Date of Award

Spring 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Evolutionary Biology

First Advisor

Vasseur, David

Abstract

Intraspecific trait variation occurs in nearly all species and is found in a variety of traits, including morphological, behavioral, and physiological traits. Unlike fields such as evolutionary biology and behavioral ecology which have long recognized the importance of intraspecific trait variation (ITV), community ecologists have historically viewed ITV as “noise” that obscures broader population- and community-level patterns of interest. However, recent work has demonstrated that ITV can profoundly affect ecological interactions and processes through a variety of mechanisms (other earlier paper, Bolnick et al. 2011). Of particular interest is the effect of ITV on competition and coexistence. Empirical work has shown that ITV can promote coexistence between competitors, with some even suggesting that ITV is “needed to explain why large numbers of intensely competing species coexist” (Clark 2010). Theoretical work struggles to support this claim, instead finding that ITV generally makes it more difficult for competitors to coexist. This dissertation seeks to resolve these disparate outcomes. In particular, I ask are there general mechanisms that would allow intraspecific variation to promote coexistence and under what ecological and evolutionary conditions would these mechanisms be possible? First, I numerically analyzed a novel model of exploitative resource competition for two essential resources. The model presented in Chapter 2 differs from previous theoretical work in competition between populations with ITV because it explicitly models the mechanisms of competition (exploitative resource use) and assumes that the resources consumers compete for are essential rather than substitutable. I demonstrated that it is possible for ITV to promote coexistence if it allows the growth of some individuals in a population to be limited by a different resource than their competitors. I used a graphical-mechanistic approach to describe two ways in which ITV can rescue populations from exclusion either through persistence mechanisms or coexistence mechanisms. The key feature of the model that allows ITV to promote coexistence was the sigmoidal function that maps trait onto function, which emphasizes the need to understand the shapes of empirical trait mapping functions. Second, I used a stochastic simulation algorithm to explore competitive outcomes when ITV is allowed to emerge naturally as a result of individual-level processes. I simulated competition under three evolutionary regimes: one in which ITV occurred exclusively through mutations in a quantitative trait, and two in which selection acted on standing variation in either partially or fully heritable traits. ITV generated through mutations was best at promoting coexistence when competitors were on average limited by the same resource, otherwise leading to exclusion of the species with higher resource requirements. Under conditions where populations would otherwise be excluded due to a mismatch between their average resource uptake and internal stoichiometric needs, standing ITV in fully heritable traits allowed populations to persist. Although previous work has suggested that moderate heritability is a key feature that allows ITV to promote coexistence (Maynard et al. 2019), I found that partially heritable traits generated fairly continuous trait distributions that made competitors’ niches more likely to overlap beyond limiting similarity that would allow for coexistence. Patterns of evolutionary convergence and divergence between competitor populations occurred, and both were capable of promoting coexistence. Third, I created a theoretical framework for calculating interaction strengths, fitness differences, and niche differences between individuals and used this to illustrate how various assumptions about the creation and maintenance of ITV, the shape of tradeoff and trait mapping functions, and the types of resources that populations compete for can lead to different competitive outcomes. The sigmoidal trait mapping function assumed in Chapters 2 and 3 led to bimodally distributed interaction strengths, where some individuals had interspecific interactions that were weaker than intraspecific (allowing for coexistence) while others had interspecific interactions that were weaker.

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