Ecological and Genetic Contributions to Species Responses to Environmental Change

Anthropogenic change has altered the ecological landscape upon which modern plant communities have assembled, causing changes in community composition and upsetting interspecific interactions. Yet, the study of these effects has proved challenging due to the many ways in which community-scale change can be characterized, and due to the difficulty in identifying the biological mechanisms which best describe those changes across all members of the community. Despite these challenges, concepts from both functional trait ecology and population genetics offer opportunities to provide insight into which species are most capable of changing in tandem with the environment. In this dissertation, I attempt to address these challenges by testing (1) how trait differences between native and introduced species explain changes in community composition, (2) whether genetic variation and/or traits explain changes in population size within a community, and (3) whether variation in population genetic parameters is explained by environmental variables and/or demographic history. I begin by presenting an analysis of a grassland plant community showing that directional change in community composition is caused by shifts in abundance within the introduced component of this community. The native components of this community show no change in composition over time. This work uses long term data for a community experiencing extreme drought for which previous work linked declines in species richness to declines in specific leaf area (SLA), a trait associated with drought resistance. I also find that changes in community composition were correlated with SLA. Importantly, the species most strongly associated with compositional change are also some of the most abundant species in this community, which demonstrates how abundant species shape environmental change and raises the question of how well these analyses describe the dynamics of less common species in the community. In my second chapter, I present further analysis of the same community, which suggests that the relationship between changes in abundance and SLA only applied for the most common species. Other mechanisms may then have stronger explanatory power for changes in abundance of less-common species. Metrics of genetic diversity are the strongest predictor of abundance changes for five species in this community. My results suggest that it will be valuable for future work to explore whether genetic diversity generally predicts the performance of populations across species and communities, and whether genetic diversity is beneficial because of the direct effect it can have on individual fitness or because of its contribution to rapid adaptation under stressful conditions. Finally, in my last chapter, I show that effective population sizes (Ne) across the range of an invading weed, Centaurea solstitialis, are correlated with both population age and the climatic environment. This pattern matches theory for the genetics of range expansion, which predicts increased genetic drift closer to the expansion front. Variation in adaptability produced during range expansion has implications for invading species, but also for species undergoing range shifts in response to climate change. In both cases, it is expected that lower Ne from colonization dynamics should reduce the ability for expanding population to evolve adaptively to novel environments. Additionally, lower Ne in more ecologically marginal habitats should further reduce the likelihood of adaptation to changing environments in these populations. Together with the previous chapters, these analyses reveal important effects of both trait-environment mismatches and genetic variables on the potential for populations to persist and grow under changing environments