The Impacts of Demographic Stochasticity on Populations and Communities

Legault, Geoffrey B.

Graduate Thesis Or Dissertation

The Impacts of Demographic Stochasticity on Populations and Communities Public Deposited

Analytics

Citations

Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/r207tp39b

Abstract

Accurate descriptions of ecological processes often require accounting for demographic stochasticity, the variation that arises in populations and communities as a result of probabilistic demography (e.g., birth, death, migration). Much theory has been developed for understanding the population-level effects of demographic stochasticity, but ecology largely lacks rigorous community-level descriptions of its consequences. Furthermore, how demographic stochasticity affects other complex biological systems, such as populations responding to continuously-changing environments or populations undergoing range expansion, is not well understood. Here I address some of these gaps using theoretical and experimental approaches. First, I examine how demographic stochasticity affects competitive dynamics in two-species communities and find, both experimentally and using simulations, that demographic stochasticity can produce outcomes not predicted by traditional deterministic models. In the next section, I consider the effects of continuously-changing environments and describe a continuous-time simulation approach that combines environment-dependent demography and demographic stochasticity. I simulate the approach for multiple ecological models and environmental change scenarios and find that accounting for environment-dependent demography in stochastic systems is often necessary for avoiding bias. In the last two sections, I examine the applied issue of geographic range shifts, a multi-faceted phemonemon affecting many species globally and one with substantial economic and social costs. Recognizing the significant variation in range shifts arising from, in part, probabilistic demography, I use highly-replicated experimental approaches to understand the effects of two different processes thought to affect range shifts: spatial selection, and interspecific competition. In the first of these sections, I find that the effects of spatial selection depend on both the intrinsic dispersal ability of expanding organisms as well as the environment into which they expand. In the second section, I show, for the first time empirically, that interspecific competition can effectively halt range expansion for multiple generations. Throughout this work, I illustrate both the role of demographic stochasticity as an emergent driver of ecological dynamics and the importance of using controlled, replicated experiments for understanding highly stochastic biological processes.

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