From Trait Patterns to Species Lifetimes: Effects of Niche Differentiation on Coexistence and Community Structure.

Abstract

One of the most enduring questions in ecology is what accounts for coexistence among trophically similar organisms. Niche differentiation is one answer, but so is neutrality: species can coexist either because of ecological differences or ecological similarities. Despite being diametrically opposite, these two theories can be difficult to separate in nature. Because neutral theory posits that species traits are irrelevant for ecological performance, trait patterns are commonly used in niche inference, but results are mixed. This dissertation argues that widely accepted ideas about trait pattern driven by niche differentiation must be updated in light of recent findings, and takes the first steps in that direction. We contend that the current theory of trait patterning is incomplete, and progress requires exploring patterns across a variety of niche models. It emerges from this exploration that stochastic niche dynamics may result in the spontaneous formation of species clusters, under the qualification that pattern will not form if idiosyncratic regulation mechanisms allow arbitrarily similar species to maintain distinct ecological strategies. We provide a new metric for identifying and quantifying species clusters, which outperforms existing metrics in rejecting neutrality in our pseudodata. Another major theme is that the null hypothesis is critical in inference tests. Process-based null models are superior to statistical null hypotheses based on randomization of data because the latter destroys pattern caused by forces unrelated to niche differentiation. For example, we show that when species can randomly mutate into similar species, clusters may occur even in the absence of niche differentiation. A final key theme is that the effect of niche differentiation on species dynamics and pattern may be more complex than is currently appreciated when stochastic dynamics under immigration is considered. Species richness, lifetimes, and extinction rates in niche-differentiated assemblages may be lower or higher than neutral assemblages, depending on the ratio between regional diversity and the number of niches available. The findings of this dissertation contribute to our theoretical understanding of niche differentiation as an important coexistence mechanism, and to realizing the potential of trait patterns in assessing its prevalence in nature.