Species Range Shifts in Dynamic Geological and Climatic Landscapes: Studies in Temperate and Tropical Trees

Abstract

Spatial patterns in the geographic distributions of ecosystems, species, and genetic variation are the result of both ecological conditions and evolutionary dynamics that have unfolded in a long, historical process. Attempts to account for spatial patterns in biodiversity provided some of the earliest inspiration for the development of the theory of evolution, but more than a century and a half later, biologists are still discovering precisely how and why these patterns arise. As a field, historical biogeography has emphasized the importance of Earth’s geological and climatic history in understanding dynamic spatial biodiversity patterns. It is well known that species and populations respond to geological and climatic change, but the details about how these responses unfolded are often vaguely specified, and the precise ecological mechanisms mediating them are often unknown. My dissertation tests specific hypotheses about historical range shifts and their genetic consequences, using recent advances such as next-generation sequencing, demographic and coalescent modelling, Approximate Bayesian Computation, and large datasets of species occurrence records and species traits. This work spans several arboreal study systems, and aims to provide new insight into unanswered questions and controversial historical biogeographic hypotheses.

I begin by considering the effects of glaciation on range shifts in two hickory species from eastern North America. It is not precisely known where most temperate deciduous tree species from this region survived the Last Glacial Maximum, and whether northern populations existed and contributed to postglacial recolonization. I show that both species were likely fairly geographically widespread and genetically connected, and that postglacial recolonization occurred from a northern source in one species and a southern source in another. Next, I develop and test competing hypotheses about the ecological factors mediating postglacial range shifts in canyon live oak from California. I aim to attain mechanistic insight into the drivers of historical range shifts in this geographically complex Mediterranean-climate region, and find that summer drought tolerance was likely a key ecological factor mediating these shifts. Lastly, I zoom out in scale and ask how long-term migrational responses to geological and climatic change affect the assembly of entire communities. I examine the biogeographic distributions of >1,000 Neotropical rainforest woody plant species, and show that traits such as drought tolerance and elevational range have impacted the ability of species to disperse across or around the biogeographic filter created by Pliocene uplift of the northern Andes and formation of dry habitats in northeastern South America.

Overall, my dissertation work has revealed the importance of species-specific responses to geological and climatic change, and how these responses affect the geographic distribution of biodiversity (from genetic to species to community levels). I test hypotheses concerning the geographic locations from which range shifts occurred (in hickories), the ecological factors mediating range shifts in complex environments (in canyon live oak), and the community-wide impact of species traits on biogeographic dispersal (in Neotropical trees). This work contributes to a growing body of knowledge helping transform historical biogeography from a realm of broad patterns, into a field where new insight can be gained by accounting for species-specific histories and the ecological processes that mediate range shifts.