Diversity maintenance in annual plants and stream communities: The effects of life history and environmental structure on coexistence in a variable environment

Species diversity and coexistence have long been central foci of ecology, but field studies are often limited to describing diversity patterns, while theory frequently ignores environmental variation. Scale transition theory is an ideal framework for studying species diversity, as it explicitly accounts for environmental variability and allows for the quantification of coexistence mechanisms. Each coexistence mechanism arises from specific types of biotic and abiotic interactions. Moreover, mechanism magnitudes provide information about how these interactions contribute to coexistence. Studying how the natural history of a community determines these biotic and abiotic interactions provides insight into how that natural history influences coexistence.;Environmental variation is a central hypothesis for the maintenance of diversity in both desert annual plants and streams. This dissertation uses scale transition theory to develop theoretical understanding of how life history and environmental structure in these communities influence coexistence mechanisms and diversity.;In desert annual plants, the focus is on how germination depends on environmental conditions. I analyze how this life history interacts with variation in the environment to affect coexistence. The germination responses of desert annual plants to an unstudied type of environmental variation, soil moisture duration, generate species-specific but highly structured patterns of germination variation. Although the environmental variation is one-dimensional, the nonlinearities that arise due to germination biology generate sufficient germination variation to promote coexistence by the temporal storage effect.;In stream communities, I examine how the physical structure of stream environments affects coexistence. This dissertation demonstrates that both patterns of diversity along streams and the strength of coexistence are primarily determined by the relative scales of dispersal and environmental change. Downstream drift has relatively minor effects on coexistence. Branched stream networks have no effect on community dynamics if tributaries have identical environmental conditions, habitat size increases additively at confluences, and demographic stochasticity is unimportant. Any effects of branching on coexistence caused by violating the environmental condition are asymptotically eliminated as streams increase in size. These studies provide a theoretical, mechanistic foundation for the study of stream communities that addresses environmental and life history factors long recognized as important by empirical stream ecologists.