Date of Award

Spring 1-1-2017

Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Brett A. Melbourne

Second Advisor

Aaron Clauset

Third Advisor

Kendi Davies

Fourth Advisor

Daniel F. Doak

Fifth Advisor

Christy M. McCain


Ecological variability, both across space and through time, plays a critical role in maintaining the incredible range of biodiversity observed in nature and is foundational to coexistence theory. Using a combination of analytical techniques, simulation models, and novel paleobiology databases, I examine how ecological variability contributes to coexistence, or how competing species are able to persist in a given environment. I quantify the strength of coexistence in spatially structured communities and the relative importance of species similarities (equalizing mechanisms) and species differences (stabilizing mechanisms) in maintaining biodiversity across space and time. I find that trait-tradeoffs among species, abiotic heterogeneity, stochasticity, and dispersal limitation within spatially structured communities all have unique signatures of coexistence mechanisms. I additionally show that coexistence theory allows us to analyze more complex communities with multiple assembly mechanisms. While coexistence theory exhibits robust, unique signatures across assembly mechanisms, other, more commonly used methods, such as the beta-null deviation measure, are unable to infer community assembly processes from patterns in beta-diversity. This is especially evident for the presence-absence based measure when compared to the abundance-based measure. Finally, I extend the stabilizing and equalizing framework of coexistence theory beyond its use in community ecology to examine macroecological patterns in mammalian diversity. I show that body mass diversity and its evolution can be well-approximated using primarily equalizing mechanisms. This trend holds across terrestrial mammals, a mammalian sub-clade (Equidae), and even across vastly different environments. Both marine (cetaceans) and terrestrial mammals exhibit similar body mass distributions and evolution, where environmental constraints determine the minimum size above which equalizing processes interact in a predictable way to create a right-skewed body mass distribution. Across this work, I find that stabilizing mechanisms are critical for coexistence in spatially structured communities, but equalizing mechanisms effectively predict patterns in mammalian body mass diversity across evolutionary time scales. Generally, the equalizing and stabilizing framework of coexistence theory provides a mechanistic understanding of patterns in biodiversity, both at the community and macroecological scale.