Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Ecological mechanisms proposed to explain community assembly and the maintenance of biodiversity are hypothesized to fall along a theoretical continuum bounded at one extreme by deterministic processes (“niche assembly”) and at the other extreme by stochastic processes (“dispersal assembly”). In this dissertation, I explore the idea that the position of ecological communities along the niche-dispersal assembly continuum is dynamic in space and time. Using field experiments in a high-diversity longleaf pine savanna, I test the general hypothesis that “ecological filters” (competition, disturbance, and resource availability) contribute to niche assembly through their effects on established plant species and recruitment from the species pool. Consistent with dispersal-assembly theory, I found that dispersal from the species pool strongly limited local species diversity regardless of the presence of these three niche-based ecological filters. Importantly, however, some ecological filters (e.g., space limitation in communities with low-intensity fire disturbance and establishment limitation imposed by drought and high-rainfall conditions) limited the extent to which community assembly was influenced by dispersal, suggesting ecological conditions that reduce stochastic community assembly in high-diversity communities. I examined the generality of these patterns by conducting a meta-analysis of >60 published experiments. I found that dispersal strongly limited species richness in a wide range of plant communities, but that dispersal had a stronger positive effect on species richness in more disturbed communities and when the species pool contained high species diversity and functional-trait diversity, supporting the hypothesis that community assembly reflects a dynamic interplay between species-pool diversity and local environmental heterogeneity. My results suggest a conceptual model for community assembly in high-diversity pine savannas, with implications for other species-rich plant communities. I propose that characteristics of high-diversity communities (large species pools and pervasive recruitment limitation in populations of many rare species) generally contribute to stochastic community assembly, but that niche-based ecological filtering of resident species and immigrating species can shift high-diversity communities towards more deterministic community assembly. This conceptual framework has broader implications for understanding the maintenance of biodiversity and species coexistence in communities of contrasting diversity and for conserving biodiversity in longleaf pine communities threatened by habitat loss, fragmentation, and environmental change.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Harms, Kyle E.