Davidson)
I

the

and t h i s

conclude

im portant

four

m ultispecies

in

in teractio n s

of

in teractiv e
This

obtaining

so-called

we m u s t u n d e r s t a n d

conducted

forces
neglect

conclusive

in com m unities.

prim ary cause of

species

resu lts

number

w ith s p e c ie s

three
was

the

in
in

as being

a nd

number,

possible

o n ly one of

m ixtures.
a

community e c o l o g y h a s n e g l e c t e d

com m unities.

d ifficu lty

w hile

th eir

(although

F inally,

natural

community

s t u d y was n o n - e x p e r i m e n t a l .

that

group

And

studies

com petitors

interpreted

indirect

studies

between

th eir

is

im portance

probably

prim arily

experim ental

I suggest

"em ergent

that

that

a p o t e n tia lly very

due

do
to

evidence

occur

in

the

extreme

of

complex

c o m p l e x e f f e c t s may be t h e

properties"

i n c o m m u n i t i e s a nd

to u n d e r s ta n d

that

community s t r u c t u r e .

Thesis O rg an izatio n
This
occur
This

thesis

among
first

an d

an d

chapter

the

given

general

from

of

presented

has

the

resu lts

using

sim ple

a

methods

sim ple

an

critica l

used

in

and

to

plant

and

of

problems

review

of

descriptions

of

species

two

the

years

a

that

community.

the g e n e ra l

of

sim ple c o m p e titiv e
field

tw o-species

com petitive

m onocultures

the

interactions

old-field

term s,

2 provides

two y e a r s
a nd

complex

introduction

for d i r e c t ,
of

and

first-y ear

of

C hapter

m onocultures
the

in

d efin itio n s

field

the

provided

the p o t e n t i a l

possible

nature

found

literatu re.

Chapter 3,

the

species

questions,

p ertin en t

investigates

interactions

tw o-species

In

involving
Chapter

between

m ixtures

study.

interactions

experim ents

m ixtures.

the

the

In
are
all

4,

the

species

is

m aintained

at

a

21

range

of

designed

densities

of

to

the

study

conspecifics

and o t h e r

presents

resu lts

five
to

the

each

sp ecies

as

in

d irect,

in d irect,

species

in

conclusions
fu rth er

in teractio n

the

a

species

for

of

a model

the
of

component

effects
sp ecies

sp ec ie s m ixtures

construct

the

all

of

community.
and

study

years

the

in s t r u c t u r i n g

of

four-species
the

t h e mean

the

to tal

model

higher-order

preceeding
on

individuals.

This

five-species
the

on t h e g r o w t h o f
possible

in teractio n s

community.
chapters

roles

of

used

p l a n t com m unities.

Chapter 5
and

individuals

to

the

6

of

all

of

other

estim ate

the

between

the

summarizes

the

occurring

Chapter

a nd

of

yield

an d d i s c u s s e s
sim ple

both

The d a t a a r e u s e d

success

then

f r om

m ixtures

study.

combined
is

were (

experim ents

com petition

describes

function

The

increasing

the

from b o t h
that

of

species.

their

complex

im plications
pathways

of

Chapter 2

EXPERIMENTAL METHODS

F ield S ite
The p l a n t c o m m u n i t y s t u d i e d was i n a r e c e n t l y p l o w e d a r e a o f B a i l e y
Yard,

located

County,

a nd

calf

arrays.

and,

pen.

is

ppm).

sprayed w ith

K ellogg

B iological

previous

to

of

basis

that,

the

to

S tatio n

for

the

was

field

m aintain

are

The

1.6,

portion

the h e r b ic id e

to tal
of

N -

the

Rou nd -Up

in

as

plowed

early

1979,

for

sm all garden
an

annual,

successional

species

is

used

Kalamazoo

12+ y e a r s
a

in

on

r e l a t i v e l y acid

.09 % dry w t.,

field

in

last

used

a s an d y Kalamazoo loam t h a t

% organic m atter -

135.5

K.

Portions

quadrennial

The s o i l

5.9,

K -

or

W.

B a i l e y Y a rd h a s b e e n u s e d

research

biennial,

-

the

M ichigan.

eco logical
plot

at

PO^ -

this

two y e a r s

1 4 . 8 ppm,

study
before

(pH

has

been

the study

began.

P l a n t C ommun it y
The

community

contributed
large

m ajority

perennials;
are

less

also

than
of

the study.
five

of

the

thirty

total

sp ecies

are

the

seedlings

of

A list

1981-1983 i s g iv e n
species

1% t o

over

these

however,
found.

Six

contains

of

the

plant

sp ecies,

autumn b i o m a s s
weedy

most

in

annuals

occasional
common

most
the

species

which

field.

and

invading

of

The

short-lived
tree

for

species

the

years

in Table 2 .1 .

made

In each of

up
the

over

90% o f

the

two y e a r s o f t h e

t h e s e m ajo r s p e c i e s were

biomass
study,

investigated.

22

during
the

Three

the

period

interactio n s
species

of

among

e a c h made

TABLE 2 . 1 .
S p e c i e s a b u n d a n c e s i n B a i l e y Yard f o r t h e y e a r s 1 9 8 1 - 1 9 8 3 . ^ S p e c i e s d e n s i t i e s a nd
y i e l d s were m easu red in S ep tem ber o f a l l t h r e e y e a r s .
Y i e l d i n g/m
i s above ground d ry
w e i g h t , w h e r e s a m p l e s w e r e d r i e d a t 65
f o r 72 h o u r s b e f o r e w e i g h i n g .
S p e c i e s d e n s i t i e s and
mean b i o m a s s w e r e n o t d e t e r m i n e d f o r t h e r h i z o m a t o u s g r a s s e s A g r o p y r o n r e p e n s and Bromus
inerm is.
1981
, 2
g/m

#/m2

A m b r o s i a a r t e m i s i i f o l i a L.
Agropyron r e p e n s ( L . ) Beauv.
P l a n t a g o l a n c e o l a t a L.
C h e n o p o d i u m a l b u m L.
T r i f o l i u m r e p e n s L.
L e p i d i u m c a m p e s t r e ( L . ) R. B r .

1.03 g 8 0 .2 8
—
66.38
0.64
24.31
0.07
15.68
8.41
0 . 12
0. 13
2.03

77.2
—
38.2
223.6
67.6
15.9

A c h i l l e a m i l l i f o l i u m L.
Bromus i n e r m i s L e y s s .
D a u c u s c a r o t a L.
O x a l i s s t r i c t a L.
P a n i c u m c a p i l l a r e L.
P o l y g o n u m c o n v o l v u l u s L.
P o t e n t i l l a r e c t a L.
T r i f o l i u m h y b r i d u m L.

1. 52
—
0.28
0.04
0.04
0. 15
0.03
0.41

Species

Mean Wt.

1 .21
14.46
2.86
1. 76
1.37
1. 28
1.37
8.71

1982

0.8
—
10. 3
46.2
39.0
8. 8
51. 7
21.4

Mean Wt.

1983

g/m2

///m2

1.53
—
0.87
0.06
0 . 16
0.05

137.17
36.85
32.07
2. 56
18.64
1. 96

89.8
—
36.9
45.3
113.3
40.4

1.60
—
0.45
0.07
0.06
0.08

0.03

0.04
8.85
4.93
0.60
2.27
0. 25
3.69
1. 74

1.3
—
26.2
28 . 0
32.9
5.3
120.9
9.3

.003
—
0 . 17
0.04
0.05
0,08
0.02
0 . 20

—

0. 19
0.02
0.07
0.05
0.03
0. 19

Mean Wt.

g/m2

# / m2

140.55
5 0. 10
11.79
8.80
2.36
1.38

88.0
—
26.0
126.0
38.0
15.4

.006
8.41
2.04
2 . 56
1.42
0 . 16
3.94
1. 31

2.0
—
12.0
58.0
28.4
2.0
176.0
6.5

24

up

over

10%

of

the

artem isiifo lia,
fourth

total

biomass

Agropyron

( Chenopodium

repens,

album)

was

in

of the

s p e c i e s wa s d i f f e r e n t

fifth

portion of
in

the

years

repens) .

(1st

year

Ambrosia

annual,

and

and

-

Chenopodium

A gropyron,

biom ass

dom inant

2.1 ).

It

disturbed

often
in

an

Yard

annual

often

and

a nd

Crompton

investigated

along

1975).

that

is

A pril,

exhibit

very

in itiate

flow er

buds

July.

male

flow ers

produce
plants
and
by

a

in

produce

single

copious

rather

p ro d u ce anywhere

September
seeds,

rad iu s).

(D ickerson

which
The

detrim ental.

are

only
It

is

three

found

in

is

Trifolium

Lepidium
are

common
years

ragweed,

of

the

America.

North

It

an u p r i g h t h e r b

is

of

the

six

I n d i v i d u a l s em erge from s e e d

pollin ated

yellow

wind

to 60,000+ seed s
1971).

The

dispersed

w hile

per

the
to

hayfever

June,

and

and

Female

species

near

species

and

pollen.

( 3. 5mm X 2 . 5 m m ) ,

of

Am erica

one

is

cause

open

only

species

this

the

(Table

field s,

E astern

the

was

study

The

of

a

follow .

May

seed

is

perennials.

through

Sweet

m ajor

T rifolium

-

growth

amounts o f

im portance

year

annuals,

in

tap ro o t.

from 2 , 0 0 0
and

species

rapid

large

a

The c h o i c e

ag ricu ltu ral

roadsides

generally

the

and

all

to

2nd

spring

Ambrosia

1-1.5 m high w ith a sh o rt

late

cam pestre,
are

native

while

abundant

were m o n ito re d .

six species

in

lanceolata)

most

( Com positae),

B ailey

h abitats,

(B assett
species

is

in

L.

( Ambrosia

in th e abundances o f minor s p e c ie s

P lantago,
these

artem isiifo lia

the

year

t h e two y e a r s o f t h e e x p e r i m e n t a l

Lepidium

B rie f d e s c ri.p tions of each of
Ambrosia

of

for

each

Plantago

abundances

th e s tu d y due to v a r i a t i o n

field

w inter

three

a nd
one

o
(num bers/m )

all

consistently

flow ers

individual

plant

in August

reproduces

adult

the

only

(w ithin

lm

h u m an s a p p e a r s

to

in

North

the

eastern

A m erica and a r e a s o f Europe a s w e l l as b e i n g a m a j o r c r o p weed.

be

25

Agropyron

repens

( L . ) Beauv.

most abundant

species,

2.1).

a

It

is

ag ricu ltu ral
is

now

weed

stands

found
that

species

in

the

In

fact
f r om

in itiate

nutrients

it

such as

of

cloning

species'

than

in

the

seedlings.

study

site

(Table

native

to Europe b u t
1977).

often

is

dicotyledonous
few

through rhizom es
this

It

in v ery dense

produces

seeds,

( W e r n e r and

s t u d y t h a t emerged

com pletely
spring,

second

a v e r y common

Yard,

in

the

a nd

Rioux

generally

rather

early

is

was

herbaceous

was t h e o n l y s p e c i e s

growth

the

and

Bailey

Agropyron

rhizom es

in

grass,

(W erner

establishm ent

1977).

emergence o f o t h e r
up s o i l

the

,

perennial

d istrib u tio n
land

2

Quackgrass

p r i m a r i l y by v e g e t a t i v e

spring

rhizom es

in
fallow

(W erner

Rioux 1977).
in

spreading

in N orth America.

exclude

reproducing

terms o f biomass/m

tenacious,

circum polar

commonly

in

(P oaceae), quackgrass,

from

seed.

The

before

the

generally

Agropyron re p en s

and c o m p e t e s s t r o n g l y w i t h c r o p s .

can r a p i d l y

However,

it

is

tie
also

o c c a s i o n a l l y used as a f o r a g e o r hay s p e c i e s .
P lantago
was t h e
of

L.

t h i r d most ab undant

the

study

lanceolata

to tal

biomass o f

(T able

2.1).

abandoned

native

Europe

but

et
to

al.

t h e most

1980).

100+

basal

ro sette.

The

flowered
produce

if
up

to

in each of

is

a

fields,

lanceolate
first-year
reached

50+ s c a p e s

in

leaves,

and
in

along

a critical
w ith

plantain,

each

3-40

observed

densely

of

in

in

about

flowered

in

also

is

some c o n s i d e r

it

It

the world (C avers

long,
this
.190

spikes,

the

found

an d a d u l t s
cm

years of

perennial

roadsides.

species

A pril

size

three

d istrib u tio n ;

late

individuals

the

prostrate

common n o n c u l t i v a t e d
emerge

narrow -leaved

t h e B a i l e y Y a r d , m a k i n g up 10-12%

now c i r c u m p o l a r

Seedlings

they

in

community

P lantago

law ns,

t o be one o f

species

the

pastures,
to

(P lantaginaceae),

consist
forming

study
g.

o f up
a

flat

generally
P lants

usually

in

may
late

26

July

or

A ugust.

Individuals

can

ind iv id u als
usually
as

P lantago
produce

are

local

is

pastures

up

generally

(w ithin

considered

10,000

this

album

t h e most numerous
did

not

2.1).
all

is

it

It

to

Bailey
the

Ya rd

total

in

first-y ear

dispersal

is

et al.

is a lso

1980).

p articu larly

quite

p alatable

to s t a b i l i z e

wa s

term s o f numbers/m
of

the

in
to

soils.

lam bsquarters,

biomass

This

one

of

, but

it

community

(T able

a n e r e c t a n n u a l w h i c h i s a v e r y common a g r i c u l t u r a l weed i n

tem perate

known.

in

much

S e ed

to a g r i c u l t u r e ,

( C henopodiaceae),

sp ecies

contribute
It

L.

though

i s w e l l known ( C a v e r s

legumes; however,

pollinated.

d i s p e r s a l b y a n i m a l s and

l i v e s t o c k and i s o f t e n u s e d a s a p i o n e e r s p e c i e s
Chenopodium

wind

seeds,

however,

be d e t r i m e n t a l

an d e s t a b l i s h e d

and

productive.

parent);

seed

to

gynodioecious
to

not

1 ra o f

a contam inant of crop

species

is

is

areas

of

the

considered

w orld.

The

origin

to be a a n t h r o p o p h i l i c

of

Chenopodium

species

is

not

found grow ing in

a s s o c i a t i o n w i t h o t h e r weeds i n any open d i s t u r b e d

h abitats

( B a s s e t t and

Crompton

A pril

or

had

July,

and

growth
and

1978).

phase

Seedlings

through

September.

local

up

to

100,000

(w ithin

1 m of

ag ricu ltu ral
a nd

Brown

Crumpton

wind

in

1978).

buds

in

produces

(Stevens

parent),
and

but

the

sugar
It

album
b eet,

is

is

Individuals

can

also

S eed

often
than

problem

weed

carrot,

an d

toxic

to

a

rapid

in August
and

is

potentially

dispersal

are

f o r more

corn,

flower
flow ers

seeds

a

May,

perfect

1932).

remain v i a b l e

Chenopodium
potato,

late

pollinated.

seeds

practices

1946).

particu larly
and

and

in

in itiate

Chenopodium

self-com patible
produce

June,

emerge

is

usually

dispersed

40 y e a r s
in

(Toole

many

soybean

livestock

by

crops,

(B assett
due

to

c o n c en tratio n s of oxalic acid.
Lepidium

cam pestre

(L. ) R.

Br.

( B rassicaceae) ,

field

cress,

is

an

27

annual

or

field s,

and

fluctuated
never
Like

The

roadsides,

a g re at deal

contributing
most

w ith

biennial

occasional

North

America

emerge

A pril

or

oblanceolate

but Lepidium seeds
wheat.

in

the

al.

In

older,

native

to

and

cm i n

of

to

the

in

S eed

field

areas

unplowed

did

Europe

but

has

most

areas

1963).

M os t

seedlings

ro settes

flow ers,

appears

set

tw o-year

basal

individ uals

to

lo cal,

though

seed,

old

be

i m p u r i ti e s of

flow ered,
a nd

of

of

f l o w e r s on d e n s e

the m ajor

plants

spread

in

bearing

flower

where

community,

dispersal

no

Ya rd

the

it

com prise one of

B ailey

of

p ro strate
When

in

fallow ed

study (Table 2 .1 ) ,

occur

C ronquist

length.

plots

the

biomass

now

produce

I960).

study

my

elsew here

is

it

a nd

known t o

are

total

newly

cam pestre

up t o 60 cm i n h e i g h t ,

stem,
et

L_.

the

species

May

of

three years

(G leason

10-15

leaves,

(B uchholtz

individuals

weeds,

early

p ro d u c e an u p r i g h t

w inter

5% t o

Am erican

North

racemes

than

m or e

tem perate
in

the

w inter-w heat,

to

abundance

during

ag ricu ltu ral

certain

common

rare

especially

individuals

were

present.
T rifolium

repens

L.

(Fabaceae),

perennial

commonl y p l a n t e d

abundance

and

2.1).

im portance

While

North

native

A m erica,

to

C ronquist

May

they

This

is

if
the

during

reach

only
the

1963).
a

on

study

flow ered.

In

much l a r g e r

a nd a p p e a r e d

other

in

site

areas

of

three

of

emerge

field,

also
the

an d
in

l ow

creeping

fluctuated
study

attracts

along

late

flow er

reduced

a

now commonly

is

field s

size,

that

to be o l d e r .

years

species

were v e r y
the

is

It

Seedlings

su fficien t

clover,

and law n s ,

fallow

species

study

Individuals

the
this

Europe,

p articu larly

( G l e a s o n a nd
and,

in p a s t u r e s

white

in

A pril
July

insect

in s i z e

ind iv id u als

of

in

(T able

found

in

roadsides
and e a r l y
or

August,

p o llin ato rs,

a nd o n l y r a r e l y
T rifolium

were

G eneral Approach
The a n a l y s i s o f s p e c i e s
of

each

species
and

sp ecies

as

in v a r io u s

assum ptions

success
to

function

w ith

this
it

reproductive

individuals
I

to

the

only

type

Ideally,

fitness

of

to

future

contribute

in ab ility
used

to

fo llo w the

individual

of each s p e c ie s
survive

to

biom ass

assumes
than

of

biomass

seeds

to

that

be

to

larger

in

the

seedbank.

sm aller

plants.

th a t dem onstrate

that

This
seed

set

is

be r e l a t e d

is

this

because

o f my

Instead,
the

I have

response

p l a n t s must
Use o f

m o re

plant

successful

supported

a straightforw ard

for

the a b i l i t y

O bviously,

produce

in

of

species

to q u a n tif y

assum ption

species

ab ility

reproduce.

ultim ately

other

problems

However,

generations

successfully

plants

the

to determ ine

and s u r v i v o r s h i p

able

should

i.e .

to the abundance of c o m p e tito r s .

adults

offspring
studies

plant

fate

future

the

is

short-lived

w o u l d be i m p o s s i b l e
to

of

special

success

of

the success

what

generations.

individuals

of

each

F irst,

individuals,

It

to

of

There are s e v e r a l

be m easured?

followed

individuals

abundance

approach.

s i n g l e growing s e a s o n s.
these

re q u ire s observing

of

contribute

have

of

sp ecies m ixtures.

a n d how c a n

the

study,

a

in teractio n s

by

many

function

of

p la n t biomass (se e Harper 1977).
Second, what i s a r e a l i s t i c
in

each

m ixture?

studies

on

Many

animal

studies

a nd

other.

P lants

several

of

in teractio n s,

density

of

p articu larly

in div iduals

as

the

the c o m p e titiv e e f f e c t of each s p e c ie s

dem onstrate

t h e same s p e c i e s
m agnitude

species

used

to

in d iv id u als of
orders

of

populations,

abundance measure b e s t r e l a t e d
on i t s e l f

measure of the abundance of c o m p e tito rs

extrem ely p l a s t i c

i n t h e same e n v i r o n m e n t

d ifferen t

in

size.

Thus,

grow th, w ith

frequently
density

being

may n o t

29

accurately re f le c t
plants,

biom ass

uptake

rate

suggested

resource

that

the

biom ass

may b e
or

S p itters

w eight

a b etter

response

of

1983a).

biom ass/m

O

com petitive e f f e c t of a population.

of

the

effect

1 9 83 ,

production

of

populations
the

the p o t e n t i a l

individuals
that

lim its

or

yield

variable

sp ecies

In

this

is

growth.
(dry

than

in

d irectly

So,

w eight

density

I w ill

it

to

the

has

been

biom ass/m )

to

com petition

study,

related

In

use

of

in m easuring

(Goldberg

and

Werner

g e n e r a l l y use

yield

(dry

abundance

com petitor

j

)

as

the measure

of

the

of

species.
The
growth
this

third

problem

patterns,

w ith

study

being

is

annual

extrem e

com pletely

bare

seeds

below-ground

or

individuals

in

Septem ber.

The

estim ated
plants
between

field,

the

any

most

plant

w ith the

community

pair

in
of

from

In

So,

and
to

Y a rd

The
the

this

actual

the

in

com petitive

effect

over
of

th e whole
the

season.

asso ciate

species

This
is

of

are

from

w eights

f r om

the

in teractio n

biomass

assumes

reflected

a

until

of

s p e c ie s w ith the growth of i n d i v i d u a l s of a fo c a l s p e c ie s

accum ulated

in

either

biomass

study

final

is

season

plant

understand

seasonal

o ne u s e d

contained

in

am c o r r e l a t i n g

the

B ailey

throughout

discussed

m ids ummer

I

A pril,

exhibit

as

Agropyron) .

increases
and

such

biomass a l l

(for

Septem ber.
species,

com m unities

com m unities

plant

rhizom es

w eights

plant

examples.

d ataanalyzed

harvested

associate
has

plant

that

by

that

that
its

an

the
final

( o r midsummer) b i o m a s s .

F ie l d E x p erim e n ta l Design
S im ilar
several

experim ental

new t r e a t m e n t s

designs

were

were

included

in

used

in

both

1983 a n d

the

1982 a n d

1983,

but

two y e a r s d i f f e r e d

in

the

n um be r

and

size

of

A g ro p y ro n were p e r f o r m e d .
and

consisted

species

all

com binations,
control.

com binations of

possible

the

five

paired

species

The m i n o r s p e c i e s

t r e a t m e n ts used
the d esig n

in

how

The b a s i c d e s i g n was t h e

of d i f f e r e n t

alone,

r e p l i c a t e s a nd

five

were

sp ecies,

removed

and

ail
the

from a l l

field

methods

are

including

the

four

natural

plots.

in each y e a r of th e s t u d y a r e l i s t e d

and g e n e r a l

removals

of

same i n t h e two y e a r s

com binations,

control,

the

presented

each

species

community

The d i f f e r e n t

in Table 2 .2 .
in

this

Only

chapter;

the

approxim ately

1/4

!

methods o f a n a l y s i s a r e d e s c r i b e d

in C h a p te r s 3-5.

F i e l d D e s i g n 1982 ( Y e a r 1) - I n Nove mbe r o f
hectare
was

of

Bailey

smoothed

Ya rd

in

was

Mar ch

1982

q u a d r a t s were e s t a b l i s h e d
was

randomly

replicated

assigned

three

Removals
as

spring

of

plants

growth

significantly

disturb

( Agropyron re p e n s

continuous

were

York

of

20

rake.

s e c tio n of
treatm ent

cm.

The

same a r e a

S ixty-nine

this
a nd

area.
each

2

x

2

m

Each q u a d r a t
treatm ent

was

were

performed

the

the

at

were

root,

soil

soil

by h a n d

removed

preventing

surface.

soil

not re s p ro u t

surface

after

throughout

t wo o r

the

in

by g e n t l y
regrow th,

The

i n e r m is ) could
disturbance.

the

beginning

These g r a s s e s

three

the

and

this

did

up
not

grasses
way due

w e r e r e m o ve d b y
growing

clippings.
prevent

A pril

pulling

rhizom atous

n o t be removed

throughout

late

summer

to

species

from t h e m i d d l e

season.

The q u a d r a t s

re-invasion

by

species.

On J u l y
quadrat

single

and Bromus

clipping

m aintained

undesired

a

a depth

a

in a l e v e l

r e m o ve d

extensive

Most c u l m s d i d

using

began.S e e d lin g s

This

potential

to

tim es.

in d ividuals.

to

p l o we d

1981,

were

25,

10 p l a n t s

measured

to

of each
estim ate

treatm ent

effects.

?
1 m of each
From

previous

31

TABLE 2 . 2
Experim ental
d e s i g n f o r e x p e r i m e n t s o f 1982 a n d 1983
showing t h e s p e c i e s
present
in each tre a tm e n t.
+ indicates
the
presence of the s p e c ie s in the tre a tm e n t.
(G = A g r o p y r o n r e p e n s , A =
Ambrosia
artem isiifo lia,
P = Plantago
lanceo lata,
C = Chenopodium
album , L = Lepidium c a m p e s tre , T = T r if o liu m r&pens) .
R eplicates are
lm x lm p l o t s i n 1982 a nd . 6 m x . 6 m p l o t s i n 1 9 8 3 .

Treatm ent
code

G

S pecies P resent
A
P
C
L

G
A
P
C
L
T

+

GA
GP
GC
GL
GT
AP
AC
AL
AT
PC
PL
PT
CL
CT

+
+
+
+
+

GLP
GCP
GAC

+
+
+

-G
-A
-P
-C
-L(82)
-T ( 8 3 )

+
+
+
+
+

+
+
+
+

+
+
+

+

+

+

+

+

ontrol

T

Number o f
R eplicates
1983
1982
3
2
3
2
2

+
+
+
+
+

2
2
3
2

+
+
+
+
+
+
+
+

+
+
+
+
+
+
+

+
+

+
+
+
+
+•

+

+

+
+

+
+
+

+(82)
+(82)
+(82)
+(82)
+

+
+
+
+

+

—

18

—

+

15
18
18
18

—

3
2
2
—
3
3
—
2
—

15
15
15
—
15
15
15
—

15
15
—
15
—
15

2
2
3

—
—
—

+(83)
+(83)
+(83)
+(83)

2
2
2
3
3
—

5
5
5
5
—
5

+(82) +(83)

2

- 5

32

experience,
Ambrosia

d ifferent

values

artem esiifo lia

longest

leaf

were

were

a nd

chosen

Chenopodium

m easured,

for

for

d ifferent

album

height

P lantago

be

for

length

a nd

of

Lepidium

were m easured.

No

n o n - d e s t r u c t i v e m easure of Agropyron re p e n s abundance co u ld

determ ined

a nd

this

so

there

species.

are

species

from n e a r b y n o n - e x p e r i m e n t a l

72 h o u r s ,

the

mid-summer

of

The a b o v e - g r o u n d

At

no

effects

for

a nd

lanceolata

c a m p e s t r e number o f l e a v e s and l e n g t h o f l o n g e s t l e a f
satisfactory

sp ecies:

p arts of these

a nd

weighed

to

same

tim e,

estim ates

th irty

for

ind iv id u als

a r e a s were a l s o

measured

p l a n t s were t h e n h a r v e s t e d ,
obtain

individual

treatm ent

dry

of each

as above.

dried

w eights.

a t 65°

M ultiple

r e g r e s s i o n a n a l y s i s was u s e d

to d e te rm in e m orphology-biom ass r e g r e s s i o n s

(Table

obtained

2.3).

The e q u a t i o n s

of i n d i v i d u a l s

in

10.

.5

dried

for

for

above

ground

species

tap w ate r,

plant

72 h r .

in

to

estim ate

at

65°C.

biom ass.
each

pressed,

was c l i p p e d

Each

th e biomass

dried,

were

also

of

O

1 m

(leaving

a t ground l e v e l ,

pressed,

center

plant

roots

of

harvested,

an d w e i g h e d .

f r o m S e p t e m b e r 28 t o

the

indiv idual

Subsamples

quadrat

the p erio d

from w i t h i n

m) o f e a c h q u a d r a t

a nd

each

took p lace du rin g

Each i n d i v i d u a l

a border of

used

the e x p e rim e n ta l q u a d ra ts .

Autumn h a r v e s t i n g
O ctober

were

was

ten

washed

The r e p r o d u c t i v e

th e n weighed

individuals

of

thoroughly

in

state

of each

i n d i v i d u a l was a l s o n o t e d .
By m i d s u m m e r , when v e g e t a t i o n was d e v e l o p e d ,
vegetation

in

the

rest.

This

the

autumn

of

1981,

each

of

p l o we d

for

P lants

in

this

the

north

area

area

end

had

of

not

been

whereas
the

four

tended

the

the
years

to

be

study

area

plowed

in

m ajority
previous
larger

and

four

the

to

became o b v i o u s

was q u i t e

the

of

it

years

study

in itiatio n

included

d ifferen t

that
than

previous

area
of

several

had
the

to

been

study.

perennial

33

Table 2.3.
E q u a tio n s used to e s tim a te the biomass of i n d i v i d u a l s of
each
species
usin g m orphological m easures.
The e q u a t i o n s
were
determ ined u sing re g r e s s io n a n a ly s is of t h i r t y in d i v id u a ls of each
sp e c ie s h a r v e s tin g near the ex p e rim en tal p lo ts in Ju ly of both y e a rs.
I l l = len g th of lo n g est l e a f , l i b = len g th of lo n g e st branch.

R egression Equation
Year

1,

1982

Ambrosia

log(w eight)

Plantago

lo g (w e ig h t) = -0.975

Lepidium

log(w eight)

Chenopodium

lo g (w e ig h t) = -1.387

fe ar 2,

R_

= -1.237

+ .0 1 9(height)

.84

+ . 163(stem d ia m e te r )

.89

= - 1 . 2 7 8 + . 047 (/ / l e a v e s )

.85

+ . 026(height)

.89

1983
= -1.260 + . 020(h e ig h t) + .068(111)

Ambrosia

log(w eight)

Plantago

l o g ( w e i g h t ) = - 1.111

+ .035(#

Trifolium

log(w eight)

= -1.711

+ . 2 00(/ / b r a n c h e s ) + . 0 3 9 ( l l b )

.75

Chenopodium

log(w eight)

= -2.353

+ .433(111)

.90

leaves)

+ .028(111)

.88
.70

species

not

found

canadensis

from

differences,
the

a

all

the

rest

previous

plots

analysis.

rep licates

in

in

this

resulted

being a v a ila b le

the

study

experim ent).

located

This

of

in

area

(including

Because

of

Solidago

these

apparent

n o r t h e nd a r e a w e r e e x c l u d e d
only

two

of

the

f o r many o f t h e d i f f e r e n t

fr om

original

treatm ents

three

f o r Year

1.

F ield
Care
had

was

D esign
taken

a single

both

the

n um be r

to

of

grouped

into

performed

as b efo re.
the

we a k

(3%)

grass

by

hand

early

May

a nd

undesired

solution

necessary

in

in

on

the

of
a

early

June

plots.

survivorship

a nd

study s p e c ie s but Agropyron.

the

changed
a nd

blocks,

A gain,

removal

each

in

1982.

field
to

to

were e s t a b l i s h e d

that

increase

increase

the

(Table

2.2).

in e a rly

April

treatm ent

was

treatm en ts of p la n ts

S e e d l i n g s w e r e r e mo v e d by h a n d

herbicide

applied

quite
It

to

treatm ent

were

to

also

followed

Follow ing

the

'R ound-up'

individual

plants.

A

individual

cu lm

of

was

performed

both

in

removed

have

m onitored

in

(the

each

effectiv ely

appeared

plots

to

t h e summer t o p r e v e n t
was

of

treatm ent

W ithin

This

The

each

was a p p l i e d

was

grasses.

was a l s o

contact

sponge.

area

as

amount o f work i n v o l v e d w i t h c l i p p i n g

the

Round-up

an

prepared

treatm ents

A pril.

the la rg e

for the r e s t of

Species

late

of glyphosate)

using

of

for

plot.

grasses,

rhizom atous

individuals

types

blocks.

beginning

1983 was

w ithin

1.3 x 1.3 m p l o t s

five

salt

in

design

quadrats

To e l i m i n a t e

isopropylam ine

The

to a s i n g l e

rhizom atous

F ield

quadrats

different

rep licate

randomly a s s ig n e d
were

all

history.

Two-hundred and e i g h t y
1983,

B ailey

place

plowing

number
of

1983 -

all

no e f f e c t
and

of

the

on o t h e r

w e e d ed

where

re-invasion.
Year

2 for

i n i t i a l weeding,

all

of

the

the d e n s ity

of in d iv id u als
center
the

f o r e a c h o f t h e c o m p o n e n t s p e c i e s was d e t e r m i n e d f r o m t h e

. 5m x . 5m o f e a c h p l o t .

study

cotyledon

sp ecies
or

had

first

leaf

d e t e r m i n e d when t h e
used

to

determ ine

emerged

Most,
by

stage.

if

not a l l ,

this

The

time

density

p l a n t s were h a r v e s t e d
the

survivorship

of

and
of

the
were

still

ind iv id u als

in Septem ber.

probability

in d iv id u a ls of

of

in

was

the
again

The v a l u e s w e r e

each

sp ecies

in

each

plot.
During J u l y

11-15,

5 plants

m o f each q u a d r a t were m easured
measures
(Table

and

techniques

2.3).

m easuring
branch.

to

n u m be r

of

R eproductive

main

state

to e s t i m a t e

estim ate

Biomass e s t i m a t e s

the

of each s p e c ie s

treatm ent e f f e c ts .

biomass

used

for T rifo liu m
branches

was a l s o

and

noted

from t h e m i d d l e . 5 x . 5

in

The same

1982 w e r e

repeated

r e p e n s w e re d e t e r m i n e d by
the

for

length

all

of

the

longest

i n d i v i d u a l s measured

of each s p e c ie s .
From

September
x

.6

25

to

2
m (leaving

center

.6

level,

pressed, dried,

O ctober

5,

a border

all
of

individuals
.35

m)

and w eig hed as d e s c r i b e d

were
for

from

w ithin

clipped

1982 a b o v e .

at

the

ground

Chapter 3

COMPETITIVE EFFECTS AND RESPONSES OF SPECIES

To b e g i n
community,

to

it

understand

was

in teractions

necessary

between

Interactions

between

Comparisons

reduction

of

different

associate

in

the

growth

and

other

by

of
in

of

for
having

species

and

role

pair
have

a

species
d istin ct
effect

focal

a

com bination

could

of

low c o m p e t i t i v e

a

response

community.
effect

and

contrasting

the

inth e

species

response

The c o m p e t i t i v e

a

high

ab ility

the

of

com petitive

com petitive

to

presence

when e a c h i s

com petitive

have

large

the

com petitive

species.
the

com petition

when

focal

this

for

components,

of

several

species

in

species

determ ine

structuring

involve

common a s s o c i a t e

species

exam ple,

a

in

in

potential

Comparisons

growth

a

some

t wo

single

a single

the

of

com petitive

of

of

of com petition

determ ine

species.

response

species;

ab ility

each

reduction

presence

effect
the

the

to

species

response.

contrast

the

effect

presence

of

on

these

species.
It
of

would

species

because

of

be b e s t

w hile
the

in

to

measure

the

complex

full

the

com petitive

com munity;

interspecific

two

sp ecies

that

could

confound

pair

of

sp ecies

has

been

extracted

to

that

conditions

in tw o -sp e c ie s

in

community.

the

conditions

Combining

investigate th eir

the f u l l
all

possible

however,

interactio n s

than

community

the
out

of

of

36

in

m ixtures

species

in

responses

im practical

m ixtures
each

full

must

may

is

So,

the
It

an d

it

resu lts.

interactions.

pairs

effects

not

of

more

possible

m ultispecies

be k e p t

i n mi nd

always

reflect

com petition

experim ents

of

37

is

sim ilar

to the d i a l l e l

agronomy.

D iallel

com petitive

effects

species

or

the

been

natural

and

1 96 7 ,

performed

extended

of

T ripathi
a

much

to u n d e rsta n d in g

as

m entioned

community c h a n g e s
in teractin g .
population

above,

the

level
and a t

a

w hether

ther e s u l t s

lim ited

this

although

in

total

each

yield

can be e x t e n d e d

using

Caputa
can

of

provides

a

in

Fowler

(biom ass/m ) ,

be

com m unities.
f r om

the

are

it

at
is

full

norm ally

measure

species

to o th e r d e n s i t i e s

further

species

only c a u tio u s ly

sp ecies

of

crop
1963,

1 9 70 ,

two s p e c i e s

pair

of

T h is method has

m ultispecies

pair

method

between

are discussed

a nd

in
the

Welbank

1977).

in which the

this

cu ltiv ars

(e.g .

method

a

investigate

extent

Jaquard

iso latin g

p articu lar

These r e s t r i c t i o n s

more

interactions

interactions

density

Trenbath

using

to

crops

1968,

environm ent

Second,

used

d ifferent

on

1 9 62 ,

obtained

been

between

weeds

(W illiam s

The r e s u l t s

have

responses

to

assem blages

1982).

F irst,

experim ents

effects

N orrington-D avies
also

c o m p e t i t i o n s t u d i e s m o s t co mmon ly p e r f o r m e d

of

the

a single
not

or to ta l

known

yields.

i n C h a p t e r 4.

Methods
F ive m ajor sp e c ie s

from a f i r s t - y e a r

a l o n e and i n t w o - s p e c i e s
1983.

The p e r f o r m a n c e o f

function

of

the

measures

of

perform ance

(Years
general

1 and

2)

species

and

com binations
individuals
w ith
were

percent

which
used,

c o m m u n i t y w e r e gr own

i n two c o n s e c u t i v e

years,

1982 and

of e a ch s p e c i e s were a n a ly z e d
they

were

growing.

above-ground

survivorship

f i e l d methods a r e g iv e n

o ld -field

in

biomass

each

plot

in C h a p t e r 2 and w i l l

as a

Two d i f f e r e n t
of

individuals

(Year

2).

The

o n l y be s u m m a r i z e d

here.

Year 1 (1982) - The natural, unmanipulated density of each species

37

is

sim ilar

to

the d i a l l e l

agronomy.

D iallel

com petitive

effects

species

or

the

been

experim ents
and

1967,

perform ed

of

a

much

natural

assem blages

(W illiam s

1982).

The

obtained

extended
F irst,

to

as

understanding

m entioned

community c h a n g e s
interacting.
population

above,

the

level

and

at

a

whether

the

resu lts

lim ited

this

iso latin g

although

in
pair

to tal

each

yield

can be e x t e n d e d

using

Caputa

can

of

only

provides

in

Fowler

the

species

to o t h e r d e n s i t i e s

or

full

are norm ally

a measure

it

be

com m unities.
fr om

( b i o m a s s / m )»

further

species

cautiously

species

of

crop
1963,

1 97 0 ,

two s p e c i e s

pair

of

T h is method has

m ultispecies

method

between

are d iscussed

a nd

in
the

Welbank

1977).

in which the

this

cultivars

(e.g .

method

a

investigate

extent

Jaquard

interactions

particu lar

These r e s t r i c t i o n s

crops

more

using

to

different

Trenbath

1 96 2 ,

in teractio n s

density

on

used

1 96 8 ,

environm ent

Second,

been

between

weeds

T ripathi

to

resu lts

have

responses

effects

N orrington-D avies
also

c o m p e t i t i o n s t u d i e s m o s t c ommonl y p e r f o r m e d

at
is

of

a

single

not

total

the

known

yields.

i n C h a p t e r 4.

Methods
F ive m ajor sp e c ie s

from a f i r s t - y e a r

a l o n e and i n t w o - s p e c i e s
1983.

The p e r f o r m a n c e o f

function

of

the

measures

of

performance

(Years

1 and

general

field

2)

species

an d

com binations
individuals
w ith
were

percent

which
used,

c o m m u n i t y w e r e g r ow n

i n two c o n s e c u t i v e y e a r s ,

1982 and

o f ea c h s p e c i e s were a n a l y z e d
they

were

growing.

above-ground

survivorship

methods a r e g iv e n

old-field

in

biomass

each

plot

in C h a p t e r 2 and w i l l

as a

Two d i f f e r e n t
of

individuals

(Year

2).

The

o n l y be s u m m a r i z e d

here.

Year 1 (1982) - The natural, unmanipulated density of each species

38
was u s e d
for

in

all

ambient

between
w ill

m onoculture

species

plots

was

to

used

including

of

two

species

removing

the

unwanted

com binations

Each
2,

species

Table

but

nu mb er o f

Both

the

no

for

each

nested

the

estim ated

plant

of

w eights

of v a ria n c e

were

in

v ariance

in d e n s i t i e s ,

that

1.

A gain,

and

all
were

3

times

each

plot

(July

23)

leaf,

determ ined

and
(see

of

all

using

height,

for

or

plants

in

and w e i g h e d .

the

m i d s um me r
harvest

test

and w i t h
used

(see

an u n b a l a n c e d

a nd an a l l - p a i r s

in Year

longest

data

were

the

the

each

four

The

that

there

five

analyze

C hapter
design

1979).

hypothesis

of

was

treatm ents

asso ciates.

the

data,

treatm ent
2)

plant

log-transform ed

between

to

a nd t h e

(Downing

in each p l o t n e s te d w it h i n
rep licates

F ifteen

weeding

or

m i d s u mm e r

w eight

was

density

th e above ground p o r t i o n s of a l l

to

type.

A

w ith
Due

an d

the

natural

in

the

analysis

used

a n a l y s i s was n o t a t t e m p t e d .

Y e a r 2 ( 1 9 8 3 ) - The e x p e r i m e n t a l
to

in

d istrib u tio n s

used

of

some

ind iv id u als

dried,

w eight

m onocultures

of

2

autumn

differences

p la n t w eights

eith er

of

in

Treatm ents

through

2.1

densities

m onocultures

in terest.

which were

f r om

(in

analysis

loss

weight

the

the

five

(length

w eights

from

all

in

in

analysis.

sp ecies

estim ated
size

plant

of

p l o t s were h a r v e s t e d ,

sp ecies

variations

were

the

replicated

At t h e end o f S e p t e m b e r ,

sig n ifican t

individual
to

and

analysis

individual

The

plant

determ ined

follow ing

were

w eights

leaves)

in a l l

w eights

2 .2).

between

nearby p l o t s .
plants

was

Table

variations

in

of

(see

v ariatio n

random

variance

treatm ent

Agropyron

regressions

sm all;

were

possible

Chapter

quite

experim ents

natural

error

treatm ents

clipping.

The

the

d ifferen t

by

tw o-species

densities).

generally

contribute

established

a nd

the

design

natural,

i n Y e a r 2 was v e r y

unm anipulated

sim ilar

d e n s i t i e s of each

39

species

were used

tw o-species

in

fifteen

com binations.

individual

p lots

in

the

treatm ent.

In

Year

2,

repens

in

the

determ ined
estim ate
the
1,

of

due

Year

1,

ind iv id u als
(July

regressions
nearby

of

performed

included

as

m ortality,
w eights

plant

of

P lants

individuals
impact
an d

each

analyzed u sing

of

an

the

follow ing

that
five
an

rep licate
analysis

transform ation

survivorship,

of

m id s umm er

th e Welsch a l l - p a i r s

in
of

were

were

of

in Year

2.1).

As i n

were m easured
effects

were d e te rm in e d
biomass

at

using
f r om

in

late

transform ation

was

in

The

end
the

the

each

plot

each

the

as most

died

(T able

each

for

w eight

used

treatm ent

from

zero

resu lts,

rates

species

having

T rifolium

1.

each

before

each

a l l o w i n g an

those

arcsine

w eights

of

The mean d e n s i t i e s

Agropyron

in Year

of

by

a nd

locate

each sp ec ie s

above-ground

died

the

of

from

the

which

individuals

Follow ing
in

on

but

for

ind iv id u als
plant

replaced

period.

to

rep licates

w eight which

harvested

individual

log-transform ed

treatm ents

vs.

survival

w eights

fr om

1979).

estim ate

used

September h a r v e s t ,

emergence

to

were

percent

plot.

little

species

the

mean

rep licate

all

size

was

d ifferen t

in

an aly sis,

untransform ed
generate

slig h tly

statistica l

on

the
that

t h e n d r i e d a nd w e i g h e d a s

B efore

blocked

variation

11-15)

P lants

five

m onocultures

was

The d e n s i t i e s

over

each

all

design

cam pestre

design.

were

of

plant

plots.

Septem ber,

has

species

including

block

using

Lepidium

survivorship

year-to-year

mi ds ummer

field,

i n m i d - M a y a nd a t

plant

four

to

A random

experim ental

in both

other

treatm ents,

plot.

were

used

species
of the

in

each

summer

were

f a ll analysis.
sp ecies

to

This

had v e r y

low

quite

sm all.

The mean p l a n t

plots

of

treatm ent

of
the

the

analysis

distrib u tio n s

data,

w eight,

each

and

(Sokal

( Do wn in g

differences
final

between

w eight

and R o h l f

were

1981).

were

40

RESULTS
Y e a r J_:
an d

The g r o w t h o f

tw o-species

response

to

p lots

the

growth o f o th e r
is

the

five

s p e c i e s when g r o w n i n m o n o c u l t u r e s

exhibited

very

different

presence
species

rem arkably s im ila r

of

other

(F igure

to

that

species

3.1).

p atterns,

and

in

However,

from t h e autum n.

both

th eir

in

effect

the d a ta

their
on

the

f r o m m ids ummer

A m b r o s i a d e m o n s t r a t e d no

s i g n i f i c a n t r e s p o n s e when g r o w n w i t h o t h e r s p e c i e s , g r o w i n g a s w e l l w i t h
an a s s o c i a t e
the

species

autumn

associate

as

this

sign ifican tly

species.

In

yield

when

respectively).
species,
autumn
than

does

wa s

w eight biom ass/m 2 ) of
of

it

p articu larly

20%

of

w ith

also

wh en g r o w n
of

grow th

in

w ith

Lepidium

no

P lantago.

also

suppressed

by

on
the

somewhat s u p p r e s s e d
In

probably

the

presence

of

its

3.21

presence of o th er
A m brosia,

6.19

in

readied

most

extreme

and

The g r o w t h o f
of

Ambrosia

an d

yield

(dry

o v e r 60%

(93

86

g/ m ,

w ith

other

and

grown

A gropyron.

reduced
(w ith

d ifferen t

achieved

P lantago
Ambrosia

2

By t h e
to

less

-

1. 16

Chenopodium

and

L e p i d i u m was v e r y
Agropyron

species,

being

example

of

com petitive

the growth of Lepidium

m onoculture

or

when

a

in

and

was

by t h e p r e s e n c e o f P l a n t a g o .

(w ith

g /in d iv id u al).

A gropyron,

w ith

g /in d iv id u al).

presence

o f Ambrosia red u ced

grow th

m onoculture -

the

had

m onoculture

-

strongly

grown

Plantago

Ambrosia

Ambrosia

m onoculture
effect

or

suppressed

g /in d iv id u al,
had

when

Agropyron

Ambrosia

was

presence

its

Agropyron h a r v e s te d

169 g / m 2 > w h i l e A g r o p y r o n n e v e r
grown

the

suppressed

m onoculture,

Plantago

harvest,

in m onoculture.

Ambrosia

.12

C h e n o p o d i u m wa s a l s o
strongly

P lantago.

suppressed

N either

to

suppression,
less

t h a n 4%

g /individual,
affected

by t h e

by t h e p r e s e n c e o f

Lepidium

or

Chenopodium

41

F igure 3.1.
The mean g r o w t h o f i n d i v i d u a l s o f e a c h s p e c i e s i n
t h e m i ds u mm er and a u t u m n o f Y e a r 1 when g r ow n i n m o n o c u l t u r e s
and a l l p o s s i b l e t w o - s p e c i e s m i x t u r e s .
Each s e t o f h i s t o g r a m s
r e p r e s e n t s the perform ance of a s in g le fo c a l s p e c ie s (a c ro s s
t o p ) when g r o w n w i t h a l l p o s s i b l e a s s o c i a t e s ( a c r o s s x - a x i s ) .
E a c h b a r r e p r e s e n t s t h e m ea n g r o w t h o f t h a t f o c a l s p e c i e s when
in m onoculture or a tw o -sp ecies m ix tu re.
The a s t e r i s k s p r o v i d e
t h e r e s u l t s o f an a n a l y s i s o f v a r i a n c e t o d e t e r m i n e i f any
d i f f e r e n c e s e x i s t b etw e en t h e f i v e t r e a t m e n t s on e a c h a x i s (* P < . 0 5 , ** - P < . 0 1 , n s - no s i g n i f i c a n t d i f f e r e n c e s f o u n d ) .

Figure 3.1

AGROPYRON

PLANJAGO

mson g/indMduaf (g/m3 for Agropyron)

PERFORMANCE OF FOCAL SPECIES. YEAR 1

AMBROSIA
MIDSUMMER

ASSOCIATE SPECIES

LEPIDIUM

CHENOPODIUM

43

appeared

to h a v e any e f f e c t on one a n o t h e r .
The m o s t e x t r e m e e f f e c t o f c o m p e t i t i o n w o u l d r e s u l t

Y e a r 2_:
death

of

the

affected

species.

There

was

little

no

or

in the

m ortality

in

A m b r o s i a , P l a n t a g o , and C h e n o p o d i u m when t h e s e s p e c i e s w e r e g r o w n e i t h e r
in

m onoculture

significant
(74%

a nd

or

in

m ixtures

reduction
59%

survivorship
in ab ility

to

the

respect iv e ly ).

only

determ ine

Trifolium

3.1).

I

species,

grass

was

unable

A gropyron,

densities.

precise

exhibit

to

a

for

determ ine

because

However,

s h o o t s was e v e r o b s e r v e d

individual

did

when g ro wn w i t h Ambros i a o r A g r o p y r o n

survival

survival,
of

senescence of

in

(T able

this

no

of

my

v isib le

species u n til

the f a l l .
The
again
by

mean

s h o ws

the

1,

other
w ith
at

very

The g r o w t h

an d

of

the

g/m^.
only

either

of

(Figure

th e y always
over

A g r o p y r o n was

At

yield

both

sp ecies
these

of

to

Lhe

species

strongly

which

suppressed

was
by

not

used
the

effect

a nd

by

and a u t u m n .

by t h e

g/individual
by t h e

from

growth
the

155.6

of

growth

place

of
of

at

any

As i n

p r e s e n c e o f any

of

and

The

presence
of

Ambrosia

Year

of

P lantago

T rifolium ,

.5 g / i n d i v i d u a l ,
in

to

Agropyron were

in d iv id u als

Lepidium

A m brosia,

in m onoculture

P lantago.
of

autumn h a r v e s t ,

presence

Ambros i a

or

1.0 g / i n d i v i d u a l

the

g/ m^

2

response

exhibited

approxim ately

autumn,

presence

from Y e ar

Ambrosia grew in m o n o c u ltu re s

to a p p ro x im a te ly
in

species

pattern

affected

affected

and

reduced

f r om ov e r 2 . 5 g / i n d i v i d u a l

4.0

the

five

b e t w e e n mi ds ummer

Agropyron

affect

sp ecies

The

averaged

only

m i d s u mm er

the

com petitive

of

ind iv id u als

slig h tly

the

of

3. 2 ) .

A m b r o s i a was

W hether

which r e d u c e d
57.6

of

species,

mi ds umm er

patterns

was c o n s i s t e n t

growth

species.
other

individuals

species

sp ecies

the

of

differen t

various

p articu lar
Year

growth

2,

and A g r o p y r o n .

was

also

By t h e

1

T a b l e 3. 1
P e r c e n t s u r v i v a l o f f o c a l i n d i v i d u a l s when g r o w n i n
c o m b i n a t i o n w i t h d i f f e r e n t a s s o c i a t e s p e c i e s i n Y e a r 2.
V alues in
the
same
row b u t
followed
different
letters
are
significantly
d i f f e r e n t ( p < . 0 5 , Welsch S t e p - u p T e s t ) .
A dash in d ic a te s t h a t the
m o r t a l i t y c o u l d n o t be d e t e r m i n e d f o r t h a t s i t u a t i o n .

Focal
Species

Amb r.

Ambrosia

-

Agropyron

-

P lantago
T rifolium
Chenopodium

1.00a
.74ab
1.00a

A s s o c i a t e S p e c i e s _______________
n on e
Agro.
P l a n .T r i f .
Chen, (m o n o c u ltu re )
1.00a

1.00a
.59a
1.00a

1.00a

-

1.00a

1.00a

1.00a

1.00a

1.00a

1.00a

.93bc

-

.97bc

1.00c

.97a

.89a

-

1.00a

Figure 3 .2.
The mean g r o w t h o f i n d i v i d u a l s o f e a c h s p e c i e s i n
t h e m i d s um me r a nd a u t u m n o f Y e a r 2 when g r o w n i n m o n o c u l t u r e s
and a l l p o s s i b l e t w o - s p e c i e s m i x t u r e s .
Each s e t o f h i s t o g r a m s
r e p r e s e n ts the perform ance of a s in g le fo c a l s p e c ie s (a c ro s s
t o p ) when g r o w n w i t h a l l p o s s i b l e a s s o c i a t e s ( a c r o s s x - a x i s ) .
E a c h b a r r e p r e s e n t s t h e me an g r o w t h o f t h a t f o c a l s p e c i e s when
in m onoculture or a tw o -sp ecies m ix tu re.
Below t h e b a r s on e a c h
a x is are the r e s u l t s o f a l l - p a i r s a n a ly s is fo r the e f f e c t of
d i f f e r e n t a s s o c i a t e s p e c i e s on g r o w t h o f t h e f o c a l s p e c i e s .
S p e c i e s a n d m o n o c u l t u r e t r e a t m e n t s u n d e r l i n e d b y t h e same l i n e
do n o t d i f f e r s i g n i f i c a n t l y ( p < . 0 5 , W e l s h s t e p - u p t e s t ) .

Figure 3.2

AMBROSIA

AGROPYRON

PLANTAGO

CHENOPODIUM

62-

2-

mean g/individual (g /m J for Agropyron)

PERFORMANCE

OF FOCAL SPECIES, YEAR 2

3-i

TRIFOLIUM

llll

1-

■ „

4-

nil L i
o»a;

O)

<

nil

c.

O

AUTUMN
3-!
4-

2-

160120-

4-

2-

I-

BO-

Jl

J

I # 01 6

I * * '

40-

£e:F6

I ^6|

61

ASSOCIATE SPECIES

47

autumn,

the

T rifolium

presence

of

these

two

from . 5 5 g / i n d i v i d u a l

A m b r o s i a and

.15 g / i n d i v i d u a l

significant

effect

However,

the

in

significant
Ambrosia

of

autumn,

com petitive

dem onstrating

Chenopodium' to

less

had

in m onocultures

w ith

the

species

of

the

Chenopodium e x h i b i t e d

other

sp ecies

at

A m b r o s i a , A g r o p y r o n , and

Plantago

effects

of

the

on

the

greatest

t h a n 9% o f

its

growth

effect,

growth

growth

of

to .1 2 g / i n d i v i d u a l w ith

Agropyron.

presence

reduced

midsummer.
dem onstrated

Chenopodium,

reducing

the

in m onoculture

no

w ith

growth

of

( w i t h Ambros i a

- .18 g / i n d i v i d u a l , m on o cu ltu re 2.13 g / i n d i v i d u a l ) .

For b o th

years,

the

consistent

differences

sp ecies

focal

effect
1)

on

data

in

the

species.

on e a c h o f

the

indicate

com petitive

Ambros i a

focal

focal

C om petitive
very

consistent

sen sitiv ity
Ambrosia

of

was

pattern.

never

associate

species

(Year

or

1)

by t h e

response

is

as

effects

had

indicated

by

focal

affected
w hile

there

the

six

the g r e a t e s t

the

least

by

the

consistent

the

to

presence

T rifolium

an d

is

large

species

populations

and

grow th,

were

of

and

associate

com petitive

effect,

the

also

(Year

if

any,

differences

of

any

in

a
the

effects.

of

the

p o ssible

Chenopodium

and

Lepidium

very

strongly

2)

were

species.

exact

exhibited

com petitive

(Year

p r e s e n c e o f a lm o s t any o t h e r

very

of

strong

C h e n o p o d i u m and L e p i d i u m

( Y e a r 2)

Again,

various

Chenopodium

affected

a nd

were v e r y

species.

responses,

the

there

a l w a y s had

species

o r C h e n o p o d i u m and T r i f o l i u m

on t h e d i f f e r e n t

that

opposite

This h ie r a r c h y of
of

the h ie r a r c h y

of e f f e c t .
These d a t a
community

is

indicate

very

g reat,

that

the

potential

as

the

presence

for
of

com petition
an

asso ciate

in

the

species

full
may

48

reduce

the

also

growth

some

various

a

insightful

focal

species

patterns

in

com petitive species.

some c o m b i n a t i o n
but

of

being

of

little

very c o n s is te n t

having

If

the

by

that

"com petitive

com petitive a b i l i ty

a large

affected

by g r e a t e r

the

com petitive
presence

ranking of a b i l i t y

in

of

90%.

There

ab ility "

of

is defined

effect

these s p e c ie s

is

the

as being

on o t h e r

potential

are

species

com petitors,

a

observed:

Am brosia > A g ro p y r o n > P l a n t a g o > Chenopodium = L e p i d i u m o r T r i f o l i u m

Note

that

this

sp ecies

in

pattern

is

the
(see

per

hierarchy

the

full

m atches

community.

the
It

ranking

is

c a u s e d by p o p u l a t i o n e f f e c t s

individual

or

per

amount

G o l d b e r g an d W e r n e r 1 9 8 3 ) .

seem t o be a t r a n s i t i v e

of

im portant

biomass/m
to

2

r e me m b e r

for

these

that

this

and d o e s n o t n e c e s s a r i l y r e f l e c t

com petitive

ab ility

N evertheless,

of

each

com petitive

species

ab ility

does

property of these populations.

DISCUSSION
D ifferences

in the co m p etitiv e e f f e c t s

Several

conclusions

com petitive
consistent
little
all
also

in teractio n s
hierarchy

sp ecificity

other
s hows

usually

can

species
that

hierarchy

all

of

these

of

and

found

species

species

a nd

lack
are

of

drawn
this

about

ju st

effects

losers'

lim ited

community.

that

is,

them.

are

of

that

(1)

The
is

affects

There

in teractio n s.

in teractio n s
for

the

This h ie r a r c h y

asym m etric.

b y an d c o m p e t i n g

of

there

Ambrosia
(2)

in c o m p e titiv e

sp ecificity

nature

dem onstrates

some o f

interactio n s

the

plant

in teractio n s;

not

pairw ise

'w inners
the

in

com petitive

strongly,

most

d efin ite

The

of

be

and r e s p o n s e s of p o p u l a t i o n s

suggest

are
(3)
that

t h e same r e s o u r c e

49

or resources.
The

asymmetric

t h e way p l a n t s
o f Ambros i a
th is

obtain

has

community

access

to

nature,

or

suggests

that

a greater

uptake

resources

other

through

to

scenario,

it

the

second

the

between

suggests

found
that

of

access

that

about

the p o p u la tio n

the o th er

population

lim iting

to

tallest

species

has

resources.

a

in

greater

Due t o t h e i r

a b o u t a m o re s y m m e t r i c

species
in t h i s

there

are

On

may u s e

may be

and

some

so

the

to

resource,

W atkinson,

light

forth.

et

u n i m p e d e d by
which

filters

In t h i s

sim ple

to c o m p le te ly asym m etric

other

access

(e.g .
freely,

the

lead

interm ingled.

study w ith

the lim itin g

the reso u rce

to l i g h t w i l l

in d ividuals.

is

the r e s o u r c e

plant

to a more e q u a b l e

different

hierarchy

rate

unimpeded by t h o s e b e n e a t h ,

m o i s t u r e may l e a d
of

than

Ambrosia

p l a n t may u s e

the o rd ered ac c e s s

interactio n s

ab ility

F o r e x a m p l e , when l i g h t

The t a l l e s t

plants,

The f a c t

something

c o m p e t i t i o n b e t w e e n i n d i v i d u a l s w h i l e o t h e r s w o u l d be

t o do s o .

1983).

the

suggest

w o u l d s ee m t o b r i n g

i n d i v i d u a l s have an " o r d e r e d "
al.

resources.

com petitive

some p l a n t

lik ely

lim iting

in teractio n s

a greater

or evenly balanced
less

interspecific

hand,

the
The

com petition

resource

if

consistent

for

the ro o ts

com petitive

the n e c e s s a ry asym m etric i n t e r a c t i o n s

sort

of

ordered

access

also

suggests

to

the

lim iting

resource or re so u rces.
The
are

not

that
of

the

hierarchy

in

species

specializing

are
Much

community s t r u c t u r e

1973;

com petitive

partitioned

resources.

use of

of

of

this

the

proposed

some r e s o u r c e s

ab ility

com munity;

t o be a b l e

is,

there

to use d i f f e r e n t

previous
that

that

work

in

that
is

resources

th eo retical

resources

no

evidence

or
and

classes
animal

individual

s p e c i e s must have e x c l u s i v e

to

in

p ersist

f o r p l a n t s - Van d e n B e r g h a nd B r a a k e h e k k e

t h e community
1978,

(e.g .

Silvertow n

May

1982).

50

Thus,
of

species

all

should

b io tic

abundance

of

and

a

of

abiotic

species.

d iversificatio n
because

occupy d i f f e r e n t
factors

Several

hypothesis

the

very

'n ic h e s ',

is

lim ited

controlling

authors

not

have

potential

for

1976,

Goldberg

and

plants

Werner

than

lig h t,

phenologies

nearly

all

1977,

m oisture

the

in

of

found

in t h i s

established

address

are

p o ssib ility

to

that

set
and

niche

communities

when t h e r e

is

a

( S c h a f f e r and L e i g h 1 9 7 6 ,

Huston

1979,

Newman

correctly

b io tic

as

the

this

plant

or

1982,

argued

that

abiotic

environm ent

such as

d ifferent

m icrosites

early

successional

community m i g h t

be

species,

considered

to

be

The c o n s i s t e n t h i e r a r c h y o f c o m p e t i t i v e

study su g g ests

seedlings

the

the

fact,

this

occupying " d i s t u r b a n c e n i c h e s " .
ab ility

of

In

argued

quite

a nd n u t r i e n t s ,

1977).

plants

have

being

d istrib u tio n

p artitioning

1978,

Others

many a s p e c t s

(Grubb

of

Connell

1983).

may p a r t i t i o n

other
or

H arper

niche

the

applicable

s m a l l number o f n o n - d i s c r e e t , s h a r e d r e s o u r c e s
Silander

the

not

th at resources a ffe c tin g

being

that

p artitio n ed ,

other

niche

though

th e growth
it

dim ensions

does

are

not

being

p artitio n ed .
C onsistent
species

have

hierarchies

been

Pemadasa 1976,
documented

data

D urrant
(1977)
first

between

have
1965,

p revious

the

all

since

the

the

Fowler

possible

pairs

odd

reanalyzed
1967,

history

of

of the

Breese

ab ilities

(Pemadasa

1982), but t h e r e
(1962)

been

only

studies

W illiam s

id en tities

analyses,

com petitive

other

N orrington-D avies

review ed
tim e,

in

Handel 1978,

exception.

experim ents
the

found

of

performed
of

seven

several
Breese
this

H ill

sp ecies

noted

a nd

1974,

l e a s t one w e l l
com petition

of

plants

(M cG ilchrist
1973).

one o f

and
1965,

Trenbath

published,

in the s tu d y .
that

d ifferen t

Lovell

additive

and H i l l

s p e c ie s used

and

and

is at

tim es

study

of

for

the

Of a l l

the

the

species

51

dem onstrated
it

had

a

fixing

a very d if f e r e n t

sim ilar

legume,

reduced

com petitive

Trifolium

com petitive

species

This

the

other

nitrogen

for

both

on

itself

It

and

others,
was

appeared

species.

would d i s r u p t

the

sp ecies

which

on

interactio n s

effect

response.

subterraneum ,

effect

s p e c i e s was p r o v i d i n g
sp ecific

com petitive

to

a

nitrogen

have

may be

its

though

a

much

that

this

neighbors.

S u ch

any h i e r a r c h y o f c o m p e t i t i v e

ab ility .
V a r i a t i o n W i t h i n _a S e a s o n
The
m onths,

plants
but

period.

in

they

this

probably

Im m ediately

occupy v e ry l i t t l e
to

their

are

in

A gain,

in

after

At

not

such

periods

it

most

for a tem p o ra rily very abundant re s o u rc e ,
resources,

such as l i g h t ,

were u s e d
these

in both y ears

dates

and a f t e r
Two

c a n be u s e d

of

this

to

compare

t h e m id s umm er d a t e s
questions

can

community

about

growth

of

individuals

before

and

Ambrosia

may s t i l l

be

v ariatio n

after

continued

the
to

and

w ithin
are

there

midsummer
grow

a

a f t e r midsummer.

this

are

sm all

and

they a f f e c t or respond

the

season,

some r e s o u r c e s

com petition

is

occurring

although com petition

for o th er

that

O n l y two c e n s u s d a t e s

( m i d s u m m e r and a u t u m n ) , b u t

im portance o f c o m p e titio n b e fo re
3.1,

3.2).

each

season:

In
the

of

are

differences

census?

h a l f of the growth in m o n o cu ltu res

during

abundant.

for

throughout

five

is

Figures

addressed

tim es

some

that

be o c c u r r i n g .

the

of

when r a i n

experim ents

(cf.

all

periods

seems u n l i k e l y

period

ind iv id u als

during

during

a

at

unlikely

tim es
as

for

interact

g erm ination,

other

abundance,

these

do

coexist

s p a c e , making i t

neighbors.

great

community

the
there

in

both

summer,

and m i x t u r e s

species

in

differences

the
in

species

interactions

Year

and

]

w ith

Year

2,

approxim ately

fo r both years o cc u rrin g

P l a n t a g o and C h e n o p o d i u m e x h i b i t e d

little

o r no g r o w t h

after
no
the

midsummer;

change

mean p l a n t

between

patterns

the

are

in

appeared

1,

m i ds u mm er
in

Year

autumn

to
2,

established

when

to

the

mi ds umm er an d d i d
So,

w hile

the

in

both

in teractio n s

to

alm ost
and

are

also

exhibited

indicates

the

in

in

during
the

size

that

of Lepidum ,

the period

autumn.

f r om

m ixtures.

T rifolium ,

mi ds ummer

However,

evident

nearly

of

that

in

the

too

between

suggests

were

firm ly

surprising

rapid

growth

during th is

the

to

the

pattern

tim ing

hierarchy

unchanged

species
not

a period

expected

the

The d a t a

is

fr om

of

before

t h e autumn h a r v e s t .

differences

most

This

is

Individuals

sm aller

harvested

double

in

were

between

June

bec o me

were

years.

m id s u m m e r d a t e s .

be

This

m i d s u mm e r .

speciesin v estig ated ,

periods

m ight

plants

there

five

before

not change a t

interactio n s

late

autumn.

a nd m i x t u r e s

o f o t h e r s p e c i e s on T r i f o l i u m was e s t a b l i s h e d

species

and

and

actually

m onocultures

com petitive a f fe c ts

among

m id s umm er

appeared

in

in m o n o c u ltu re s

o f t h e c o m p e t i t i v e e f f e c t s o f o t h e r s p e c i e s on P l a n t a g o and

Chenopodium
Year

size

for

all

period

the

two

of

sam pling

the com petitive

established
the

growth

and p a t t e r n

that

since

of

before

the

t i m e b e t w e e n May

of the

s p e c i e s a nd i t

r e s o u r c e s would

be

most

lim itin g .
V a r i a t i o n B e t w e e n Two S u c c e s s i v e S e a s o n s
This
grow th in

data

also

enables

two d i f f e r e n t

Chenopodium w ere v e r y
an d

so

differences

differences
tillers
The

mean

between

of

investigate

The d e n s i t i e s
in
the

I did

between the
sizes

to

years.

sim ilar

in d e n s ity .

changed

us

the

species

years

not determ ine

and

o f A m b r o s i a , P l a n t a g o , and

two y e a r s

two

interactio n s

of
are

if

the
not

s tu d y (Table
attributable

2.1)
to

th e d e n s i t y of Agropyron

two y e a r s .

ind iv id u als

of

Ambrosia

and

Chenopodium

were

greater

in

Ambrosia

Year

2

than

exhibited

in

Year

13% m o r e

1.

Individuals

growth

in

C h e n o p o d i u m e x h i b i t e d 75% m o re g r o w t h .
in Year 2 e x h i b i t e d
explain

this

60% l e s s

pattern

in

two c o n s e c u t i v e y e a r s
A possible

In

Year

and

1,

little

w ith

of

senescing.
the

of

an

rain

of

in

intense

this

September

September

Year

period

growth
in
2

that
may

A g r o p y r o n , and Chenopodium.
are being

filled

The

most

difference
the

two

between

com petitive
years.

the

hierarchy

preparation.

g r o w t h m i g h t be

the

two s e a s o n s

continuous

most

of

2,

July,
the

that

May,

there

but

plants

the

(Table 3 .2 ) .

throughout

In Year

when

June,

was v e r y

there

was

were

an

already

J u n e and J u l y Y e a r 2 c o i n c i d e s w i t h

P lantago,

year.

w h i c h may h a v e

However,

delayed

the

the

lim ited

the

e x t r e m e l y wet month

senescence

of

A m brosia,

S enescing p l a n t s o f te n drop le a v e s as seeds

resu lt

patterns

despite

It

the

appears

of

is

that

species

differences
to

be

t h e i r h a rv e ste d w eight.
there

is

no

com petitive
in

mean

consistent

both

qualitative

ab ility

plant
w ith

between

size.
and

The
between

T h u s , e v e n t h o u g h i n Year 2 i n d i v i d u a l s o f Chenopodium w ere v e r y

much l a r g e r
Year

in

1.

t h e same s o i l

in

a nd h a r d e n e d , w h i c h r e d u c e s
in terestin g

years,

after

and

have

in d iv id u a ls of P lantago

in

June

for

1 w hile

The e x p e r i m e n t s w e r e p e r f o r m e d

in

in

Year

to

September.

p articu larly

species
in

fairly

in

of

is d iffic u lt

different

was

m onocultures

than

in Year

differences

ex trem ely dry

The v e r y d r y

period

growth of

the

was q u i t e

p recip itatio n ,

abundance

for

p recip itatio n

July,

than

resu lts.

2

However,

i n t h e same f i e l d

ex planation

p attern of r a in f a ll

the

growth

Year

in

1,

the

and

individuals

relativ e

b a s i c a l l y unchanged.

of

P lantago

com petitive

w e r e v e r y much s m a l l e r

ab ilities

of

these

species

than

in

remained

54

T able 3.2
P r e c i p i t a t i o n by m o n t h a t
S t a t i o n f o r t h e y e a r s 1982 a n d 1 9 83 .

MONTH

A pril

Year 1
1982

4. 75 cm

the

K ellogg

Year 2
1983

Average
1967-81

12.78

10.78

May

10. 19

13.82

8.46

June

10. 54

4.85

11. 73

July

10.82

7. 26

15.91

August

5.51

7.32

9. 54

September

3. 53

11.00

9.04

B iological

Chapter 4

THE DYNAMICS OF PLANT POPULATION INTERACTIONS

The

experim ents

species

used

However,

the

densities

at

com petitive
were

year

which

species

each
of

vary

of

the

each

allowed

in r e g u l a t i n g

com petitors.

at

the

To

the

fu rther
in

m onocultures

five

ab ilities.

single

experim ents

of

of
I

each

w ill

w ith

sim ultaneous

of

a

the

range

of

potential

and

(am bient)

explore
1983

and

the

(Year

2)

tw o-species

abundances.

im portance

complex

with

chapter

as

a

The

of

intraspecific

the d e n s i t y
w ith

the

potential

density

of

of

The

u sin g sim ple m ath e m a tic a l

in

sim ple
effects

o ne

both

m onocultures,

species

held

constant,

of

both

species

rep resen tatio n s

and
section

of the

then
and

the
can

tw o-species

investigate

interspecific,
each

constant

the

of

sp ecies.

effects,

held

w hile

f r om

the success

abundance

intrasp ecific

resu lts

53

the

d en sities

effects,

in trasp ecific,

effects.

of

growth

varying

densities

to q u a n tif y

o f one s p e c i e s

interspecific

of

is

function

investigate

quantify

varying

in trasp ecific

this

sp ecies

m ixtures

estim ate

action

in

first

can

m ixtures

to

over

interspecific,

tw o-species

tw o-species

discussed

d ensities

approach used

m onocultures

com plex

com petitive

held.

species,

analysis

tw o-species m ixtures w ith

m ixtures

was

only

that

the growth of each s p e c i e s .

individuals

used

3 dem onstrated

different

applied

species

an

sim ple

The b a s i c

be

these

Chapter
very

drawn

to

intraspecific,

The

had

conclusions

responses

fin ally ,

in

The p u r p o s e was t o m e a s u r e t h e p e r - a m o u n t c o m p e t i t i v e e f f e c t s

experim ents

of

each

ab ilities

designed

m ixtures.
and

in

discussed

the

potential
w ill

resu lts.

be

56

S e v e r a l d i f f e r e n t models of p l a n t
used

to

1 9 7 7,

describe
Trenbath

(1960,1961),
yields

of

total

two

two

and

has

three

studies

However,

the

from

to

study

(Baeumer
model

has

1 96 3 , M a r s h a l l
determ ine

de

Wi t

are

and

Schaffer

1 9 8 1,

(1981)

a nd

S p itters

(1983a)

model b ased

on t h e r e c i p r o c a l

yield

description

of

individuals

their

W illey

the

density

a nd

reciprocal
of

the

has

at

Ism ail

resu lts

been

Trenbath

in

applied

of

least
1983).

to

natural

o f t h e m o d el

( H a r p e r and C l a t w o r t h y
it

param eters
Goldberg

is d i f f i c u l t
of

the

model

and W e r n e r

1983,

1983a).

W atkinson

of

the

A

which

1 97 7,

1 9 82 ,

the

relativ e

model

1 9 7 9 ) and t h a t

of

in

species

when

that

the

Wi t

proportions

(Harper

individuals

1 9 6 9 , Wu a nd J a i n

Inouye

This

Fowler

de

experim ents.

the

three

in H arp er

of

design

w hile

lim itations

meaning

model

series

dynamics
to

review s

p red icts

0:1.

1 9 68 ,

d e n s ity of

bio lo g ical

1977,

to

extended

serious

to tal

a nd J a i n
the

(D eB enedictis
S pitters

1:0

The m a j o r d i f f i c u l t i e s

a r e d e p e n d e n t on t h e

which

constant

tw o-species

and

the

experim ental

kept

successfully

is

law s,

su b stitu tiv e

varied

(see

replacem ent

are

been

com m unities.

to

a

in

is

used

1 9 77 )

is

g r ow n

m ixtures

known

gas

ind iv id u als

species

extensively

best

R aoults'

species

of

tw o-species
The

on

series

density

in

1977).

based

replacem ent

the

growth

i n t e r a c t i o n s have p r e v i o u s l y been

intercropping

advantage

over

a

total

single

of

and h a s b e e n u s e d

Heath
yield

growth

1969,

Harper

t h e de Wit model
density

of

in

in m onocultures
ag ricu ltu re

The

1983a,

that

ind ividual

1983b).
is

tw o-species

yield
as

law i s

a

a function

versio n

to d e s c rib e

t h e model

plants.

a

and e c o l o g y ( s e e

tw o-species

successfully

(S p itters

proposed

The r e c i p r o c a l

in both

1977).

law h a s been u s e d
experim ents

law.

both

It

the

of

the

resu lts

h a s one l a r g e

n o t d e p e n d e n t upon

However,

as

with

the

de

57

Wit

m odel,

resulting

the
in

terms

the

often

the

do

actual

not

have

species

any

sim ple

dynamics b ein g

b io lo g ical

obscured

meaning

by t h e

form

of the model.
The

approach

v ariety

of

taken

sim ple
no

in

models

method

makes

should

s how how t h e

this
to

study

explain

assum ptions

about

per-am ount

is

to

the
the

effects

compare

the

variance

in

mechanism

of

of

ab ility

the

data.

of

This

in teractio n ,

com petition

change

a

but

w ith

the

d e n sity of co m p etito rs.

1.
In

1983,

single-species
w ill

only

be

I

In tra sp e c ific E ffects

investigated
plots

over

summarized

the

a

intraspecific

range

here;

of

effects

densities.

a detailed

using

The

description

data

general

of

from

methods

the methods

is

by r e m o v a l

of

i n C h a p t e r 2.
M onoculture
all

but

the

May

using

contact

plots

desired

all

species

hand-rem ovals

h erb icide.

species

for

in

an

attem pt

rep licates

of

each

from

and

Three
to

s p e c i e s were o b t a i n e d

each

plot

continued
different

generate

treatm ent

throughout

in

a c h i e v e d by ra n d o m ly re m o v in g h a l f o f t h e

using

a

(natural)

density

decide

plots

the

which

P lantago.

the

seedings

The s e e d s p l a n t e d

were

of

remained

d e n s i t y p l o t s which were seeded w ith
U nfortunately,

( 1)

fate

block

a

for

each

w ith

five

design.

The

half-densitv

i n d i v i d u a l s on t h e p l o t

each

ind iv id u al;

unm anipulated;
sp ecies

successful

in h ig h d e n s i t y

using

used

approxim ately

the d e s ire d

only

were

random

plots,

to

of

a

in e a r l y

summer

range of d e n s i t i e s ,

density

toss

consisted

initiated
the

treatm ents

a wide

arranged

and w e r e

three

coin

treatm ents

five

plots

for
for

and

(2)

full

(3)

high

in e a r ly A p ril.
Chenopodium

the o th e r

and

species

failed

to

germ inate

in d ividuals.
an d

so

and

these

plots

only

contained

No s i g n i f i c a n t e f f e c t s o f t h e

four

extra

the a n a ly s is

m onoculture

for a l l of

plots

for

natu rally

t r e a t m e n t b l o c k s were f o u n d ,

each

sp ecies

the s p e c ie s bu t Agropyron.

were

the o th e r

center

fifteen

. 6m x

oven d r i e d ,

. 6m o f

five

density

curves

of

have

resource
very

of

4.1,

the

(T able

the

e nd

of

the

Septem ber,

Ambrosia

4.1);

was

however,

by

negative

the

provided

a better

field

techniques

Chapter

2).

putting

up

relatio n sh ip
significant

model

when

new
to

culms

determ ine

by

The
the

shape

an

of

species

increasing
for

the

some

response

involved

and

the

of

individuals

and

negative.

The

was

given

by

the

reciprocal

the

shape

of

the

relationship

of

a

if

w ith

extending
actual

single

plot

at

a

best
yield
was

very

low

Agropyron

and

plants.

between

m aintains

the

perform ance

the

compared

description.

w orking

Agropyron

to

was c o m p e t i t i o n

significantly

presence

barely

other

there

u p on

p e r f o r m a n c e show t h a t

response

4.1).

mean

relatio n sh ip

influenced

was

difficu lt

the

plan ts w ithin

in th e m o n o c u ltu re s .

between

of

that

Table

d e n s i t y which c o n ta in e d v e r y l a r g e

density

at

negative

depending

achieved

density

description

The

harvested

sig n ifican t

different,

relatio n sh ip

increasing

strongly

a

(Figure

range of d e n s i t i e s

equation

were

co n sp e cifics, indicating

are

The

plot

i n t h e same a r e a

All

o f d e n s i t y on t h e mean i n d i v i d u a l

species

lim iting

each

for each s p e c ie s .

in

and w e ig h e d .

The e f f e c t s
all

p l o t s used

included

T h e s e p l o t s had b e e n

p a r t o f a s e p a r a t e e x p e r im e n t and were randomly l o c a t e d
as

occurring

a

growth
to

a

This

density

or

model

and

no

a problem

in

rhizom atous

growth

underground

linear
reflects

perennial

activ e

of

throughout
rhizom es.

biomass

of

grass
the
It

is

(see
year,
very

individuals

59

•
1.0

AMBROS I A

AGROPYRON

•

•

•

•

•

0

0.5

’ .

*

100

50

150

•

r

PLANTAGO

1

1

100

200

300

400

•

500

TRIFOUUM
•
•

•

2

•

0.5

*

•

•

. •

•

•

•

I o*
• •
250

500

I
50

7 50

MEAN

PERFORMANCE

(g/individual

%• • •

100

chenopodium

•

INTRASPECIFIC
EFFECTS

150

%
p• *
• •
• •
’ • •
i
400

individuals

•

•
800

/ m"

1
12 0 0

)

F igure 4.1.
The i n t r a s p e c . i f i c e f f e c t s o f d e n s i t y on t h e g r o w t h
o f i n d i v i d u a l s o f f i v e s p e c i e s g rown i n m o n o c u l t u r e s i n Y e a r 2.

60
('

n

T a b l e 4 . 1.
The a m o u n t o f v a r i a n c e e x p l a i n e d (R ) by t h r e e
e q u a tio n s d e s c r ib in g p la n t perform ance in m onocultures as a
fu nction of d e n sity .
The l i n e a r e q u a t i o n d e s c r i b e s t h e mean
w eight of the fo c a l s p e c ie s as a l i n e a r f u n c ti o n of the
m onoculture d e n s ity ; the r e c ip r o c a l y ie ld eq u atio n d e sc rib e s
t h e r e c i p r o c a l o f t h e mean w e i g h t o f t h e f o c a l s p e c i e s a s a
linear
fu nction
of
the m onoculture d e n s ity ;
the
log-log
e q u a t i o n d e s c r i b e d t h e l o g a r i t h m o f t h e mean w e i g h t o f t h e
fo c al s p e c ie s as a l i n e a r fu n c tio n of th e lo g a rith m of the
m onoculture
density.
(* - S i g n i f i c a n t ,
P <.05,
** S i g n i f i c a n t , P<.01)

Species

Linear
**

Ambrosia

.36

Agropyron

.32

Plantago

.38

Trifolium

„,*
.34

Chenopodium

.46

« *

**

**

R eciprocal
Yield
.74
.29
.58
,.45
.63

**
*
**
**
**

log-log
. 47
.30
.75
.42
.85

**
*
**
**
**

because

of

an aly sis,
as

these

underground

I have e s t i m a t e d d e n s i t i e s

individual

plants,

underestim ates

the

I

could

not

Agropyron

density

and

d e n s ity of t h i s
processes

that

relatio n sh ip

(Table

is

a

contribute

a great deal

only

individuals

contained
packed

highly

Five
as

to

a

the

the

described
able

to

increase

than

Even s e v e r a l

the

of

of

the

estim ating

effect

of

hyperbola

seeding,

of stunted

(Figure

of

also

mean

equation.
T rifolium

these

appeared

to

p lo ts m aintained

v ery poor growth of i n d i v i d u a l s

regulate
at

a very

(Figure 4 .1 ).

high
plots

The h i g h

i n d i v i d u a l s w hich were

to

on

the

have

taken

at

w eight

high
and

I expect th a t
to

very

the curve.

suppressed

between

4 .1).

log d e n s i t y

and

observed

appeared

the

increasing

dem onstrated

shape of

humans!)

were

yield

density

also

in

between

species.

plots

non-linear

which

in itial

correlation

a greater

r e l a t i o n s h i p would have been fou nd .

density

the

problems

this

the

resu lt

relatio n sh ip

by t h e r e c i p r o c a l

more c u r v i l i n e a r
other

T rifolium

non-linear

knowing

an d

evidence t h a t d e n sity -d e p e n d e n t

found u n d e r t h e h ea vy g r o w t h .

w ith

density

One o f t h e s e h i g h d e n s i t y p l o t s

tigrinum

the

is

moist

of

actual

by a l o g mean w e i g h t -

of

than

of

the

Ambyst oma

Individuals

l ow

rectangular

a heavy cover

(other

portion

to

individual
soil

the

significant

described

together.

vertebrate

it

negative

4 .1).

of

the

whether
due

the

Without

in r e g u l a t i n g
a

best

densities

very t i g h tl y

is

this

an d p l a n t b i o m a s s b y t r e a t i n g c u l m s

in d ividuals.

w eight

resem bles

In

overestim ates

determ ine

dem onstrated

curve

plots

of

im portant

The r e s p o n s e

density

this

s p e c ie s or w hether

are less

P lantago

but

w eight

density,

density

connections.

the

plots;

an

cover

in

densities,

density
if

of

best

I had b e e n

extent

However,
growth

contained

that

a

processes
T rifolium .

low d e n s i t y d e m o n s t r a t e d

62

The

growth

densities,
density

of

Chenopodium

w ith the o v e r a ll

relatio n sh ip .
density

was

seeded

obtain

extrem e

to

in

large

part

high

established

interm ediate d e n s itie s

su m,

exhibited
plant

higher

the

highly

w eight

and d e n s i t y .

to

potentially

for

high

enough

affect

th eir

g r o w t h was n o t e d

of

is

method

is

to

neighbor
been

designs

extrem ely

elim inating

hold

constant

to

sp ecies

investigate

sim ilar

seedling

plots

being

the

between

I had been a b l e to
all

densities
mean

r e l a t i o n s h i p s might

five of

the

increase

the

species

can

of d en sity -d e p e n d e n t

for
the

the e f f e c t s

present

experim ental
a nd

Werner

number o f

experim ents
intraspecific

by m a i n t a i n i n g

(ad d itiv e

effects

interspecific

usually

a large

s i n g l e v a l u e and t h e n m e a s u r i n g
associate

no

to

which were

case,

increased

some e v i d e n c e

quantify

also

d ifficu lt

of

it

to

(Goldberg

require

Instead

if

In tersp ecific

effects

effects

this

log

response

plots

relationships

However,

use a t a r g e t - n e i g h b o r

intraspecific

high

of the sp ec ie s used.

very d i f f i c u l t

intraspecific

five

w ith

seeding

know i f

own g r o w t h :

2.
It

at

f r o m 180 t o 560 i n d i v i d u a l s / m .

sp ecies

levels.

for a l l

the
In

non-linear

I do n o t

the o th e r

to

anticipated,

which

sig n ifican t,

have been found f o r
densities

than

sp ecies

due

densities.

was

In

suppressed

P la n ta g o , the h ig h ly n o n -lin e a r

establishm ent
at

strongly

p a t t e r n b e s t d e s c r i b e d by a lo g w e i g h t -

Like

increasing

wa s

the

effects

in

the

design

1983).

density

of

I

the

the e f f e c t of v ary in g

design,

Harper

of varying

1977).

the d e n s i t i e s

One

which e l i m i n a t e s
target

which would

have

effect,

system s.

However,

rep licates
which

independent

many
have

focal

have

treatm ents.
attem pted
species

to

at

a

th e d e n s i t y of the

This
of

d e s i g n was u s e d
each of the

four

63

p o s s i b l e a s s o c i a t e s on e a c h o f t h e
To

achieve

a

range

of

five

focal

species.

densities,

three

treatm ents

each p a ir

of s p e c ie s , w ith each tre a tm e n t r e p li c a t e d

arranged

in

consisted

of both sp e c ie s a t

first

a

species

half-natural
the

random

second

obtained
using

species

plots

method

p lo ts,

from

average

the

full

in

a

All

described

com bination,

number

of
of

each

for

each

average d e n s ity
were

to

determ ine

the

mean

controls

ten

density

and

on

to

each

removed

from

were
p lot,

Because

begin

sp ecies

to over

full

these

w ith,
in

the

natural
plots

A ll p la n ts

Septem ber, oven d r i e d ,

sp ecies

by

in each

a nd w e i g h e d , a s

are given

in Table

asso ciate

success

of

individuals

in

elim inates

intraspecific

fo r each f o c a l- a s s o c ia te

found in c o n t r o l

of

than

constant

which c o n ta in e d

effect

rather

the v aria n c e

effects

plots

the n a t u r a l d e n s ity
of

the

t h e end o f

at

individual.

individuals

the

species

H alf-d en sities

in d iv id u als

densities

species

intraspecific

only the

densities

on

treatm ents

i n C h a p t e r 2.

To h o l d t h e

closest

The

second

h a n d - w e e d i n g and a p p l i c a t i o n o f a c o n t a c t h e r b i c i d e .
p l o t were h a r v e s t e d a t

for

tim es in p lo ts

at half-natural

the

fate

of

the

density.

of

the

range

other

species

half

than h a l f - n a t u r a l

density.

2 .2).

and

natural

a v ariable

resu lts
less

first

determ ine

have

Table

density

removing

to

(see

five

used

t h e i r n a t u r a l , unm anipulated d e n s i t i e s ,

natural

at

toss

design

and t h e

randomly

coin

individual
th is

the

density,

by

a

at

block

were

of

variance

effects

is

These

yield

the

densities

p l o t s were u s e d

to

( d r y w eight biom ass/m )

focal

in

species

p l o t s were used ( n a t u r a l

2.1).

species

focal

the

species.

This

in trasp ecific

included

in

the

design

effects;

e rro r variance

of the r e g r e s s io n s .
As was

dem onstrated

in

Chapter

3,

there

were

differences

in

both

64

the responses of d i f f e r e n t
species
focal

and

in

the

species

effects,

effects

(Figure

increasing

negative

effect

com binations
The

of

4.2,

the

on

statistically

species

the

to

d ifferent

Table

significant

of

in

asso ciate

4.2 ).

abundance

w eight

the p re s e n c e of a s i n g l e a s s o c i a t e

of

focal
only

As

to

individuals,

but

e f f e c t was

four

of

of

ind iv id u als

of

which

previous

study

(C hapter

A m b r o s i a was n o t

species

However,

the

over

range

as

species

of

that

Agropyron
of

any

Agropyron did
an d

in

was

on

the

change

to

associate

had

the

have

tw enty

by

This

a

possible

a

the

than

be

presence

that,

of

to s u p p re ss
other

effect.
any

at

any of

a given

the o th e r

th e growth of

associate

or asy m p to tic

by

significantly

This agrees w ith

any i n c r e a s e

large

unaffected

species.

any

threshold

already

not

e f f e c t in

in

of

affected

by

that,

y ie ld does

Agropyron

yield

species.

not

is the only

A m b ro sia , which

i n C h a p t e r 3.

not

sign ifican tly

species.

However, i n c r e a s e s

decrease

also

is

the growth of

in

agreement

changes
in

in

the

the y ie ld

of

P lantago, T rifo liu m ,

w ith

the - h ie r a r c h y

of

previously discussed.
of

P lantago

As d i s c u s s e d
of

more

wa s

dem onstrated

investigated,

significantly

growth

growth

is

the

also

com petitive a b i l i t y

A gropyron.

or

the a n a l y s i s

Chenopodium.

The

much

yields

appears

agrees w ith

yield

affected

suppression

p r o d u c e an i n c r e a s e

also

3)

The p r e s e n c e o f A m b r o s i a d o e s a p p e a r

the

the

Ambrosia

the

focal

in trasp ecific

appears

the a s s o c ia te

the

the

a given

com petitors

a f f e c t e d by t h e y i e l d o f any o f

species.

w ith

on

(Table 4 . 2 ) .

growth

density,

species

above,

P lantago,

significantly

over

responded

chan ges in

Ambrosia a p p e a rs

but

the

only to

the

m agnitude

range of

yields

of
of

to have
this

the y ie ld

of

a large e f fe c t
effect

A mbrosia used

does
in

not
this

65

Figure 4 .2 .
The i n t e r s p e c i f i c e f f e c t s o f t h e y i e l d ( d r y w e i g h t
biomass/m^)
of various
associate
species
on t h e g r o w t h o f
i n d i v i d u a l s o f f i v e f o c a l s p e c i e s in Year 2.
Each a s s o c i a t e
s p e c i e s was g r o w n i n t w o - s p e c i e s m i x t u r e s w i t h t h e f o c a l s p e c i e s
a n d t h e d a t a o r g a n i z e d on a s i n g l e a x i s .
Each l e t t e r i n d i c a t e s
the
yield
of
a
sp ecific
asso ciate
species:
A= A m b r o s i a ,
G= A g r o p y r o n , P= P l a n t a g o , T= T r i f o l i u m , C= C h e n o p o d i u m .

66

c

t

AMBROSIA

p

•tt
c, >
c c

.6

■r p

G

G

( mean g / individual )

00

O

p

AGROPYRON
,
A

rc c

p

G

i

.4

%
G

Lf

*

a

r c

*

A

**
A

r

a

.2

a.
100

50

150

PLANTAGO

100

jl

200

JL.

.

300

400

1.0

500

TRIFOLIUM

PERFORMANCE

OF FOCAL

SPECIES

.8
Tt
.6

C

^O CC
c

Cc
C

A
<f‘ c <

c *

*

.2

*

a

C

*

c

A A
A

A

*

‘
100

200

300

400

500

100

200

300

*

\

.A

400

A

500

che n o p o d iu m

IN TERSPEC IFIC
EFFECTS

A■ A1
200

400

YIELD OF ASSOCIATE SPECIES ( g /m )
F ig u re 4.2

600

Table 4 .2 .
L i n e a r c o r r e l a t i o n s b e t w e e n mean w e i g h t o f f o c a l
s p e c i e s and t h e y i e l d o f d i f f e r e n t a s s o c i a t e s p e c i e s i n t w o - s p e c i e s
m ixtures.
N=10 f o r a l l s p e c i e s p a i r s . ( * - S i g n i f i c a n t , P<. 0 5 ;
** - S i g n i f i c a n t , P < . 0 1 ; a d a s h i n d i c a t e s t h a t t h e c o r r e l a t i o n was
not determ ined for t h a t s i t u a t i o n ) .

Focal
Species

Ambrosia

Ambrosia

-

Agropyron

.0 2

Associ a te Species
T r i f o l i u m Chenopodium
Agropyron P la n ta g o
-.35
-

-.57

.04

.32

.40

-.03

.06

.49

.06

P lantago

- . 15

-.65*

T rifolium

-.43

- . 65*

.0 0

.2 2

-.60*

-.77**

Chenopodium

.0 1
-.14

-

68

study.

Increasing

growth
This

of

is

not

in

In

was

Both
of

affected

these

species

little

effect

by

the

resu lts

very in te r e s tin g
have

a

response

low

effect

on

with

of

the

any of

is

com petitive

had

least

unimpeded
why

other
by

Agropyron

species.

the

any

However,

dem onstrates

the

in

the

found

4.2

in th e d a t a .
effect

com petitive

effect

(as

argued

both

portions

of

competitive

evidence

also

of

that

obtained
ability

a re

on

for

the

t hese

determined

both

often

species
strongly

and F i g u r e

of

is

other

because

or

some

com petitive

of

have

an

and t h i s

three
to

extrem e

in

an d

effect

this

may be

the

reason

on

many

of

to

this

argument

determ ine
part

on

A gropyron grew r e l a t i v e l y

in teractio n s

effect

poor growth

com petitive e f fe c t

in

a sp e c ie s determ ine

ab ility

so c a n n o t b e c o m p l e t e l y s e p a r a t e d .

hand,

circu larity

of

does

document

which a p p e a re d

they

species

sp ecies

com petitive

response

species

had v e r y l i t t l e

other

a

4.2

T r i f o l i u m a nd C h e n o p o d i u m w e r e h i g h l y

On t h e

large

above),

effect

previously:
and

F irst,

d i d so

interdependence

the

yields

competitive

that

Table

two s p e c i e s

there

Does

species.

Plantago

significant

focal species

That i s ,

presence
a

the

of a s s o c i a t e s .

species.

had

of

of

no

reduce

three

hierarchy
presence

no

at

general

precisely

presented
patterns

at
The

on

to co m p e titio n .

the

other

T r i f o l i u m , yet

Chenopodium

presence

t o be why t h e s e

the

the

s u p p r e s s e d when gr own w i t h a n y o f t h e o t h e r
seems

significantly

here.

associates.

two

The

growth

species,

for

suppressed

the

any
as

did

experiments,

and

species

no

agreement

Trifolium
focal

Plantago

had

previous

demonstrated

two

have

complete

the

significantly

growth

of

C h e n o p o d i u m , but

ability.

e f fe c t

yields

the

that

this

community.

its

com petitive

response?

determ ine

other

one

In

fact,

another

and

Second,

there

seems

to

com petitive

effects

among

the

determ ining

the

(dry

w eight

biomass/m2 )

fact,

there

the

is

species

hypothesis
species

com petitive

no

are

the

different
to

overlap

to

of

the

yield

of

the

per-am ount

over

to

allow

a

species.

its

id en tity .

of

this

of

asso ciate

g e n e r a lly did

statistica l
for

of

of

two a s s o c i a t e

range

species

in

In

effects

A test

same

Thus,

factor

com petitive

effect

of

t o be t h e y i e l d

yield.

the

of a s s o c ia te

degree

than

com petitive

species

im portant

appears

rath er

p articu lar

equivalence

analysis

example,

of

I

not
the

cannot

c o m p a r e t h e e f f e c t o f C h e n o p o d i u m and A m b r o s i a on P l a n t a g o

these

two

associate

t wo l i n e s

com petitive

effects.

of

id en tity ,

A gropyron,

P lan tag o ,

where

species

of evidence

regardless

instance

the

su fficien t
ab ilitie s

any

compare
focal

U nfortunately,

However,

at

The

of a species

that

yield.

because

effect

per-am ount

species.

evidence

same

statistica lly

five

species

the

com petitive

general

that

be

a

a

of

would

on

be

a

T rifolium ,

sp ecies

of

growth

in

the

yield,

significant

of

in

two

amount o f
solely
is

a

variance

by t h e
factor

in growth o f

total
in

yield

determ ining

response

third
curve

pattern
of

the

the

one

associate

Table
focal

This

com petitive

4.3

species

suggests

effect,

though

gives

the

explained
that

it

yield

does

not

per-am ount e q u iv a le n c e of s p e c ie s .

exhibited
of

of

asso ciates.

com petitive

for

growth

P>.28).

individuals

of a l l

provide c o n c lu s iv e evidence
The

F=1.2668,

per-am ount

for

c o u l d be f o u n d ( a n a l y s i s o f c o v a r i a n c e o f A g r o p y r o n and P l a n t a g o
growth;

in

of equivalence

Second,

yield

yield.

effects

Chenopodium

difference

same

asso ciate

is

Chenopodium.

overlap

the

the h y p o th e s is

relatio n sh ip

and

sig n ifican t

at

wa s

on

no

occur

species

yield

found,

the

focal

significant

not

do s u p p o r t

F irst,
a nd

did

by

focal

the

data

individuals

is
as

the

shape

a

function

of
of

the
the

70

O

Table 4 .3 .
The
v a r i a n c e e x p l a i n e d (R ) by t h r e e d i f f e r e n t
equ atio n s d esc rib in g the
mean g r o w t h o f i n d i v i d u a l s
of
focal
species
at
a single
density
as
a
function
of
a s s o c ia te species y ie ld .
The l i n e a r e q u a t i o n d e s c r i b e s t h e
mean w e i g h t o f t h e f o c a l
s p e c i e s as a l i n e a r f u n c t i o n of
the a s s o c ia te sp ec ie s y ie ld ; the re c ip ro c a l y ie ld equation
d e s c r i b e s t h e r e c i p r o c a l o f t h e mean w e i g h t o f t h e f o c a l
sp e c ie s as a l i n e a r
fu n c tio n of the a s s o c ia te sp ec ie s
y i e l d ; th e lo g -lo g e q u a tio n d e s c rib e d the lo g arith m of the
mean w e i g h t o f t h e f o c a l
s p e c i e s as a l i n e a r f u n c t i o n o f
t h e l o g a r i t h m o f t h e a s s o c i a t e s p e c i e s y i e l d . N=40 i n e a c h
c a s e . ( * - S i g n i f i c a n t , P < . 0 5 ; ** - S i g n i f i c a n t , P < . 0 1 )

ecies

Linear

Reciprocal
Yield

log-log

Ambrosia

.06

.06

.0 2

Agropyron

. 10

.08

.08

P lantago

.29

T rifolium

.23

Chenopodium

.34

kk
**
**

.50
.1 1
. 18

**
*
**

**
.40
.26
.55

**
**

71

yield
are
is

of the a s s o c ia te

not

strongly

sim ilar

affected

to

the

m onocultures.
id en tity ,

A m b r o s i a and A g r o p y r o n a s f o c a l

by ch an g es

patterns

The

and

species.

observed

relatio n sh ip

focal

in

species

the

between

of

growth

for

the

ab ility

(P la n tag o ,

T rifolium ,

resem bled

negative

hyperbolic

function,

e ffe c t of a sso c ia te

yield

3.
It
occur

is

evident

in

two

that

there

is

own

growth

m ixtures
there

both

species.

in

the

both of

sp ecies

in

a

section

these

types

tw o-species

This

is

sim ple
effects
The

attem pt

in traspecific,
in each of

sim ple

of

a

an

lower

strongly
asym ptotic

in

experim ents

C hapter

then

it

the

3 dem onstrates

is

also

some

that

of

the

lim it

possible

w ith

the

that

effect.

im portance

complex

a

sim ple

intraspecific

relativ e

and

th eir

tw o-species

sim ultaneously

occurring,

intersp ecific,

of

may

indicate

to l i m i t

lim iting

interactio n s

understand

in teractio n s

analysis

effects

are

to

in u n d e rsta n d in g

the net r e s u l t s

pathways

lim it

is

having

five sp ecies

The

and

of

types

the m agnitude of th e

in i n t e r p r e t i n g

taken here

of

of

intraspecific

the t w o - s p e c ie s m ix tu r e s .

d ifficu lty

which

regardless

dem onstrating

for a ll

m ixture,

effects

e f f e c t changing
an

in

Chenopodium)

m onoculture

interspecific

interspecific
section

The

effects).

above

intraspecific

species

and

different

the p o te n tia l

significant
If

complex

several

m ixtures.

least

yield,

density

T o tal C om petitive E ff e c t

(in trasp ecific

are

focal

at

a nd

This

on f o c a l s p e c i e s g r o w t h .

that

species

of a s s o c ia te s .

growth

associate

com petitive
a

yield

species

the

to determ ine

even sim ple

of the s e v e ra l

growth

of

of

system s

is

sim ultaneous c a u s e -e f fe c t

in d ividuals.

t h e mean s u c c e s s

tw o-species

The

approach

individuals

of

the

I

have
focal

72

species

at

associate
a

priori

adequate

various

com binations

species.
models

These r e s u l t s
of

way

This

same

reflects

of

the

the

approach

been

models

p r e v io u s ly used

in p l a n t

a n o th e r model.

An i n f i n i t e
the

of

the

reasonable

describes

first

model

the

growth

a

the

model
that

species

studies

and

sim ple

provides

an

t h e model

in

in teractio n s.

using

1 97 1 , A y a l a e t a l .
either

focal

to s e v e ra l

be assum ed
of

the

laboratory

1973).

because

they

have

been

number o f o t h e r m o d e ls c o u l d have been u s e d ;

as

The

both

o r b e c a u s e t h e y were an e x t e n s i o n o f a

a n a l y s i s to

only thesefour

p o s s ib le about

do n o t d e s c r i b e d e t a i l s

If

several

used,

studies,

few a s s u m p t i o n s
have

w ill

in

Ayala

of

t h e n be a p p l i e d

nature

used

were

as

terms

it

fu nctional

has

different

I have r e s t r i c t e d

w ill

data,

p o pulations of D rosophila ( e .g .
Four

abundance

species in te ra c tio n s .

descriptio n

some

of

the

biological

in

f or m o f

an a t t e m p t

t o make

any i n t e r a c t i o n .

in terp retatio n s,

Most

although

they

of any p a r t i c u l a r mechanism o f i n t e r a c t i o n .

is

the

of

sim plest

individuals

fu n c tio n of the d e n s ity of both the

possible
of

focal

a

tw o-species

focal

model,

sp ecies

and a s s o c i a t e

as

a

which
linear

species:

w t 1 = A + B j N j + B2N2

where A r e p r e s e n t s
Bj

and

th e g ro w th o f an i s o l a t e d

B2 r e p r e s e n t

species

1 and 2,

the

individual

per-amount

effects

of s p e c ie s

of

1 and

individuals

of

respectively.

The s e c o n d m o d e l i s
by W a t k i n s o n ( 1 9 8 1 )

the

t w o - s p e c i e s r e c i p r o c a l y i e l d model p ro p o s e d

and S p i t t e r s

model

1 and

describes

as

linear

fu nction

a

constant

CD

the
of

(1983a).

reciprocal
the

density

of
of

T h is model

is very sim ilar

t h e mean g r o w t h o f
both

the

focal

to

individuals

and

associate

73

sp ecies:

l / w t 1 =

This

model

has

been

(S p itters

im plicit

effect

in

that

the

w ithout

per-am ount

knowledge

discussed

of

species

reciprocal

linear

the

effect

Nj*

of

extended

to

function

of

However,

this

species

law

of

the

a tw o-species
the

1,

yields

would
is a

constructed

yield

be

an e q u a t i o n

this

that

between

The m o d e l d o e s i n c o r p o r a t e

2 on wt
complex

or

indirect

cannot
term

effect)

be d e t e r m i n e d

was n o t

noted

that
of

rath er
the

the

its

density.

growth of

species.

1

a

This

w hich would

sp ecies

of

than

focal

not

include

or

and

combines models

species

w t^

is

a

as a l i n e a r

species

in tractab le

mean g r o w t h ,

single

y i e l d m o d e l c o u l d be

predict

(B ^ )

The

as

wt^.

the

2

(B ^ ).
yield

Instead,

of

I have

1 a n d 2:

w t } = A + B j N j + B2Y9

T h i s model d o e s

an

2
(d ry w eight biomass/m ) of the a s s o c i a t e

m athem atically

function

interactions

studies.

equation
of

describe

higher-order

species

variable

states

(2)

B 2N2

to

1983b).

However,

independent

fu nction

+

(intrasp ecific

The t h i r d m o d e l u s e s y i e l d
as

BjNj

used

1983a,

by a n y o f t h e p r e v i o u s

species

+

successfully

c o r n and p e a n u t s
complex

a

a term d e s c r i b i n g

(3)

any com plex

intraspecific

in tera ct ion.
The
explicit
effect

fourth

model

cross-product

of the y ie ld

is

identical
term

of the

which

to

model

describes

second s p e c ie s

3 w ith
a

the

complex

(Y? ) 0n t h e

addition

of

an

in traspecific

performance of the

74

focal species

(w t^):

w t j = A + BjNj^ + B2Y2 + C3N1Y2

where

represents a

constant

complex e f f e c t .

(4)

N e it h e r model

3

or

4

have p r e v io u s l y been u se d .
The
used

data

in

this

usedabove

section

to

Mean b i o m a s s

of

species

determ ined

were

unique
using

m u ltip le

using

non-linear

of

in
t wo

linear

evaluate

evaluate

in d iv id u als

com bination

P artial

to

each of
the

densities

each

of

the

species.

regression

regression

the e q u a tio n

into

amount

possible
suggest

species
that

the

is

model

At

in teractio n

in e le v e n o f

one

models

of

the

least

given

one

of
the

provided

in

were

total

also

effects.

yields

that

and

of each

contained

a

4 were

analyzed

2 was

analyzed

model

Computer

determ ined

P r o g r a m - PAR).

for

model

2

by

form:

( B j / A J Y j + ( B 2 / A ) N 2w t j

the

Table 4 .4 .

a v e r y g o od
the

models

significant

four
R

2

models

values

d escriptio n

significantly

twenty p o s s ib le
a

3,

while

e x p l a i n e d by

provides

dynam ics.

plots

1,

Biom edical

of

to tal

a l i n e a r model in t h e

of variance
pair

Models

were

w t j = 1/A -

The

and

fifteen

analysis,

(BMDP

effects

th e models

and

regressionc o e ffic ie n ts

transform ing

interspecific

fit

cases.
to

for

close

to u n i t y

of the

sp ecies

described

In g e n e r a l ,

the

each

data,

all

the

if

any

of

the

m odels were s i g n i f i c a n t .
The

relativ e

im portance

of

each

( in trasp ecific/in tersp ecific/co m p lex

individual

term

intraspecific)

in

can

the
be

equations
determ ined

75

Table 4 .4 .
C o effic ien ts of d eterm in atio n (v arian ce explained) for
f o u r d i f f e r e n t models o f t w o - s p e c i e s m i x t u r e s .
(* - S i g n i f i c a n t ,
P < . 0 5 ; ** - P C . 0 1 ; *** - P C . 0 0 1 ) .

Mod el
Focal sp.

A ssociate

sp.

1

Ambrosia
Ambrosia
Ambrosia
Ambrosia

Agropyron
P lantago
T rifolium
Chenopodium

.58**
**
* 3 9 * **
• 6 8 **
.58

Agropyron
Agropyron
Agropyron
Agropyron

Ambrosia
P lantago
Trifolium
Chenopodium

Plantago
P lantago
P lantago
P lantago

2

3

4

* 7 2 * **
8 7 **
.62

*
*4 6 **
* 5 8 ***
• 5 7 **
.57

. 51
.64
.58
.61

.05
.02
.09
.28

.05
.00
.16
.30

.02
.01
.11
.08

.03
.09
. 18
. 10

Ambrosia
Agropyron
Trifolium
Chenopodium

• 2 5 **
• 59*
.38
.24

.69
.36
.07

• 0 8 **
.52
*28*
.42

. 14
.58
.28
.42

T rifolium
T rifolium
T rifolium
T rifolium

Ambrosia
Agropyron
Plantago
Chenopodium

*3 9 **
• 6 5 *k
.38
.01

• 2 3 **
, 4 2 ***
.75
.04

• ° 3 **
.58
.25
.11

.07
. 63
.30
.11

Chenopodium
Chenopodium
Chenopodium
Chenopodium

Ambrosia
Agropyron
Plantago
T rifolium

.34
. 2 9 **
..
.63
.27

.39

.23
• 32 . . .
.68
.23

.23
.32
.77
.36

**

* 2 ^***
. 61
.17

76

using

p artial

zero
the

correlation

suggest
m odel,

portion

that
w hile

of

b io lo g ical

of

4 .4).

which

appear

near

1 or

v ariance

to

models

In

only

example,

contributes

values
to tal

the

in trasp ecific
3,

term

(T able
little

-1

is

4 .5 ).
to

C oefficients

the

suggest

overall

that,

explained,

success

because

this

near

term

of

a

large

has

some

significance.

None
(T able

the

a

co efficien ts

appears

particu lar,

to

be g e n e r a l l y

models

2

and

effects,

do n o t

appear

include

sim ple

interactions.

apply

somewhat

better

o n ly model 2 acco u n ted

better

4,

which

to be g e n e r a l l y

for

complex

than

different

species

f o r a h i g h amo u nt o f

any o t h e r

include

better

However,

sp ecific

than

1 a nd
models

pairs.

For

the v a r ia n c e

in th e

e f f e c t s o f P l a n t a g o and T r i f o l i u m on T r i f o l i u m .
A ll of
Ambrosia

the e q u a tio n s

as

a

com petitor.
that

the

strong

The

intraspecific
also

v ariatio n

in

the

on t h e

previous

suppressed

effect
the

growth

the

presence

of

any

of

the

on

other

of

other

fact

species,

described
an d

coefficients
is

the

(Table

own p e r f o r m a n c e .

other

4.5)

and

at

the

of

a

indicate

O nl y A g r o p y r o n and
On t h e

an a s s o c i a t e
species.

dem onstrated
species.

abundance

t h a t A m bro sia g e n e r a l l y had a

Ambrosia as
the

the growth of

that

This

had v e r y
is

Ambrosia

However,

as

o th e r hand,

high

range

as

sig n ifican tly

discussed

of

little

suprising

t h a t most s p e c i e s have a n o n - l i n e a r

Ambrosia

hand,

abundance

resu lt
its

that

perform ance of Ambrosia.

had

Ambrosia u s e d , v e r y l i t t l e
On t h e

own

this

grow th of any of

t h i s may be d u e t o t h e
to

its

abundance of

experim ents

the

significant

correlation

for

affected
the

of

p artial

prim ary reason

Plantago

effect

function

were

above,

response

d ensities

of

r e s p o n s e would be e x p e c t e d .

A g r o p y r o n was n o t

including

itself.

affected
There

was

by t h e
no

abundance of

indication

fr om

Table
4.5.
P artial
correlation
co efficients
from
m ultiple
re g re ssio n a n a ly sis of tw o-species m ixtures.
The v a l u e s g i v e t h e
p a r t i a l c o e f f i c i e n t a s s o c ia te d w ith d i f f e r e n t e f f e c t s : i n t r a s p e c i f i c
/ i n t e r s p e c i f i c / h i g h e r - o r d e r ( o n l y m o d e l 4 ) . F o r m o d e l s 1, 2 , a n d 3 ,
c o e f f i c i e n t s g r e a t e r than .62 a r e s i g n i f i c a n t (P < .0 5 ) .
F o r model 4,
v a lu e s g r e a t e r than .67 a r e s i g n i f i c a n t (P < .0 5 ).
The .m. i n d i c a t e s
independent v a r ia b le s
w h ic h c o u l d n o t be- e n t e r e d i n t o m u l t i p l e
r e g r e s s io n e q u a tio n s because of high c o r r e l a t i o n s w ith the v a r ia b le
f i r s t e n t e r e d i n t o th e model.

f o c a l sp.

asso ciate

sp.

1

2

Model
2

4

.65/.52
.76/.62
.9 3 /.1 8
.7 9 /-.0 4

-.6 5 /-.3 2
-.7 3 /-.5 3
- . 7 6 / .m.
-.7 1 /.2 9

-.0 4 /.1 8 /-.2 9
-.6 9 /-.5 2 /.3 7
-.4 0 /.0 8 /-.0 9
-.3 7 /.3 9 /-.3 3

Agropyron
Agropyron
Agropyron
Agropyron

Ambrosia
P lantago
Trifolium
Chenopodium

- . 1 0 / - . 17
. 14/.15
-.1 2 /.0 4
.0 4 /-.0 5
- . 1 4 / - . 3 0 -. 16/.29
-.1 7 /.4 7
.23 /-.47

-.1 1 /.0 5
- . 1 2 / .m.
-.2 2 /-.3 2
-.2 6 /-.0 6

-.2 8 /-.2 3 /.2 8
-.3 0 /-.4 2 /.2 8
-.2 9 /-.1 6 /.1 4

P lantago
P lantago
P lantago
P lantago

Ambrosia
Agropyron
T rifolium
Chenopodium

-.0 3 /-.4 8 -.0 6 /.3 9
- . 5 8 / - . 66
.57/.78
-.5 8 /-.3 9
.60/.23
-.3 5 /.2 2
.26/.11

-.2 6 /-.2 4
-.5 6 /-.5 9
-.3 4 /.2 1
- . 6 4 / - . 52

-.3 2 /-.3 4 /.2 6
-.5 6 /-.5 6 /.3 7
-.2 5 /.0 4 /.0 9
-.6 3 /-.4 0 /.0 2

T rifolium
T rifolium
T rifolium
T rifolium

Ambrosia
-.3 9 /-.5 7
Agropyron
.6 0 /-.6 0
P lantago
-.5 0 /-.5 1
Chenopodium - . 1 2 / - . 0 3

-.0 8 /.4 8
.10/.62
.83/.75
-.1 9 /-.1 4

.0 9 /-.1 3
. 12/-.57
-.5 0 /-.2 1
-.2 8 /-.3 1

. 2 0 / . 1 3 / - . 19
.3 6 /.0 7 /-.3 6
-.4 0 /-.3 1 /.2 6
-.2 0 /-.2 3 /.0 6

Chenopodium
Chenopodium
Chenopodium
Chenopodium

Ambrosia
Agropyron
P lantago
T rifolium

-.5 8 /.4 0
-. 15/.39
-.0 3 /.7 4
.38/.35

. 4 5 / . 13
.04 /-.56
-.4 8 /-.8 3
-.4 3 /-.3 9

.1 3 /.0 5 /.0 1
-.0 2 /-.2 3 /.0 5
-.6 1 /-.7 7 /.5 0
-.5 0 /-.4 9 /.4 3

.5 2 /-.4 0
-.0 8 /-.5 3
-.5 4 /-.8 0
-.4 7 /-.4 6

•

o

-.6 4 /-.5 4
-.6 3 /-.5 4
-.8 1 /-.5 1
-.7 5 /.3 3

1

Agropyron
Plantago
T rifolium
Chenopodium

0
00
1
•
0

Ambrosia
Ambrosia
Ambrosia
Ambrosia

78

either

the

growth

of

2

R

However,

perform ance

general

or

the

A g r o p y r o n was

species.
the

values

the

p artial

affected

correlation

by v a r i a t i o n

abundance

of

coefficients

in

the

Agropyron d id

abundance

with

the

interspecific

studies

This

the

of

sign ifican tly

o f A m b r o s i a , P l a n t a g o , an d T r i f o l i u m .

agreement

that

any

affect

also

is

in

above

in

abundance

of

discussed

s e c tio n 2.
The

growth

o f P l a n t a g o was

A gropyron.

P lantago

effect

gr own

when

Chenopodium.
dominant

all

w ith

in

did

a

com petitive

the

abundance

the p re v io u s s t u d i e s ,

P lantago

lim ited

dem onstrated

the

V ariation

fr om

P lantago.

also

strongly

dem onstrate

intraspecific

subordinates
A m brosia,

no

effect

significant

T rifolium
the

on

and

com petitive

the

growth

of

com petitive

effects

on

s p e c i e s b u t A g r o p y r o n , w i t h model 2 d e s c r i b i n g

e ffe c t of

the

significant

of

had

by

a p articu larly

strong

t h e a b u n d a n c e o f P l a n t a g o on T r i f o l i u m and C h e n o p o d i u m .

The g r o w t h o f i n d i v i d u a l s o f T r i f o l i u m and C h e n o p o d i u m was s t r o n g l y
suppressed
N either

by v a r i a t i o n

Trifolium

abundance.

The

in

or

the

abundance

Chenopodium

abundances

of

of

appeared

T rifolium

Agropyron
to

an d

ever

on t h e g r o w t h o f

P la n ta g o ; however,

effect

apparent

of

resu lts

only

are

suggested

also

that

in

in

one

agreement

Chenopodium

and

w ith

the

four

in

were

P lantago.
th eir

own

appear

each c a se ,

possible

my p r e v i o u s

T rifolium

lim it

Chenopodium d i d

h a v e some e f f e c t
was

and

models.

experim ents

com petitive

to

this
These

which

had

subordinates

(C hapter 3).
The

term

exhibited

a

interpreted
effects

are

for

complex i n t r a s p e c i f i c

significant
two

very

sim ply

p artial

different
not

effects

in

correlation

co efficien t.

ways.

is

im portant

in

It

these

possible
two

model

4

This
that

sp ecies

never
can

be

complex
m ixtures.

79

However,

the

range

abundances

also

of

models

possible

only
of

that

the

complex

are

not changing

in

not

possible

separate

to

abundance o b ta in e d
This g e n e ra l
models

is

a

particu lar
that

they

two v a r i a b l e s

v ariable

im portant

the

ranges of

two

but

tos e v e r a l

the

types

actu ally

causes

these

It

abundance used.
using

the

m ixtures

the

resu lts

of

to

is
but

It

is

ranges

of

s e v e r a l _a p r i o r i

errors.

F irst,

circum stantial

That

is,

the

claim

a change

that
in

a

models

change

another

in

among

evidence

in

the

for

any

may d e m o n s t r a t e

in a p a r t i c u l a r m ath e m a tic a l

not

the

The e q u a t i o n s may p r o v i d e

in terrelatio n sh ip s

only

mechanism.

do

w ithin

experim ents.

in

altern ativ es

of the

provide

arranged

they

the

change

study.

description

biological

correlated,

are

over

in

approach i s only c o r r e l a t i o n a l .

but

that

effects

approach o f comparing

m athem atical

variables,

dynamics
used

these

in t h i s

the

two s p e c i e s

in ten sity

su scep tib le

cu rv e-fittin g

reflect

for m a r e h i g h l y

the

value

of

on e

th e manner d e s c r i b e d

by

th e model.
Second,
both
is

occur

the
and

p articu larly

study

a nd

change

understand

dynamics t h a t

i m p o r t a n t when e x p l o r i n g

non-linear

system s.

(e.g .

range

to

investigated.

others

the

used

abundances

the r e c i p r o c a l

a negative hyperbolic

com petitors.

very l i t t l e

over

o n l y be

of

many

plants e x h ib it
of

models can

change

This

suggests

that

response
at

very

yield

law)

Both t h i s

suggest

t o an i n c r e a s i n g
high

yields,

i n t h e e f f e c t o f t h e c o m p e t i t o r on t h e

to

be o c c u r r i n g ,

w ith Chenopodium ( F i g u r e

4 .2).

d e p e n d e n t upon th e r a n g e o f t h e

for

that

abundance

there w ill

be

focal sp ec ie s,

e v e n t h o u g h t h e o v e r a l l m a g n i t u d e o f t h e e f f e c t may b e v e r y l a r g e .
p h en o m e n a a p p e a r s

This

This

e x a m p l e , when A m b r o s i a was gr own

Models b ase d

on m u l t i p l e

regression

param eter v a lu e s: v a r i a t io n

are

in the y i e l d

80

Ambrosia d e m o n s t r a t e s
because

a high

range

no s i g n i f i c a n t e f f e c t
of

yields

was

used,

u p on t h e

g r o w th Chenopodium

not becau se Ambrosia does not

s u p p r e s s Chenopodium.

CONCLUSIONS
The s p e c i e s
a

web

of

in teractio n s

reciprocal

cause-effect

associate

species a f fe c ts

yield

the

of

species

on

species,
species

target

itself

etc.

in a m u l t i s p e c i e s m i x t u r e ca n be view ed as

its

which

reciprocal

I have measured

to varying

changes
effect

the abundance o f

alone,

interspecific

interspecific

com petitors

response

exhibit
curve

a

com petitors.

highly

lower

to tal

many

yields.

studies,

by

reciprocal

yield

reducing
This

both

in

which th en changes

the

effect

of

the

the o r ig in a l

This

target

asso ciate

individuals

of e a c h

h as been done in

alone,

and b o t h

i n d i v i d u a l s o f most s p e c i e s

non-linear

dem onstrated

sp ecies

an

the abundance of i n t r a s p e c i f i c

com petitors

that

resem bles a n e g a tiv e h y p e rb o lic
an a s s o c i a t e

u p on

abundance

in tra-

and

sim ultaneously.

The e x p e r i m e n t s s u g g e s t
community

the

of

t h e mean r e s p o n s e o f

i n c r e a s i n g l y complex c o m m u n itie s , v a r y i n g
com petitors

The

the growth of a t a r g e t ,

species,

a nd

pathways.

P lantago,

to

com petition.

T rifolium ,

and

response, w ith in c re a se s

in the y i e l d

success

of

of

pattern

has been p re v io u s ly

ag ricu ltu ral
and

in

focal

m onocultures
natural

individuals

(as

The

Chenopodium

the

type

equation)

response

in t h i s

expressed

of

m o re

at

noted

in

by

com m unities

(e.g .

explain

hierarchy

the

W einer

1982).
This

non-linear

response

may,

c o m p etitiv e e f f e c t dem o n strate both
Ambrosia

in

this

community

is

in

p art,

the

i n C h a p t e r 3 a nd h e r e .

always

quite

high,

as

of

The y i e l d o f
the

growth

of

individuals
species.

Ambrosia

The

generally

resu lt

past

f o r most
growth

of

the

is

other

not

that

point

focal sp ecies:

of

is

of

affected

the

yield

the

presence

of Ambrosia as

in flectio n

the

by

of

the

This

in

turn

non-linear

may e x p l a i n

other

an a s s o c i a t e
response

p re s e n c e of Ambrosia s t r o n g l y

species.

of

curve

suppresses

why o t h e r

is

the

species

h a v e no c o m p e t i t i v e e f f e c t on A m b r o s i a ; t h e y e x h i b i t a l a r g e

com petitive

response

which might

affect
very

to

Ambrosia

and

never

th e growth of Am brosia.
almost

all

strongly

suppressed

by

presence

of

species,

which

low

which

in

in

how

the

turn
the-

effect
much

a

In c o n t r a s t,
as

presence
very

allow s
growth

biomass

t h e y i e l d o f Chenopodium i s
of

most

species.

little

of

large

individuals

of

these

and r e s p o n s e

each

achieve

situ atio n s,

Chenopodium h a s

influences

com petitive
clear

so

other

effect

other

on

this

the

species

to

Thus,

the

of

achieve

So,

inversely

correlated,

a c tu a l ly determ ines

are

growth

Chenopodium.

are

species

it

the o th e r .

other
a

size

appears
but

it

These

that
is

not

processes

c a n n o t be e a s i l y d i s e n t a n g l e d .
I can

find

no e v i d e n c e

that

i n a ny way i n d e p e n d e n t o f t h e i r
to

have

test

of

a

general

this

different

associates

because

of

d ifficu lt

it

the
to

is
large

increase

individuals

never

com petitor.

It

is

at

achieve
may

be

yield

com petitive

yield
to

achieved
of

by

the
on

a

size

the

im possible

to

yield

of

and

it

achieve

and

in

of

equal

Ambros i a
is

very

because

presence
yield

two

species.

T rifolium

the

c a n n o t be s e p a r a t e d

appropriate

effect
focal

in d iv id u als

when

species are

e f f e c t . The

single

reduce

of

examined a p p e a r

com petitive

Chenopodium

a large

s p e c i e s b e c a u s e c a u s e and e f f e c t

effects

The f i v e s p e c i e s

examine

d ifficu lt

size
the

to

of

t h e s ame

very

com petitive

yield.

equivalence

hypothesis

U nfortunately,

the

of

of

a

these

82

F in ally ,
intraspecific
generally

no s t r o n g e v i d e n c e was f o u n d f o r
in teractio n s

explained

interspecific

effects.

in

tw o-species

su fficien tly

by

the

the

im p o rta n c e of complex

m ixtures.
sim ple

The

resu lts

were

in trasp ecific

and

Chapter 5

ANALYSIS OF MULTISPECIES INTERACTIONS

The m a j o r
the

intrasp ecific

m ultispecies
can

challenge

be

system .

lim ited

effects,

but

(sim ple
for

sim ple

how

the

the

d ifferent

a nd

the

d ifferen t

effects.

S p ecifically ,
of

the

interspecific

species

mesh

a nd

is

are im portant

in t h i s

plant

to

generate

of b io tic

in tra-

full

community

several

the

to

full

t o be a d d r e s s e d a r e

complex

of

d e r i v e d from

chapter

types

in teractio n effects

interspecific

this

the questions

and

a

p o te n ti'a lly

complexe f f e c t s

combine

various

how a l l

to g e th e r in

each sp e c ie s

The p u r p o s e o f

the

and

1)

in teractio n s
in tersp ecific)

and

2 ) how do

asso ciatespecies

the

"sum-up”to a

on a f o c a l s p e c i e s ?
fact

that

ecologists

have

the

different

types

of

is

due

in

m ethodological
"m ulti-body
im possible

planetary
and

part

problem

problem ".
to

find

many i n t e r a c t i n g

bodies

of

understand

intraspecific

in

im portance
system s

sim ple

when

net e ffe c t
The

system s,

interactio n s

strengths

in tra-

each of

host

to

in teractio n s

types of s p e c ie s i n t e r a c t i o n s

a nd

are

only

by a

community s t r u c t u r e .
what

intersp ecific

not

between

d e te r m in e what

community e c o l o g y i s

In m u l t i s p e c i e s

by

also

interactio n s

community

a nd

for

m otion
in

to

Simply

of

in

put,

understanding

poor

understanding

in teractio n s
The

complex

it

generala n a ly tic

affected

very

problem s.

inherent

variables.
is

two

a

is

very

solutions

first

difficult
to

the

83

problem

of

is

termed
and

problems

a
the

sometimes

which

have

t o u n d e r s t a n d how

g ra v ita tio n a l influences

th e m otion

the

m ultispecies

system s,

Examples i n c l u d e a t t e m p t s
by

in

of

charged atom ic

of

other

p articles

in

relatio n

to

in teractin g
analytic

one

another

v ariab les

are

The

of

second

as s e p a r a t e ,

interspecific
direct

focal

effect
This
focal
a

This

the

turn

occur

E ffect

-

changes

of

the

asso ciate

species

and a f o c a l

the f i r s t

asso ciate

sp ecies

e ffe c t of

the

asso ciate

separately

first
type

in

the
on

same
the

respond

focal

focal

of

this

direct e ffe c ts,

the

but

involve

species

previously

species
been

(Figure

noted,

the
but

another

direct

three

associate
first

sp ecies

the

l.ld ).

asso ciate

species

on

the

species

has

species

focal
might

first

the abundance of

changes the d i r e c t

species.
that

do n o t

of

the

I

w ill

involve

discuss
a loop of

both o p e r a tin g

has
the

3) There i s

on

a direct effect
focal

species.

species

Once a g a i n ,

to
the

changes th e e f f e c t of the second

species.
be

has

between

sp ecies

asso ciate

different

(Figure

focal

ab ility

on a

species

two o r m o re d i r e c t e f f e c t s

changes

o r m or e o f

direct

in teractio n

associate

or

a

This a ls o

Compound E f f e c t s

not

intra-

d istin ctly

in teractio n

1. I f ) .

do

effect

- A second a s s o c i a t e

s p e c i e s on t h e

associate

on

when o n e

associate

of

presence of the f i r s t

presence of the f i r s t

(sim ple

sp ec ie s w ithout a f f e c tin g

A second

that

of

the

higher-order

species.

to th e

associate

of

mechanism

chapter.

the

are the r e s u l t

occur

least

2) H ig h e r-o rd e r E f f e c t
on

d istin ct

at

first

effect

effect

a

in

effects

effects

A second
a

the

and

effects

complex e f f e c t s

m agnitude

can

species

interspecific

direct

complex

the

the v ario u s

ecology,

interactio n s.

complex

more

•' The

abundance

species.

direct

or

community

to d e t e r m i n e would i n c l u d e te rm s f o r

in teractio n s;

changes

Indirect

on

in

t wo

effects).

effects

1)

that

d istin ct
of

species.

ways.

is

In

abundances of

types of sp ec ie s

problem

com binations

the

the

1961).

s o l u t i o n we a r e a t t e m p t i n g

each of the d i f f e r e n t

exist

(Pines

quite

Compound
im portant

effects
in

have

species

not

which

experience hig h ly p l a s t i c
A ll

three

types

of

quantified

by d e t e r m i n i n g

species

changed

the

is

m athem atical

species

is

associate

things

a

b it

d ifferen t
In

by

complex

that

the

presence

included

species

m or e

it

third

species.

chapter,

I

a

effects

model

individuals

as

of

model i s d i s c u s s e d

of

used

and c o m p l e x i n t e r s p e c i f i c

focal

make

that

or

more

the

conceivable

resu lts

p o ssible

of

species

pairs

that

estim ate

effects

in

of

th eir

growth

in

the

species.
the

1

then

success

com petitors.

of
This

of the ex perim ental

the r e l a t i v e

the f u l l

t wo

and t h e

pro v id e a measure

describes

abundance o f

first

sim ultaneously.

These e x p e rim e n ts

all

the

to

the

to

and s u p p o r t e d u s i n g d a t a f r o m a l l

The m o d e l i s

on

F inally,

interference

a function

case,

1 / ( N 1+N2 ) ) .

present

plant

each

or

between

to tal

In

the

c o m p l e x i n t e r a c t i o n s m i g h t be o c c u r r i n g
this

two

abundance of

quite

effect

between

the

is

complex

be

both

of

plots.

in teractio n s

can

the

experim ents.

propose

of a

(e.g .

complex,

direct

in teractio n s

of

f o u r - and f i v e - s p e c i e s
the

interspecific

how much t h e

rep resen tatio n

a term

second

g row th su ch as found i n most p l a n t s .

im p o rta n ce of sim ple

five-species

community.

A l l e x p e r i m e n t s were perform ed in B a i l e y F i e l d d u r i n g

1982 ( Y e a r

Summary o f M e t h o d s

and

1983

(Year

2).

The

general

methods

w ill

be

summarized

here;

1)
a

d e t a i l e d d e s c r i p t i o n o f t h e m eth o d s can be found i n C h a p t e r 2.
Most

of

the

analysis

in

this

chapter

g r o w t h i n f o u r - an d f i v e - s p e c i e s m i x t u r e s .
the

species

Am brosia,

fifth

species

being

other

species

were

A gropyron,

Lepidium
removed

in
from

P lan tag o ,
Year

1 and

the

plots.

uses

the

resu lts

of

species

Both y e a r s o f t h e s t u d y used
and

Chenopodium,

T rifolium
Six

in

Year

different

w ith

the

2.

All

treatm ents

86

were

used,

consisting

(resu ltin g

in

community.

Three (Year

of

all

four-species

were m a i n t a i n e d

the

m ixtures)

1) o r f i v e

in p l o t s

r e m o v a l s were i n i t i a t e d

possible
a nd

(Y e a r 2)

arranged

one-species

the

removals

five-species

rep licates

"full"

of each tre a tm e n t

i n a random b l o c k d e s i g n .

The s p e c i e s

by h a n d w e e d i n g i n e a r l y May a n d t h e n m a i n t a i n e d

t h r o u g h o u t t h e summer t h r o u g h w e e d i n g o r t h e u s e o f a c o n t a c t h e r b i c i d e .
At

the

e nd

section
dried,

of

September

the

plots

and w e ig h e d .

analysis
The

of

Step-up

w ith

(Sokal

in

Rohlf,

all

for

from Y e ar

in d ividual

used

and

years,

harvested

The d a t a

rep licatio n

test

both

were

of v arian ce,

greater

in

1981)

2
to

w ithin

above-ground

w eights

allow ed
determ ine

using

nested

the

the

center

biom ass,

1 were a n a ly z e d

plant

Year

plants

use

oven

a nested

w ithin
of

pairw ise

plot.

the

Welsh

differences

in trea tm e n t e f f e c t s .

A n a ly s is of th e F i e l d D ata
S p e c ie s can respond
associate
this

sp ecies.

study:

a

from d i f f e r e n t
but

significant
species

A gropyron,

growth.

survivorship

(T able

removal

or

5 .1 ),

increased

either
the

although

full

survival

to

quantify

the

t h e p r e s e n c e o f an

types
the
to

of response

only

species

com petitive

survival

that

effects

p ro b ab ility

c o m m u n i t y (26% s u r v i v a l )
(2%

dominant

of

T rifolium

and

27%

to

over

a
of

and w i t h

respectively).

sp ecies,

i n any o f t h e

survivorship

in

Chenopodium d e m o n s t r a t e d

the

A m brosia o r P l a n t a g o had any m o r t a l i t y
im possible

The

removed

of

is

response

pattern.

Plantago

of

t wo d i f f e r e n t
Trifolium

e x t r e m e l y low i n t h e

Chenopodium

However,

and

non-significant

Trifolium is
only

s e v e r a l d i f f e r e n t ways t o

have m easured

survival

dem onstrated

sim ilar

I

in

Ambrosia
60%.

treatm ents.

of Agropyron g e n e ts

d ue

or

N either
It

was

to

its

87

Table
5. 1
Percent
survival
of
focal
individuals
f o l l o w i n g th e rem oval o f d i f f e r e n t a s s o c i a t e s p e c i e s in Year
2.
V a l u e s i n t h e same r o w w i t h d i f f e r e n t l e t t e r s a r e
s i g n i f i c a n t l y d i f f e r e n t ( p < . 0 5 , Welsch S t e p - u p T e s t ) .
A
d a s h i n d i c a t e s t h a t t h e m o r t a l i t y c o u l d n o t be o b s e r v e d f o r
that s itu a tio n .

Focal
Species

Ambr.

A s s o c i a t e S p e c i e s Removed
Agro.
Plan.
T rif.
Chen.
1. 00a

Ambrosia

1. 00a

full
community

1. 0 0 a

1. 0 0 a

1. 0 0 a

1. 00a

1. 0 0 a

1. 0 0 a

Agropyron
P lantago

1. 00a

1. 0 0 a

T rifolium

.67a

.61a

Chenopodium

. 88a

1.00a

.27 ab

-

. 02ab

.26b

.92a

1.00a

-

.78a

88

rhizoraatous

growth,

but

no d e a t h o f s h o o t s o r c u l m s was n o t e d

in any of

the tre a tm e n ts .
These e x p e r i m e n t s d e m o n s t r a t e
sp ecies

to

the

removal

different

w a y,

the

d ifferen t

focal

species.

species

in

com binations
large

effect

not

shown

the

removal

2 by

alm ost

o f the stu d y .

of

of

900% ( f r o m

P lantago

has

experim ents

on

the

>

large

Agropyron
in

com petitive
studies,

com petitive e f f e c ts ,

This

found
>

the

a

is

tw o-species
no

response

extreme

Chenopodium

in

Ambrosia d id
either

year

at

all

>

of

Lepidium

studies,

P lantago

=

the

effect

T rifolium

Agropyron g e n e r a l l y
had

lower

species

com petitive

on

in

a

sm aller,

the

only have

but

hierarchy.

either

In

very sm all

t h e y h a v e no e f f e c t a t a l l .

experim ents.
to

the

had a s g r e a t a n e f f e c t on

hierarchy

on s p e c i e s

these

is

in g e n e ra l agreement w ith

tw o-species
a nd

each

P lantago,

t h e most
of

of

o n ly th e removal of A g ro p y ro n ,

The h i e r a r c h y o f c o m p e t i t i v e r e s p o n s e

alm ost

growth

of

of Agropyron in

P lantago

effect

of

pattern

growth
In

for

a

four-species

g /in d iv id u al).

yield

fact,

the

5 .1).

the

growth

obvious

at

growth

( w i t h L e p i d i u m ) , h ad a n y e f f e c t

effect

both
or

In

which

com petitive

the

on

.479

the

possible

The m o s t

(Figure

to

species.

had

in

the

looked

on

in d ividual

of

5.1.

.054

species.

However,

removal

all

increased

effect

Chenopodium.

the

mean

Am b r o s i a

Ambrosia

any f o c a l

Ambrosia

significant,

the

and,

associate

The r e m o v a l o f no o t h e r s p e c i e s

the growth of

a

on

and C h e n o p o d i u m

instance,

tw o-species

single

and

the growth of a fo c a l

associates

a

Figure

removal

the growth of a fo c a l

w ith

in

a significant

and i n one

Data

are

the

have

of

five-species

Lepidium , T rifo liu m

Year

different

the

effect

case,

of

the response in

the

is

Ambrosia

removal

of

also

sim ilar

and

Agropyron

any

other

to

that

found

dem onstrated

species,

while

89

Figure 5 .1 .
The mean g r o w t h o f i n d i v i d u a l s o f e a c h s p e c i e s i n
t h e f u l l community and i n a l l p o s s i b l e s i n g l e s p e c i e s re m o v a l s
( f o u r - s p e c i e s t r e a t m e n t s ) i n Y e a r 1 ( t o p ) a nd Y e a r 2 ( b o t t o m ) .
B el o w t h e b a r s i n e a c h f i g u r e a r e t h e r e s u l t s o f a l l - p a i r s
a n a l y s i s f o r th e e f f e c t of removing d i f f e r e n t a s s o c i a t e s p e c ie s
on g r o w t h o f t h e f o c a l s p e c i e s .
S p e c i e s m i x t u r e s u n d e r l i n e d by
t h e s ame l i n e do n o t d i f f e r s i g n i f i c a n t l y ( p < . 0 5 , W e l s h s t e p - u p
test).

PERFORMANCE OF FOCAL SPECIES

Tl
H-

09

mean g/individual (g/m for Agropyron)

C

i-t
fD

■<

JSL

ro

AMBROSIA

Ag
PI
Tr
Ch
none

5U
Ag
PI
Le
Ch 1
none

I
none [

/
r

Am

Ch
none

Am
Ag
Le
Ch
none

O
00

Am

Am
Ag
PI

Ch
none

Ch
none

Am
Ag
PI
Tr

Am
Ag
PI
Le

none

none

06

3

V

PLANTAGO

ASSOCIATE SPECIES REMOVED

PI
Tr
Ch
none

AGROPYRON

Am

91

C henopodium , L e p i d i u m , and T r i f o l i u m
to th e removal of A m brosia.
it
the

is

in

the

full

as th ey did

There

were

in

respond

were

some d i f f e r e n c e s

sm aller

respectively).
larger

perform ance

two

the

response

not as c l e a r - c u t as
the

focal

to

the

Year

opposite

species
This

exhibited

Year

than in

two

years.

in

the
1

growth

by

of

the

of Ambrosia in

1 than

s ho we d

other

patterns

dram atically

in

Year

in

The

in .the

betw een-year

in

size

respectively).

as

Individuals

Plantago

individual

In g e n e r a l ,

a strong

species

in

removal of

an

t h e o n e - and t w o - s p e c i e s e x p e r i m e n t s .

two y e a r s o f t h e s t u d y .

community

dem onstrated

the h ie ra rc h y i s

experim ents.

c o m m u n i t y do n o t

asso ciate

the

tw o-species

Again,

all

2

species

the f i v e - s p e c i e s

(1.72

g and

response,

Year

2 (.57

2.98

g,

obtaining
g

dem onstrated

is in

between

an d

very

g e n e ra l agreement

the m onoculture p lo ts

(see

.26

a
g,

sim ilar
w ith

the

Chapter

3).

A Model o f P l a n t I n t e r f e r e n c e
The

response

associate

species

asso ciate

in

of

a

focal

provides

suppressing

('p re s s '

experim ents,

complex

interspecific

removal

of

an

measured.

measure

the

effects

total

of

focal

Bender e t

associate,

The

a

s p e c ie s tothe

al.

are

the

species

1984).

of

to tal
in

can

is

the

be
only

a

effect

the

full

Because d i r e c t

sim ultaneously re lie v e d

neither

effect

removal

d irectly

and

in teractio n

single
of

this

community

(sim ple)

and

follow ing

the

independently
that

we

can

e x p e rim e n ta lly m easure.
To

estim ate

the

higher-order

components

general

of p la n t

model

interspecific
of

the

d irect

total

interference

that

(sim ple),

effect,

I

describes

have

in d irect,
constructed

an d
a

t h e mean p e r f o r m a n c e

92

of

each

the

species

as

community.

effects

of

m ultispecies

the

provides

species,

allow ing

the

different

community.

co n ta in in g one,

of

The m o d e l

between

im portance

a function

two,

The

four,

focal

species

sp ecies.

In

a

different

m ulti-body

the

model

asso ciate
is

is

the

the

resu lts

overcome
may

be

Werner

this
a

of

the

dilemma.

The

predicted

general

1983).

That i s ,

occurring

in

data

plots

equivalence

any s i n g l e

representing

difficu lty
response

in

provide

individual

com petitive

effect

d ifferen t

upon

is

experim ents
of

suggest

effects

focal species

a

focal

sp ecies

of

com petitive

equivalence
there

is

an

equivalence

space

th a t give

into

a single

the

to tal

do

of

associate

the

most

focal

respond

effect,

im portant

(see

in

not

combined

Figure
yield

5.2
(dry

the

response.
the

dem onstrates

method
that

axes

there

the

species,

no

of

the

determ ining

its

be

noted

ways,

i.e .

it

in

and

to

should

If

to

Goldberg

biomass

factor

different

then

w eight

The

It

the abundance of each a s s o c i a t e

axis.

in

effect
by

seems t o re s p o n d

species.

in d ividual.

effect,

the

a

iden tity

neighboring

by

( n +1 ) - d i m e n s i o n a l

m atter

the

a

associate

expressed

t o t h e p r e s e n c e o-f a s e t am ou n t o f a n y a s s o c i a t e
of

fr om

be r e p r e s e n t e d

same d e g r e e
the

relativ e

com binations.

would

experim ents

tw o-species

per-am ount

using

the

of d iffe re n t

axis

in

to d eterm in e .

tw o-species
The

of

in vario u s

response

independent

space is u s u a lly ex trem ely d i f f i c u l t
The

supported

species

s i m p l e and c o m p l e x

interactions

abundances

sp ecies.

that

the

understanding

of

increasing

each

of

the o th e r

b a s e d on t h e r e s p o n s e d e m o n s t r a t e d

system ,

space,

problem

an

and f i v e s p e c i e s

a n-species

(n+1)-dim ensional
of

to

estim ates

types

The s t r u c t u r e o f t h e m o d e l i s
each

abundance of

that
is

an

we c a n a s s u m e t h a t
the

n-dim ensional

s p e c i e s c a n be c o l l a p s e d

t h e g e n e r a l model in w hich

biomass/m )

of

the

asso ciates

93

CO
LU

o
LU

a.

co
_j

<

o
o

Li_

a
O
LU

D)
w.

O
2

<

CO

o

Ll

DC
LU

a

TOTAL YIELD OF ASSOCIATE SPECIES (g /m 2 )
Figure 5 .2 .
The h y p o t h e t i c a l r e l a t i o n s h i p b e t w e e n t h e mean g r o w t h
o f i n d i v i d u a l s o f t h e f o c a l s p e c i e s and t h e t o t a l y i e l d o f t h e
a s s o c i a t e s p e c i e s ( b i o m a s s / m 2 ) . Yf i s t h e t o t a l y i e l d o f a s s o c i a t e
s p e c i e s i n t h e f u l l c o m m u n i t y , Yr i s t h e t o t a l y i e l d o f a s s o c i a t e
s p e c i e s wh en a s e l e c t e d a s s o c i a t e s p e c i e s h a s b e e n r e m o v e d f r o m t h e
c o m m u n i t y , a n d Yp r e p r e s e n t s t h e p r e d i c t e d t o t a l y i e l d o f a s s o c i a t e
s p e c i e s i f t h e s e l e c t e d a s s o c i a t e s p e c i e s h a s no i n d i r e c t e f f e c t s on
the fo c a l sp e c ie s .
vm i s t h e e x p e c t e d mean g r o w t h o f i n d i v i d u a l s o f
t h e f o c a l s p e c i e s when no c o m p e t i t o r s a r e p r e s e n t , w h i l e w f , w , a n d
wr r e p r e s e n t t h e e x p e c t e d me an g r o w t h o f i n d i v i d u a l s o f t h e f o c a l
s p e c i e s a t Y f , Yr , a nd Yp , r e s p e c t i v e l y .

94

determ ines
to tal

the

combined y i e l d o f

describes
full
the

mean b i o m a s s

the

mean

community
focal

species

in d iv id u als

the a s s o c ia te

success

(wf ) .

of

of

of

species

in d iv id u als

The v a l u e

the

in the

of

the

wm r e p r e s e n t s

when no a s s o c i a t e s

are

fpcal
full

present

The

c o m m u n i t y (Y^)

focal

the

species.

species

expected

(w ithout

in

the

success

of

com petitors;

see Table 5 .2 ) .
Note t h a t
associates

in t h i s m odel,

does

not

m ethodological
which

can

yield.

independent
yield

of

an

of

mean

I could

able

which

s p e c ie s does n ot a f f e c t

its

This

might

the

the

effectiv ely
This

if

sizes

either
of

allow s

d irect

(sim ple

effects

o f an a s s o c i a t e

By

a

not

a

function

effect
the

of

focal

all

of

the

the

affected

it

by

own s u c c e s s
density
were

the

associates

species:

the

c a n be

yield

of

quantified

the

effect

of

the

is

restricted

to

that

intraspecific

the

focal

that

effects).
was

the

l ow

plants

individuals.
of

in tersp ecific

species.

the

the

to tal,

com petitive

Given e q u i v a l e n c e ,

species

these
as

c a n be

abundance of

species

quan tificatio n

indirect

and

species

in Figure 5.2

so

a

density,

focal

strong

sm all,

w ith c o n sp e c ific

a nd

a

(no i n t r a s p e c i f i c

of

is

perform ance

of

model

would s u g g e s t

s p e c i e s on a t a r g e t

sum o f

yield

This

species

mean

including

the e f f e c t of a p a r t i c u l a r group of a s s o c ia te
is

species.

control

both

the

straightforw ard

interspecific)

focal

on

itself,

ind iv id u als

h ad no i n t e r a c t i o n s

model

the

curve such as t h a t

sp ecies,

this

and/or

effect

not

a response

for a focal

of

only d i r e c t l y

on

are

2
( d r y w e ig h t biom ass/m ) of

yield

to m a n ip u la te

species

However, i f

occur

yield

perform ance.

focal

species

established

the

to tal

intrasp ecific

I wa s n o t

the

analyzing
effects.

problem.

have

So,

include

the

on a f o c a l

asso ciates.
percent

species
This

reduction

net
of

95

Table 5.2
D efin itio n s
in teractio n s.

=

o f t e r m s f o r a model o f m u l t i s p e c i e s

2
yield
(biom ass/m )
of
the
asso ciate
species
of
interest
when
grown
in
the
full
(five-species)
community.

Y^

=

t o t a l y ie ld of a l l
f u l l community.

associate

s p e c i e s when grown i n

Y^

=

t o t a l y i e l d o f a l l a s s o c i a t e s p e c i e s when gr own i n
community
w ith
the
asso ciate
species
of
in terest
removed.

Yp

=

p redicted
grown i n
occur.

w
m

=

mean w e i g h t o f i n d i v i d u a l s o f t h e f o c a l s p e c i e s when
no i n d i v i d u a l s o f t h e a s s o c i a t e s p e c i e s a r e p r e s e n t .

wf

=

mean w e i g h t o f i n d i v i d u a l s o f t h e f o c a l s p e c i e s when
g r ow n i n t h e f u l l c o m m u n i t y a nd a s s o c i a t e y i e l d = Y^.

w
^

=

p r e d ic t e d w eight of i n d i v i d u a l s of the fo c a l s p e c ie s
i n t h e f u l l c o m m u n i t y i f no i n d i r e c t e f f e c t s o c c u r a nd
asso ciate yield = Y .
P

t o t a l y ie ld of a l l
t h e f u l l community

the

a s s o c i a t e s p e c i e s when
i f no i n d i r e c t e f f e c t s

96

e f f e c t of t o t a l

w h e r e w a nd
in
in

are

t

m onocultures

The

associate

the

of

full

each

p r o p o r t i o n a l to

associates.

So,

focal

the

sp ecies

focal sp ecies

can

Yn

is

the

negative

in fin ity

facilitativ e

the

of

model

o n l y on

indirect

effects

asso ciate

(Y )

this

the

focal

the

to tal
w ith

the

5.2).

effect

is

yield

of

species

on

total

asso ciate

percent reduction

associate

effect

of

values

can

selected

sp ecies

of

the

sp ecies:

(2 )

species

species

less

in

n w ill

that

0

the

full

range

fr om

indicating

a

t h a n 0 i n d i c a t i n g an a n t a g o n i s t i c

on t h e

as

of

to tal

to tal

the

the

growth

associate
as

the

yield

species

species

F irst,

the

p redict

understood

asso ciate

ways.

model
is

be

to

yield

asso ciate

increases

by t h e

us

to tal

species

different

d irectly

quantified

the

of a s p e c ific a sso c ia te

sp ecific

allow s

ad d ition of a selected
d istin ctly

to

the

Table

com petitive

p articu lar
as

(see

species

Y
w -w f
o f s p e c i e s n = (-—-n- - ) ( — ------ )
Yf
w
f
m

to 1,

based

when

net

focal

( 1)

effect.

Because

found

this

a

e f f e c t a nd v a l u e s g r e a t e r

(com petitive)

selected

of

quantified

y ie ld ofthe

The v a l u e

and

be

to

---------w
m

of the

respectively

contribution

effect

effect

community.

species

its

c a u s e d by t h e y i e l d

to tal

where

to tal

ind iv id u als

community,

associate

d irectly

the

on t h e f o c a l s p e c i e s =

t h e mean w e i g h t o f

a nd

contribution

yield

yield

y ie ld of

sp ecies,
effect

of

of

not

to tal

the

selected

asso ciates

by

this

change

in

the

the

focal

the d i r e c t
adding

yield

The
i n two

associate

(Yr )*

This

ofin d iv id u a ls
to tal

a

previously

present.

the

how much t h e mean g r o w t h

affected

of

associates

was

affects

of

yield

of

is

of
the

97

asso ciate
species

species.
can

Yr ) .

change

This

is

obtained

data

the

the

yields

presence

of

as

the

the

of

other

the

indirect

effect

associate

(and

of

so

the

change

selected

focal sp ec ie s.

to

estim ate

the

from t h e d i r e c t

selected

a s s o c ia te s in

selected

asso ciates

direct

and

indirect

f r o m t h e f o u r - and f i v e - s p e c i e s m i x t u r e s .

quantified
of

the

quantified

a s s o c i a t e on t h e
The

Secondly,

increase

associate

in y ie ld

species.

the f i v e - s p e c i e s m ix tu re is

is

thus

The d i r e c t e f f e c t

expected

The

effects

from t h e

expected

total

addition
yield

effects

this
of

to tal

yield

of

e f f e c t of the s in g le

(3)

associates

th e whole a s s o c i a t e
selected

of

then:

Y = Y + Y
p
r
n

Using

is

species
asso ciate

as

an

group,

estim ate

of

an e s t i m a t e

s p e c i e s on t h e

of

focal

the

direct

the d i r e c t
species

can

be o b t a i n e d :
Y
e f f e c t = (— —
Yr
f

direct

where w i s
P
5.2).

the

predicted
p

The e s t i m a t e d

indirect

of

indirect effect

the

focal

(4)

sp ecies

a t Y (s e e Table
P

c a n b e f o u n d s i m p l y by d i f f e r e n c e :

Yn
w -w
e f f e c t = t o t a l e f f e c t - d i r e c t e f f e c t = ( — ■ ) ( —— — )
ic
w
f
m

Both t h e d i r e c t

an d i n d i r e c t e f f e c t

maximum p o t e n t i a l
when t h e r e

success

w -w
—— - )
W
m

are

growth

of

no i n d i r e c t

4 are equivalent.

rep resen t a percent reduction

in d iv id u als
effects,

of

a

focal

species.

^5 )

in

Note

Y =Yf and w =wf a n d e q u a t i o n s
p f
p f

the
that

2 and

98

E v a l u a t i o n o f t h e Model
The

critical

requirem ent

focal sp ecies

is

To

assum ption

test

this

a fu n c tio n of

indirect e ffe c ts,
Year

a nd

tliis

the
to

model

total

allow

all

w i t h any a s s o c i a t e

function

the p lo t

of

the

f o r Year

The

data

total

an

were

to

combined

equations

c u r v e and a t

portion

of

curve

portion

of

curve

necessary,
whole

length

of

of

used

the

for

w

of the a s s o c ia te

estim ation

the

of

L epidium ,
fu n c tio n of

the

the

to tal

and

in both

are
of

sp ecies.

of

e a c h c u r v e was s o m e w h a t s u b j e c t i v e .

I

used

curve

support

com petitive

T rifolium ,

species

a nd

choice

regression

other

an aly sis.

to

is

the

w eight

(generally

presented
the

and

focal

in

use o f

Chenopodium

y ie ld of a s s o c ia te s

in

shape of

effects.

right-hand

side:

the

residuals

F igures

the

The
where

over

the

transform ation).
5 . 3 and 5 . 4 .

in

the

absence

The
of

each sp e c ie s m aintained

t h e model
The

the

indirect

sp ecies

hierarchy.

The

th e v a r ia n c e over the

log-log

Chapter 3 ).

in

direct

and Y e a r 2 ( F i g u r e 5 . A).

at

low er

species.

in

w hich would d e m o n s t r a t e

were

each curve

curves

the

species

predictions

response

den sity (see

the

t h e s ame t i m e m i n i m i z e

for

success

of

from m o n o c u l t u r e s o f

The r e s p o n s e

the

of

s p e c i e s was m e a s u r e d a s

all

a s s o c i a t e s , was d e t e r m i n e d
the n a t u r a l

of

for a l l

focal

to e s t i m a t e d i r e c t

transform ations

The e q u a t i o n s
value

used

yield

using

to d e s c r ib e

whole r e s p o n s e
the

success

s p e c ie s or com binations of a s s o c ia te

analyzed

determ ine

the

e x p e r i m e n t a l p l o t s which i n c l u d e d each f o c a l

1 (Figure 5 .3 )

the r e l a t i o n s h i p used
wished

that

yield

The mean p e r f o r m a n c e o f i n d i v i d u a l s o f t h e
a

is

r e s p o n s e c u r v e s were e v a l u a t e d

1 and Y ear 2 u s i n g

species

of

for

s p e c i e s which a r e

responses

both

(Figures

years

of
are

P lantago,
clearly

5 . 3 and 5 . 4 ) .

a

And i n

99

Figure 5 .3 .
The e f f e c t s o f t h e t o t a l y i e l d o f a s s o c i a t e s p e c i e s
on t h e g r o w t h o f i n d i v i d u a l s o f t h e f i v e f o c a l s p e c i e s i n Year
1.
The p o i n t s i n d i c a t e t h e mean w t / i n d i v i d u a l o f t h e f o c a l
species across
a l l t r e a t m e n t p l o t s (grown w i t h e i t h e r o n e ,
t h r e e , or a l l fo u r of the a s s o c ia te s p e c ie s ) .
The v a l u e w^ i s
t h e y i e l d o f a s p e c i e s when f o u n d i p m o n o c u l t u r e ( n o c o m p e t i t o r s
p resent).
The e q u a t i o n s
and
r
values d e sc rib e
the lin e
i n d i c a t e d on e a c h f i g u r e .

100

200

AMBROSIA

AGROPYRON

Wm

cut

= “ .2 4 7 ( y i e l d ) + 1 4 5 .1 8 4

.42
100

1

CO
UJ

O

UJ

Q.

CO

_l

<

C

LL

o

LU

O

300

400

a

□>

<

LEPIDIUM

PLANTAGO

.Wm ► %

log

cu t

= - .0 0 3 (y ield ) + .8 0 3

e
O)

cut

=

2 3 4 .3 9 6 (y ie ld )

-

1

.39*

***

*5

D
W m *■1

’>

^5
C

cr
ou_

C
CO
0)

cc

200

100

400

300

o

z
<

200

>»

o

o
u_

100

ok
.

r f iT»»

E

100

200

300

400

200

100

300

LU
CL

CHENOPODIUM
cut

=

1 0 .1 1

K yield)

r = .3 8 * * *

100

200

TOTAL YIELD OF ASSOCIATE SPECIES
g /m ’

Figure 5.3

300

400

101

F igure 5 .4 .
The e f f e c t s o f t h e t o t a l y i e l d o f a s s o c i a t e s p e c i e s
on t h e g r o w t h o f i n d i v i d u a l s o f t h e f i v e f o c a l s p e c i e s i n Y e a r
2.
The p o i n t s i n d i c a t e t h e mean w t / i n d i v i d u a l o f t h e f o c a l
s p e c i e s a c r o s s a l l t r e a t m e n t s p l o t s (grown w i t h e i t h e r o n e ,
t h r e e , or a l l four of the a s s o c ia te s p e c ie s ) .
The v a l u e w i s
t h e y i e l d o f a s p e c i e s when f o u n d
m onoculture (no c o m p e tito rs
present).
The e q u a t i o n s
and
r
values d e sc rib e
the l in e
i n d i c a t e d on e a c h f i g u r e .

102

AMBROSIA
ujt=

AGROPYRON
200 r V

7 . 4 3 5 ( y i e l d ) ,5B
. 11C
6

r

***

= -

'r

' '

I *
I . ;
•

J ________I________I________L

LU oc

' "

•

UJ

O

.2 5

100

Wm ►

C/5

,212(yield) + 1 2 8 . 0 8 7

.

b
I**

u jt

200

400

*

•

_______ La______L

600

200

400

600

CL

>
C/5
0
_ J 01
a

<

<

C5

u it =

O

u. E
u_ oi
O

1

«. .

1.0

PLANTAGO

u» r= 4 6 .6 2 4 (y ield )'

1 1.9 5 4 ( y i e l d )
r = .48 * * *

3

Wm

LU = Wm«■ kt

O

tr
o

ll

DC

TRIFOLIUM

Q5

■co
c

eo
o
E

200

400

200

600

400

600

LU
CL

CHENOPODIUM
. .

u»T=39.319(yield ) 973

Wm i

•
.c
I ~ a "- ■
200

.7

m 11
400

TOTAL YIELD OF ASSOCIATE SPECIES
g /m ‘

Figure 5.4

a

1 - ____ ]

103

each c a s e ,
were

t h e r e s p o n s e c u r v e a p p e a r s t o be h i g h l y n o n - l i n e a r .

generally

described

d a t a , which a l s o

provided

F igures

5.4).

the

5.3

total

there

yield

was

growth

and

no

in

o

The Y
the

p redict

total

of

to tal

the

(Figure

of

to tal

yield

Since

in

a nd

the

five-species

absence

relatio n sh ip

5.3

and

no

the

a nd

were

focal

strong

year

5.4)

total

species

effect

of the

then

of

study,

it

of o th er a s s o c ia te
effects

for

presented

f o r b o t h Year

represents

the

direct

indirect

zero),

or

growth

an a n t a g o n i s t i c

a facilitativ e
that

all

indicating

yield

of

of

the

Y •

success

determ ine

absence
growth

of

was

rem aining
2,

4,

to
The
and

w^> the
indirect

found

for

to e stim a te

the

s p e c i e s on A m b r o s i a .

the

are

maximum

on

effect,

was i m p o s s i b l e

The r e s u l t s

Note

i n Year 2

a nd u s e d

species

to

the

yield

p a i r s were d e te r m in e d u s i n g e q u a t i o n s

indicate

o n mean

p a i r were d e t e r m i n e d

experim ents

used

in

species

indicate

For A m b r o s i a ,

range in the

between f o c a l

in d irect,

the

the

fu n c tio n of

asso ciates

o f an i n d i r e c t

d irect,

values

of

b e c a u s e d by t h e s m a l l

four-

a nd i n d i r e c t e f f e c t s

changing

of

v a lu e s are g iv en in

was o n l y a v e r y weak r e l a t i o n s h i p

the

of

Ambrosia in e i t h e r

in

(r

in both years of the study.

part,

yield

success

effects.

value

transform ation

found w ith A m brosia.

describing

yield

direct

log-log

and Yn f o r e a c h a s s o c i a t e - f o c a l s p e c i e s

the

pred icted

in

a

o f A g r o p y r o n was a l i n e a r

effect

there

species

resu lts

equations

yield

of a s s o c i a t e s

T h i s may,

of a s s o c ia te

from

The

1 and

using

a homogeneous v a r i a n c e

significant

Year

(R = . 1 6 ) .

best

The d a t a

The

associate-focal

a n d 5.

1 and 2 i n T a b le 5 . 3 .

Each

effect

of the

associate

species

focal

species

(w ) .

P ositive

m

c o m p etitiv e e f f e c t w hile n e g a tiv e values

effect.
the

direct

a com petitive

effects

effect

of

are
the

p o sitiv e

associate

in

value

species

(or

on t h e

104

Table 5 .3
The e s t i m a t e d d i r e c t ( D) a nd i n d i r e c t ( I ) e f f e c t s b e t w e e n
s p e c i e s i n Y e a r 1. a n d 2.
Values r e p r e s e n t th e p e r c e n t r e d u c t i o n in
maximum p o t e n t i a l g r o w t h o f i n d i v i d u a l s o f t h e f o c a l s p e c i e s .
P ositive
values
indicate
a com petitive
effect;
negative
values
indicate
a
fa c ilita tio n effect.
YEAR 1.
Ambro s i a
Focal specie s D
I
Agropyron
Plantago
Lepidium
Chenopodium

.428
.598
.580
.591

-

.063
.038
.024
.011

As s o c i a t e S p e c i e s
Agropyron
P lan tago
Lepidium
I
D
I
D
D
I

—

—

.276 - . 0 3 0
.257 - . 0 1 2
.260 - . 0 0 6

.062 -.0 1 1
.079 - .0 0 3
.081 - . 0 0 1

.001
.001
.001

.000
.000
.000

Chenopodium
D
I
.006 -.0 0 1
.008 -.0 0 1
.007
.000
-

YEAR 2 .
Ambrosia
Focal species
D______ I_
-.097
-.008
-.012
-.003

_

218 - . 0 0 6
228 - . 0 0 5
240 - . 0 0 2

.066 -.0 1 0
.072 -.001
.076

s
T rifolium
D
I

Chenopodium
D
I

0 00
000
000

.007
.007
.008
-

.000
.000
.000

O
0
•
1

.560
.563
.596
.627

•H
0
O
•
1

Agropyron
P lantago
Trifolium
Chenopodium

A ssociate Speci
Agropyron
Plantago
I
D
D
I

.000
.000
-

105

focal

sp ecies,

w hile

the

indicating a f a c ilita tiv e
large;

for

example,

indirect
effect.

the

cases

the

effects

The m a i n
the

factor

id en tity

of

growth of o t h e r
percent,

P lantago

6

to

Chenopodium n e v e r r e d u c i n g
rem arkably
iden tity

consistent

of

the

Chenopodium

h iera rch y of
Of

negative (or

zero)

The

(Kermack
effects,

an d

described
years,

between

Haldane

the

the

of

affects

the

most

In

T rifolium ,

direct

Ambrosia

a nd

was

reducing

the

Lepidium ,

1 percent.

each
the

effect

A g r o p y r o n by 22 t o 28

T rifolium ,

le a s t affected

is

that

and

This i s

asso ciate.

value

of

affected.

Once

a

The

the

by a n y a s s o c i a t e

the

indirect

direct
species

again,

the

are

both

effects

co n trib u tin g very l i t t l e

between

the

1950).

relatio n sh ip
of

relatio n sh ip

t h a n one (Y e a r 1 - . 9 5 ,
near

and

effect

and v e r y s m a l l ,

by a l i n e

the

five-species

the

direct

and

the

to the t o t a l

total

effects

was

t h e r e d u c e d m a j o r a x i s method t o d e t e r m i n e an i n d e p e n d e n t

line

the

the

the

63 p e r c e n t ,

also

being

in terest

relatio n sh ip

examined u s i n g
regression

zero),

c o m p e t i t i v e e f f e c t a nd r e s p o n s e was a p p a r e n t .

p articu lar

effect.

(or

species.

t h e g r o w t h by m o re t h a n

sp ecies

generally

the

Lepidium ,

w ith

percent,

com petitive

focal

in

the magnitude of

sp ecies,

8

e f f e c t , w ith Agropyron being
and

sm all;

by f r o m 43 t o

by

Ambrosia

no e f f e c t on t h e o t h e r

associate

species

of

quite

determ ining

the

negative

t h e g r o w t h o f C h e n o p o d i u m by 63 p e r c e n t .

were

Chenopodium had v i r t u a l l y

were

Some o f t h e d i r e c t e f f e c t s w e r e q u i t e

presence

community i n Y ear 2 r e d u c e d
many

effects

origin

(Figure

pairs

of

If

there

between

the

45 d e g r e e s
was d e f i n e d

passing

So,

no

the

through
w ith

still

the data

direct

significant

and d i r e c t

by l i n e s
yet

an d

were

total

Year 2 - . 9 6 ) ,
5.5).

total

effects

the

origin.

slopes

passing

indicate

effects
indirect
would

be

In both

slig h tly

less

through or very

not only th a t

the

106

Figure
5.5.
The r e l a t i o n s h i p
b e t w e e n t h e d i r e c t and t o t a l
effects
of
associate
species
on
focal
species
for
each
asso ciate-fo cal
species
pair
in
Year
1 (top)
and
Year
2
(bottom ).
The l i n e
in e a c h f i g u r e r e p r e s e n t s where t o t a l
effect = direct
effect
and
the
in d irect
e f f e c t = 0.
The
i n d i r e c t e f f e c t i s t h e d e v i a t i o n b e tw e e n t h e l i n e and e a c h p o i n t
along the a b s c is s a .

107

.7

Year 1

.6

■

■■
.5

.4

.3

.2
.1
/

U

<D
.2

D

.3

.4

.3

.4

.6

.7

.6

.7

.7

Y ear 2

.6
.5

.4

.3

.2

.1

/
.2

Di rect
Figure 5.5

.5

Effect

108

indirect

effects

effects,

but

increased
negative

in

also

(T able
or

asso ciate

this
that

community
they

5.3).

were

generally

The

fact

zero

indicates

th at,

always

decreased

or

h ad

the

this

no

relativ e

increased

that
in

sm all

as

indirect

the

on

the

direct

direct

effect

effects

community,

effect

to

the

the

were alw a y s

addition

of

abundance of

an

other

asso ciates.
T he'm easure of i n d i r e c t
in

which

an

associate

and

asso ciate
thus

focal sp ecies.

If

effects

species

changes

the

the

asso ciate,

generated.
p red icts
effects

then

That
the

is,

success

are not

changes
effect

of

5)

only in clu d es

the

abundance

the

second

of

is

the

the
of

affected

fact

the

that

focal

curves
the

to tal

species

second

associate

w ithout a f f e c t i n g

response

effects

a

t h e r e were s i g n i f i c a n t h i g h e r - o r d e r e f f e c t s ,

th e mechanism of i n t e r a c t i o n
of

(equation

could

suggests

in which

have

of

that

the

the abundance

not

yield

on

been

asso ciates
higher-order

i m p o r t a n t i n t h i s weedy p l a n t com m unity.

DISCUSSION
The

use

indicates

of

the

model

to

quantify

t h a t most of th e t o t a l

in teractio n s,

and n o t

by i n d i r e c t

5.3).

The

effects

were

the o p p o site d i r e c t io n
expected

as

the

community r e d u c e s

the

the o th e r a s s o c ia te
It
sm all.

yield

pathways
all

and

of

indirect
is

a

fa cilitativ e;

dominant

and

potential

associate

com petitive

effects

c a u s e d by th e

in teractio n

com petitive e f f e c ts .

of

they

(Table

acted

in

T h i s w o u l d be

sp ecies
effect

in

the

of each of

species.

was s o m ew ha t u n e x p e c t e d
For

also

as the d i r e c t

presence

d irect

e f f e c t between s p e c ie s

direct

indirect

the

example,

it

was

that
known

the
from

indirect
the

effects

tw o-species

would

be v e r y

studies

that

109

Agropyron

has

A m b ro s ia had
was

that

a

the

Chenopodium
reduced,

com petitive

to

a

its

curve

appears

as

the

has

very

for

effect

effect

strong

of

sm all

facilitativ e

of Ambrosia in

the

is

As t h e

changes

in

Chenopodium i n F i g u r e s

zero

At

abundance

size

of

effect

full

of

on
of

c om m u n i ty

the

size

of

Chenopodium

is

decrease.

The

dem onstrates

success

for

that

the

focal

any

affect

that

sim ply

has

th is point
Agropyron

that

abundance

reduce

5 . 2 and 5 . 3

increases.

the

to

to ta l yield

to a s y m p t o t ic a ll y approach

reducing

indirect
the

Ambrosia

and

My e x p e c t a t i o n

in reducing

size.

the

yield

on C h e n o p o d i u m

on A g r o p y r o n .

effect
the y i e l d

to

total

in

a

effect

response

curve

A mbrosia

have

of

direct

its

response

com petitive

However, b e c a u s e

large,

species

strong

because

Agropyron.

the

strong

A m b ro s ia would

Chenopodium

is

a

little

n e t e f f e c t on Chenopodium.
Two f a c t o r s
indirect
d irect
this
the

s ee m t o

effects:

effect.

the

region

on t h e

on

species

the

focal

associates

as

where

focal

restrictin g

response

neither

species.

'p u sh e s'
direct

and

again

im portance

a nd t h e

h ierarch y of com petitive

of

amo un t o f t h e

effect

found in

the response curve

or in d irect

effects

into
have

Species having sm all d i r e c t e f f e c t s

g e n e r a l l y havesm all

w ell,

curve

the

d irect

thep o t e n t i a l

effects

for

on t h e

indirect

effects

other
is

sm all.
It

effects

appears

that

the

c a n be r e l a x e d

the

im portance

A m b ro s ia had
the

the

the

in

com munity, any l a r g e d i r e c t e f f e c t

large e f fe c ts

about

im portant

shape of

Because of

asym ptotic

quite

be

the

conclusions

assum ption

equivalence

somewhat w i t h o u t c h a n g in g
of

indirect

same c o m p e t i t i v e
of

of

the

model

effects.
effect

would

For
as

still

the g e n e ra l
example,

three
hold.

of

if

com petitive
conclusions
one gr a m o f

grams of Chenopodium,
In

fact,

the

sim ple

110

model

presented

here

could

into

an a m o u n t o f A m b r o s i a

The

assum ption

of

be

used

by c o n v e r t i n g

"units"

equivalence

having

is

the

only

grams

same

of

Chenopodium

com petitive

im portant

in

effect.

allow ing

the

c o n s t r u c t i o n o f a sim p le model o f c o m p e t i t i v e e f f e c t s .
The
do

third

appear

to

difficu lt
is

type

to

given

be

of

complex

very

im portant

quantify.
by t h e

shape of

a com petitor.

response
no

of

the

If
focal

the

yield
species

the

If

the

rest

of

for

Agropyron

the

compound

asso ciate

focal

the

to

and

species

fairly

effects,

to

assumes

model
that

different

associates.

s p e c i e s on t h e
the

associate.

effects

is

on t h e

to

a general
That

focal species
Secondly,

is
the

t wo

then

the

constant

effect

only

the

any

over

total

associate

to tal

the

are

ab ility

yield

the resp o n se

that

changing

there

the

w ith

upon t h e
that

per-am ount

and

then

of

curve

of

curve

m agnitude

of

the

of

range

P la n ta g o , Lepidium , T r if o liu m ,
indicating

p a r tic u la r ly at higher y ie ld s .
critical

equivalence
is,

are

the p re s e n c e

h ig h ly n o n lin ear response cu rv es,

r e q u i r e s making

there

not

response

changes

indicate

However,

they

curv ilin ear,

indicating

t h a t compound e f f e c t s a r e o p e r a t i n g ,
The

is

per-amount

is

however

The

lin ear,

com petitors

present,

exhibit

is

is

The d a t a

effects,

to respond

w o u l d be d e p e n d e n t

investigated.

and C h e n o p o d i u m a l l

curve.

com petitors

the

linear,
if

curve

compound

i m p o r t a n c e o f compound e f f e c t s

species

response

associates.

is

yield

response

the

the

effect,

community;

response

to respond

community

on t h e

this

focal

sp ecies

fo cal species

of

the

of the

the

compound e f f e c t s .

of

in

A measure of

d e m o n stra te s the a b i l i t y
of

in tersp ecific

the

assum ptions.
of

effect

com petitive
of

a unit

of

F irst,

it

effects

by

asso ciate

c o n s t a n t , no m a t t e r w h a t t h e i d e n t i t y
model

assumes

fo c a l s p e c ie s a r e m inim al.

that

the

of

intraspecific

Ill

The

assum ption

of

equivalence

is

critica l

because

it

allow s

d e t e r m in a ti o n o f a sim p le r e l a t i o n s h i p between f o c a l s p e c ie s
the

abundance

of

com petitors.

this

community

poor

relatio n sh ip

yield.

In

Ambrosia

is

Chapter
between

fact,

the

the

relatio n sh ip
to

(see

of

between

asso ciate

4).

assum ption

focal

species
is

species.

Ambrosia
high

not

It

is

success

a nd

yields

that

were

good,

or

to

produced

for

a very

associate
except

w hether

associate

effects,

to

total

clear

to tal

lead

and

quite

not

s u c c e s s and

unreasonable

would

success

actually

intrasp ecific

species

is

N on-equivalence

relatio n sh ip

focal

non-equivalence,

range

This

the

a

yield

the
when

when
poor

is

very

due

sm all

grown

w ith

Am brosia.
A sim ilar
Peart
a

(1982)

general

in g ra ssla n d

m id-successional

form.

diffuse

plant

o ld -field

argue th a t equ iv ale n ce
growth

equivalence

individuals

w ill

lead

community.

Goldberg

a homogeneity of

a nd

strong

to

a general

B ailey

though

begin

individuals

effects

was f o u n d

c o m m u n i t i e s a nd G o l d b e r g ( i n

that

The we e dy c o m m u n i t y i n
all.

com petitive

and

review )

Werner

by
in

(1983)

s h o u l d b e e x p e c t e d among p l a n t s p e c i e s o f s i m i l a r

They s u g g e s t

com petition,

of

size

resource

asym m etries

equivalence

of

between

sim ilar-sized

competing

com petitive

F ie ld g e n e r a l l y m eets
as

requirem ents,

these

seedlings,

effect.

requirem ents:
a strong

size

asymmetry d e v e lo p s w ith 4 - 6 weeks.
The

assum ption

also

appears

to

full

community,

be
the

and C h e n o p o d i u m a r e
species

to

be

in

effects

do

appear

of

low

valid

intrasp ecific

for

most

individual

to

w ith

re stric t

the

plants

so sm all, t h a t
contact

of

it
one

the

effects

of
is

on

species.

the

focal

When g r ow n

species
in

the

P la n ta g o , L epidium , T r if o liu m ,

rare

fo r i n d i v i d u a l s of the

another.
growth

of

However,
Ambrosia

same

intraspecific

and,

to

a

more

112

lim ited

extent,

can

seriously

for

this

model

Agropyron.

bias

reason

was

not

the

and

If

response

because

used

the

intraspecific

curve.

of

the

As

pointed

possible

to i n v e s t i g a t e

effect
out

is

strong,

earlier,

non-equivalence,

the growth

of

Ambrosia

in

it

it

was

that

the

the

full

community.
Several
indirect

requiring

experim ents

Bender

et

L aw lor's
return

studies

com petitive

measures
the

other

in teractio n s.

data

used

al.

1984).

provide

in

this

evidence

In

(Lawlor

1977,

However,

follow ing

study
from

fact,

single-species

and,

a l.'s

the
in

studies

of

S ilander

a^

studies

and

fa c t, neither

al.

S ilander

and

estim ate of r e la tiv e

to tal

indirect

components.

proposed

in

an

this

in teractio n

focal species
One o f
chapter

was

all

are
as

of

very

a

it

the

proposed

sim ilar.

function

the
to

full

community.

questions

of

interactions

types

originally

understanding

The

that

the

can

species

of i n t e r a c t i o n s .

s u p p r e s s i n g most

of

the

measure

the

out

in

the

species.

provides

an

in d ir e c t
including

and
that

quantify

response

beginning
of

each

community
the

to

of

the

present.

affect

Ambrosia i s

be

effect)

studies

of

im portance

potentially

other

the

amount

species

set

this

m easures,

A ll of

relativ e

in

their

(to tal

c a n n o t be s e p a r a t e d

a nd t h e a m o u n t o f a s s o c i a t e

of

strongly

study,
effect

types

different

But

effec ts,

or

system .

plant

While

t h e community

Bender e t

any n a t u r a l

com m unities.

determ ine

Lawlor

a

co astal

1982,

T h i s would

t h e i r m easure of "removal re sp o n se "

of

to

species.

A ntonovics a p p lie d
set

sim ilar

to

that

an d

proposed

Antonovics

used

require
of

d irect,

studies

removal

removal

in

to tal,

three

th e m athem atics

a nd B e n d e r e t

equilibrium

im possible

from

here

gamma v a l u e

to

have proposed m easures

are

the

of

this

different

species

in

the

restricted

by

co m p etitiv e dom inant,
No e v i d e n c e was

found

113

that
It

the

is

presence

lik ely

The f o u r the

of

that

a nd

Ambrosia

five-species

in trasp ecific

experim ents'
reduced
full

a nd
These

sp ecies

increasing

it

end

by

response

curves.

affected

by

the

species.

the

all

the

m onoculture

Ambrosia

of

these s p e c ie s
species
of

bec ome

It

is

that

th e grow th of Ambrosia in

the

the

in creasin g ly

does

effects,

b u t n o t by

L epidium ,

T rifolium

are

that

very

by

have

sm all

these

species.

and,

Indirect

are q u i t e m inor.

p ersist

responsive
suppressed

A gropyron, the

in
to

the

the

increases

by d i r e c t

species

in

effect

they

is

not

response curves

are

on

other

sm all,
that

only
the

im portant

in r e g u la t in g

affectin g

the growth of

is

that a ll

community and t h a t

in

non-linear

species

the

pattern

first-y ear

both

strongly

This su g g ests

are not

full

highly

little

effects

from

also

these

because

The g e n e r a l
in

are

b a sis of
very

community.

P la n ta g o , Lepidium ,

the

the growth of

full

effects

community

on t h e

species

the

direct

indicated

effec ts,

these

full

as

lik ely

by

in

The g r o w t h o f

s i m p l e and c o m p l e x e f f e c t s

species
less

tw o-species
to

suppressed

5 .3).

in

are h ig h ly suppressed

these

of

lead

P lantago,

restricted

effects,

Individuals

growth

on i t s e l f .

a nd

density

r a r e l y c a n be f o u n d i n c o n t a c t w i t h one a n o t h e r .

the

effect

do n o t a l l o w - a q u a n t i f i c a t i o n

that

strongly

(T able

intrasp ecific

in trasp ecific

growth of A m brosia.

com petitive

hierarchy,

strongly

compound

fact

probable

Chenopodium

affected

and

is

of

are

are

and

experim ents

that

A m b ro s ia and A g r o p y r o n
T rifolium ,

strong

the

in teractio n .

other

Chenopodium

a

affects

s t r o n g l y a f f e c t e d by i n t r a s p e c i f i c

any o t h e r ty p e o f
the

has

However,

So,

community i s

species

effect.

suggest

growth.

At

any o th e r

field

only

com petition

of th e s e

ind ividuals

because
as

they

they
are

com petitive e f f e c ts .

the

m iddle

of

the

hierarchy,

may b e t h e

114

only

species

interspecific
an d 5 . 4 )

that
effects.

in
in

also

th a t the

be

com petitive
is

curve

an

effect

strong

of

the v a r ia n c e
be

not

Ambrosia
The

in

to

of

and

the

include

exhibits

higher-order

experim ental
but

a

a

it

strong

effect

direct

effect

It

may

significant

indirect

on A g r o p y r o n .

5.3

the su c c e ss of

tillers,

strongest

Ambrosia

intrasp ecific

due

density

does

on A g r o p y r o n .

intraspecific

fo r Agropyron (F ig u re s

may

in itial

Only

strong

curves

This

the

effect.

also

isa

plots.

response

effect

both

a h i g h amount o f

controlling

in trasp ecific

by

The r e s p o n s e

different

d ifficu lty

there

lim ited

do n o t e x p l a i n

Agropyron

(-.0 9 7 )

is

is
of

found

unknown

if

Ambrosia

on

Agropyron.
In

summary,

h ierarch ical,

reasons:
other

effects

effects

do

on

community

direct
the

not

species

a nd

species

sho w

very

dom inant

appear

species,

to

to

be

be

c a n n o t be

are

suppressed

by

by d i r e c t

fu rth er

response

w ith

Ambrosia

generally

affected

structured

effects

and

effects
to

strong

for

two

by d i r e c t e f f e c t s

indirect
to

effects,
a size

direct

by

A gropyron.

im portant

strongly affected

so

little

seems

com petitive

dominant s p e c ie s a re not

subordinate
they

plant

asym m etric

intraspecific
Indirect

this

or

of

while

a t which
indirect

com petition.

General Im p lic a tio n s
What

do t h e

resu lts

interactions

and

the

contributing

to

the

in teractio n s
exhibited

by

in

this

most

of

this

structure

of

im portance
community

species

as

study

communities?
of

are
foci

suggest

d ifferen t
1)
a nd

the
2)

The

complex

m ajor

types

non-linear
the

about

of

factors
complex

response

hierarchy

of

curve
species

115

com petitive
p h en o m en o n
may

also

ab ility .
in

herbaceous

occur

in

U nfortunately,
the

The

response

non-linear

plant

other

sp ecies

species

v e r y few s t u d i e s
curve

of

curve
(e .g .

which

seems

exhibit

species

be

reciprocal

have a tte m p te d

different

to

to

a

yield

very

general

law ),

p lastic

to d eterm ine

increasing

and

growth.

the

form of

com petition

(see

Schoener 1983).
The h i e r a r c h y o f s p e c i e s
in

determ ining

While

few

im portance

studies

ab ilitie s,
found

the

have

hierarchies

herbaceous

Pemadasa

1976,

Handel

at

plant

1978,

(e .g .

Fowler

that

system s.
these

of

these

As

w ith

the

plant

appear

to

system s

single

all

resource.

most

com petitive
This
these

would
w ill

that

not
both

m agnitude

It

is

hold.

w hile

elim inate
of

the

any

a

and

a nd

Lovell

1974,

organism s -

Connell

1961,

corals

of

It

com m unities
in

this

by and

is

compound

to

in direct
consider

response

compound

effects

curve

This

and

have

may

be

effects

where
for

for

leads

a
to

non-linear

com petitive

effects

in

1979).

also
of

to

species

that

-

lim ited

competing

ch aracteristic

hierarchies

space
the

possibly

of

in terestin g

are

study,

communities

effects.

have been

types

linear

indirect

com petitive

other

interspecific

in teresting

F irst,

(Pemadasa

s e e B us s a nd J a c k s o n

and

community.

several

this

plant

curves

this

in

lim ited

(how ever,

also

1 9 82 )

community

be

in

relativ e

S tebbing 1973).

interspecific

com m unities,

im portant.

may

of

eq u a lly im portant

com petitive re la tio n s h ip s

types

be

herbaceous

response

suggests

other

It

com petitive h ie ra rc h ie s
T he n

rankings

in tertid al

all

is

effects

com m unities

Lang 1973, r a a c r o a l g a e e p i p h y t e s note

complex

or t r a n s i t i v e

other

com m unities

of

looked

in

sessile

com petitive a b i l i t i e s

im portant

in

w ill

be

these
any

increase

situ atio n

ab ility .

predictions

focal
the
was

not

species

potential
found

for

116

Agropyron

in

this

e f f e c ts of a l l
also

produce

may n o t

study,

the s p e c ie s .

strong

influence

m ight

s p e c i e s on o t h e r
What

other

indirect

allelopathy

of

by a

strong

species

in

of

affected

response
rarely

curves

question.

It

types

complex

of

com m unities.
su fficien tly

for

appears

indirect

However,

im portance of th ese

the

it

sp ecies

effects

of

may o r

For exam ple,

on a c o m p e t i t i v e
this

subordinant

exhibit

non-linear

o r a l g a e may e x h i b i t s u c h p a t t e r n s ,
of com petitive

this

is

interspecific

in teractio n s

I am n o t

study
to

that
be

also clear

dynamics

response

P o s s i b l y s e s s i l e m arine com m unities

com m unities,
from

indirect

though i t

compound e f f e c t s .

subordinate

in teractio n s

explored

com munity,

com m unities m ight

corals,

strongest

t h e community.

a nd p a t t e r n s

determ ined

the

the

c u r v e s and c o m p e t i t i v e h i e r a r c h i e s ?
such as b a r n a c l e s ,

by t h e

a n y d i s r u p t i o n o f t h e h i e r a r c h y may

e f f e c ts in

produce

types

was

Second,

the magnitude

sp ecies-sp ecific
dominant

which

of

the

able

that

com m unities

forces.

to

answer

have
to

this

for d iffe re n t

exists

we

since

have been v e r y

potential

im portant

but

even

in

very

many
seldom

predict

the

Chapter 6

GENERAL CONCLUSIONS AND SUMMARY

The

resu lts

im portance
yielded

1.

of

species

several

The

presented

five

in

this

in teractio n s

to

species

on o t h e r

the

investigated

species

presence

hierarchy

by

Chenopodium
suppressed

of

other

Agropyron
all

by

that

The d a t a
most

abundance
of

a

had

the

five-species

a nd

no

com petitors

had

plant

exhibited

the

least

while

com petitive
This

four-,

a

Increasing

negative

zero.

3.

be

appeared
effects
the

to

among

com petitive

com petitive

com petitive
was

effect

response

follow ed

Lepidium ,
on

hierarchy

in

the

T rifolium ,

and

a nd

was

and f i v e - s p e c i e s

non-linear

a s y m p to tic a lly approaching

determ ining

community

a consistent

were

highly

established

the

effect

response
yield

on

the

m ixtures
to

by

a
the
effect

general

increasing

(d ry w eight

2
biomass/m )

success

per-am ount

species.
of

117

a

species

suggest

an

T r i f o l i u m , L e p i d i u m , and Chenopodium w i t h t h e s l o p e o f t h i s

com petitive

an d

in both years of the stu d y .

exhibit

a

year

Ambrosia

P lantago,

tw o-,

of co m p etito rs.

There

types

A m b r o s i a h ad t h e g r e a t e s t

species.

from o n e - ,

species

each

species.

alm ost

other

in

and d e m o n s t r a t e d

m id summer a n d was c o n s i s t e n t

2.

in

on

conclusions:

h iera rch y of com petitive a b i l i t y .
effect

dissertation

The
was

of

P lan tag o ,

relatio n sh ip

equivalence
im portant
the

yield

of

factor
of

that

118

species,

rath er

com petitive

than i t s

effects

id en tity .

of

differen t

s p e c i e s when t h e a s s o c i a t e

4.

S ingle-species

community
single
of

to tal

species

as

the

an

effects
an d

of

of

+

on e a c h

in

the

w ith

less

as

ab ility

full

a

single

full

species

a

to

focal

five-species

the

removal

provided

observed

of

a measure

effects)

focus.

com munity.

the

the

intersp ecific

interm ediate

in

in

experim ents

sp ecies

full

on

t h e same y i e l d .

focal

indirect

com petitive

two s p e c i e s
were

of

c o u l d be f o u n d i n t h e

species

performed

These removal

(direct

p ersisted

P lantago,

were

response

associate

hierarchy

experim ents

the

species.

effect

associate

s p e c i e s were a t

removals

measure

associate

the

that

to

No d i f f e r e n c e s

of

each

The r e s u l t s

show

in

the

tw o-species

However,

the

com petitive

com petitive
community

ab ility ,

than

in

a nd

the

Agropyron

tw o-species

m ixtures.

5.
of

The

relatio n sh ip

associate

associates,
P lantago,
used

increase
the

sp ecies,
was

highly

focal

regardless

species
the

the
into

to tal

species

a nd

suggest

com petitive
effects

were

that

a)

always

in

of

decreasing

the

very

the

of

the

focal

Chenopodium.

of a s s o c ia te

(antagonistic)

id en tity

for

effect

c o m p e t i t o r s and t h u s d e c r e a s i n g
resu lts

of

grow th

total

species

This

the a s s o c ia te

often

sm all

or

A gropyron,

relationship

was

s p e c i e s on
to

s p e c i e s a n d an i n d i r e c t e f f e c t

of

the

effects

and

or

species actin g

yield

t h e sum y i e l d

direct

yield

asso ciate

c o m p e t i t i v e e f f e c t o f an a s s o c i a t e

a direct

summed y i e l d

associate

species

sig n ifican t

Lepidium , T r if o liu m ,

to s e p a r a t e

a focal

between

quite
zero,

of

other

of a s s o c ia te
between
large

a nd

species.

species
a nd

b)

potential

were
the

fa cilitativ e,

The

always

indirect
acting

to

119

increase

6.

the growth of the

There

was

no

focal

evidence

im portant

in

im portant

f o r some s p e c i e s .

effect
focal

and

structuring

occur

species

species.

that

this

community;

the

direct

changes

the

ab ility

species.

effect
of

in d irect

however,

Compound e f f e c t s

when

another a sso c ia te

interspecific

of

the

effects

compound e f f e c t s

were
were

are a type of h ig h e r-o rd e r
o ne

asso ciate

focal

species

species
to

on a

respond

to

The h i g h l y n o n - l i n e a r c u r v e s d e m o n s t r a t e d by

t h e g r o w t h r e s p o n s e o f i n d i v i d u a l s o f P l a n t a g o , L e p i d i u m , T r i f o l i u m , and
Chenopodium
effects
the f u l l

7.

must

considered

community

com petitive

yields
to

of

com petitors

p redict

studied

effects.

A m b r o s i a , wa s r e s t r i c t e d
of

interm ediate

by b o t h i n t r a species,

be

plant

asym m etric

species

increasing

the

suggest

success

of

that

these

compound

species

in

community.

The

species,

to

Growth

structured
of

by s t r o n g

com petitive

and i n t e r s p e c i f i c

was

ab ility ,

effects.

the

by

hierarchical,

com petitively

intraspecific

dominant

effects.

A g r o p y r o n , was

The

restricted

The c o m p e t i t i v e l y s u b o r d i n a t e

P l a n t a g o , L e p i d i u m , T r i f o l i u m , and C h e n o p o d i u m , w e r e r e s t r i c t e d

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