Trait Evolution and Species Coexistence in the Hyperdiverse Tropical Tree Genus Psychotria.

Abstract

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.