Extinction rates exceed any in recent history. Simultaneously, invasive species are invading new areas and increasing in abundance. The leading proposed drivers of biodiversity loss include habitat destruction, urban and agricultural development, climate change, and biological invasions. Despite evidence that these anthropogenic changes influence both extinction of native species and invasions by non-native species, quantifying the role of different proposed mechanisms of diversity loss remains a challenge, in part due to lack of information on species responses to anthropogenic change, a need to examine species traits associated with both invasiveness and decline, and discrepancies between historical and current extinction patterns. In this dissertation, I use a combination of field experiments, historical datasets, and population modeling to examine how the dominant environmental changes facing natural populations, including habitat loss, climate change, and nutrient deposition, influence local species invasions and extinctions. Each chapter addresses a different potential cause of biodiversity decline (habitat loss, climate change, increasing herbivore densities, and nitrogen deposition) or compares the responses of more versus less successful species (i.e., invasive vs. native or locally extinct vs. extant). This approach will help us understand the species traits and responses to anthropogenic change associated with either invasiveness or extinction. I examined (i) how native and nonnative species differ in their phenological responses to climate warming and (ii) how locally extinct (i.e., species that have disappeared at a small spatial scale) and extant species differ in their species characteristics, their phenological responses to changing temperature and precipitation regimes over the past century, and their demographic responses to increasing levels of nitrogen and deer herbivory in threatened prairie habitats. I found that while non-native species flower earlier under warming temperatures, native species' flowering time does not respond to warming, potentially putting them at a disadvantage as the climate warms. I also found that locally extinct species, which are often rare, native prairie specialist species, differ from extant species in their phenological responses to climate warming and their demographic responses to nitrogen fertilization. Specifically, locally extinct species did not advance flowering under warmer temperatures to the same extent as extant species, a response consistent with the hypothesis that appropriate phenological responses correlate with species success. Finally, locally extinct species experienced higher mortality and fewer benefits to growth and reproduction under nitrogen addition than extant species, suggesting that increasing nitrogen levels may influence species extinctions in threatened prairie habitat. By providing evidence of differences in phenological and demographic responses to global change between locally extinct and extant species as well as native vs. non-native species, my research allows us to evaluate the mechanisms underlying contemporary biodiversity change.
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