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Models are central to the role of science as a tool to make sense of the natural world. In ecology, coexistence theory fundamentally relies on population models to understand diversity maintenance in biological communities. Models of coexistence historically simplify the biology of diverse systems into equations of pairwise species competition, an abstraction that is often imported into empirical tests of coexistence theory. As a result, empirical coexistence studies often overlook the role of trophic interactions, mutualisms, spatial heterogeneity, and facilitation in their formulations of what is important for diversity maintenance. My dissertation asks if and how overlooking such dynamics matters for understanding plant interactions and thus coexistence. I determine the roles of pollinators and spatial heterogeneity on plant performance in annual plant communities using (1) field tests in a system of sympatric flowering plants in the genus Clarkia (Onagraceae) and (2) simulation models. In Chapter 1, I ask how shared pollinators affect Clarkia interactions using a combination of interaction plots, pollen supplementation, and pollinator behavior experiments. I find the effect of pollinator sharing on co-occurring Clarkia is to amplify the effect of plant interactions over vegetative growth- i.e. sharing pollinators intensifies both facilitative and competitive interactions between Clarkia, implying thatsharing pollinators destabilizes plant coexistence. In Chapter 2, I use a reciprocal transplant experiment to ask how spatial variation in hydrology affects Clarkia performance. I find that hydrological variation corresponds to Clarkia spatial partitioning by driving spatially variable germination rates, resulting in highest population growth rates in the home patches in three of four species. This evidence is the first step in determining if Clarkia coexist via the spatial storage effect, an oft- overlooked coexistence mechanism. In Chapter 3, I use simulation models to ask if plant species facilitate each other by better supporting pollinator populations in two- species communities compared to single-species communities. I find that such facilitation via pollinator support can occur in two-species communities that exhibit low-germination rates and species-specific responses to the environment. My findings indicate that facilitation can be incredibly important for understanding plant diversity maintenance.

Diverse rainforests in the tropics are being rapidly modified and fragmented for human use. Plants that persist in remnant forest patches are threatened by many biotic and abiotic changes that are associated with fragmentation. Several of these fragments consequently contain fewer plant species than they once did but, in the absence of contiguous forest, remain the only strongholds of plant and animal diversity in many parts of the world. However, the future of diversity in forest fragments may be uncertain as fragmentation can influence plant-animal interactions that shape plant communities. Some of these changes may already be apparent in the youngest life stages of fragmented plant communities, which would then form the template for the future of those fragments. By examining the compositions of differently aged plant communities in forest fragments, we may be able to identify interactions that are especially influenced by fragmentation, as various plant-animal interactions play structuring roles during different stages of a plant’s life cycle. In this thesis, we primarily investigate relationships between fragment area and processes that structure plant communities in a tropical forest. In the first data chapter (the second chapter of this thesis), we examine the compositions of four life stages of woody plants across a gradient of fragment size. We show that plant species are primarily affected by fragment size during their transitions from seeds to saplings. We discuss several possible explanations for the observed patterns but suggest that altered interactions between plants and their natural enemies (insects and fungal pathogens) may be particularly important drivers. Such plant-enemy interactions have the potential to maintain plant diversity by causing the negative density-dependent mortality of locally abundant plant species, and by allowing the persistence of locally rare species (the Janzen-Connell Hypothesis). As the modification of diversity-maintaining interactions can have catastrophic long-term consequences for the plant diversity in small fragments, it was important to investigate whether plant-enemy interactions are indeed sensitive to fragment size. In the second data chapter (the third chapter of this thesis), we investigate the relationship between fragment area and (soil-borne) fungus-induced mortality of six woody plant species in a shadehouse experiment. We present evidence that the pathogenic effects of fungi on one plant species increased with increasing fragment size. Although we show that plant fungus-interactions can be influenced by fragment size, further experiments are required to investigate whether such effects (even when spread across the community) can influence the diversity maintained in a forest fragment. In the third data chapter (the fourth chapter of this thesis), we first examine the roles of insects and fungi in maintaining woody plant seedling diversity in an Indian rainforest. We then investigate whether the diversity-maintaining abilities of these natural enemies are influenced by fragment size. We present evidence that insects play important roles in maintaining gamma diversity primarily by suppressing common plant species independent of density. We show that they maintain more diversity in large fragments than in smaller fragments, as smaller fragments were dominated by insect-resistant species. We present evidence that fungi caused the density-dependent mortality of one plant species but infer the presence of more such interactions in the community. We show that fungi may be sensitive to fragment size, and that fungi maintain more beta diversity of woody plant seedlings in large fragments than in small fragments. In conclusion, I show in my thesis that insects and fungi play important roles in maintaining woody plant seedling diversity. I further show that these essential plant-enemy interactions are predictably influenced by the size of a forest fragment, and that the plant diversity in small fragments may consequently be at risk from the breakdown of such interactions.

Understanding how tree species coexist in the world’s most diverse tropical forests in the face of intense competition for resources has proved an enduring challenge to ecology. This challenge is only exacerbated when one recognizes that an inordinate number of species in the most diverse tropical forests are comprised of a small number of exceptionally species-rich genera. Such genera, however, may provide the key to understanding the origins and maintenance of tropical forest diversity, because they allow for the explicit examination of trait and niche differences between species that are otherwise likely to share important similarities owing to their shared ancestry. This dissertation aims to identify the processes responsible for the diversification of tropical forest understory shrubs in the genus Psychotria (Rubiaceae), one of the most species-rich plant genera in the world, and for their coexistence in species-dense communities such as that found on Barro Colorado Island (BCI), Panama. I have examined the phylogenetic and community patterns exhibited by physiological traits associated with drought and shade tolerance as well as secondary compounds and physical leaf traits thought to function as anti-herbivore defenses. I have sought to account for the influence of historical biogeography on these patterns by explicitly examining the relationships between species’ traits, microhabitat preferences, and biogeographic origins, and have furthermore examined the theoretical implications of plant interactions with shared enemies for stable coexistence. The results of my dissertation work suggest that hydraulic traits that dictate species’ tolerance of drought strongly influence species distributions, both regionally and locally. Species that exploit similar hydraulic environments on BCI are likely to be physiologically similar and closely related, and to have migrated from the same biogeographic center of origin. The lack of phylogenetic signal exhibited by secondary compounds, on the other hand, coupled with the even distribution exhibited by Psychotria anti-herbivore defenses and insect herbivores, is consistent with the view that secondary chemistry has played an important role in the evolutionary diversification of the Psychotria lineage and the coexistence of species-rich assemblages by allowing even closely related species to reduce the extent to which they share natural enemies.