Predators affect prey populations both through consumption and by inducing antipredator trait responses. In the mere presence of predators, many prey modify traits in order to reduce their risk of being consumed. Predation risk-induced trait responses (hereafter 'trait responses') are numerous and universal across ecosystems and across different taxa, from protists to large mammals. Increasing attention is being given to the proposition that trait responses can have large effects on prey fitness, with ensuing effects on prey population growth and interacting species. A thorough understanding of the role of such predation risk effects is important for the ecological theory of basic properties such as resilience and biodiversity, and for ecological models used in natural resources management.While there are many studies that demonstrate a variety of trait responses in different taxa and examine the drivers of trait responses, it is still difficult to predict when trait responses will translate to population and community-level effects. The majority of theories and studies of trait responses have been conducted in simplified food webs such as predator-prey pairs. However, to examine the contribution of predation risk effects in addressing ecological questions, there is a need to understand how trait responses operate in larger food webs. To scale up from simplified systems, fundamental properties of populations and communities need to be considered including whether there is variation and contingency in trait responses among life history stages and similar species of prey. While there is a theoretical basis for expecting variation, empirical examples in a natural setting are lacking.My dissertation research empirically examines the variation and contingency of behavioral trait responses induced by a fish predator within a diverse assemblage of zooplankton prey. Experiments were conducted in mesocosms with and without fish kairomone (produced by caged fish); the effect of kairomone on the position of zooplankton is used as a measure of behavioral response. Chapter 1 examines variation in behavioral responses among life history stages of copepods. The responses were highly stage-dependent, with nauplii shifting in the opposite direction than copepodites and adults. Chapters 2 and 3 examine variation in cladoceran behavioral responses and assess if the expression and magnitude of responses is contingent on differences in predation risk among taxa. In trying to understand the variation in trait responses among prey, it might be expected that more vulnerable prey would exhibit larger trait responses. Such positive relationships between trait responses and predation risk have been exhibited in some systems. We compared the relationship between behavioral responses and metrics of predation risk across cladocerans. Metrics included relative predation rate and net effect of the predator on density on each taxon (measured from a treatment with uncaged fish) as well as cladoceran body size and taxonomic identity (family). While cladocerans exhibited strong variation in behavioral responses, we did not find larger trait responses in more vulnerable prey.Taken together, the chapters within this dissertation demonstrate there can be considerable variation in trait responses among prey and reinforces the complex nature of factors underlying trait responses. Explicit consideration of variation in trait responses and trade-offs that govern them can lead to better insight when scaling up the study of predation risk effects and their incorporation into models.
Read
