UC Riverside

Ecology has long been thought to influence the evolutionary process by determining the selection pressures that cause adaptive evolution. The reciprocal influence of evolution on ecological processes has received less attention, but interest in its potential influence is increasing with the recognition that ecological and evolutionary processes can occur on congruent timescales. When these two fields are combined in theoretical models, the results are predictions about population and community dynamics not obtainable when modeled alone. Despite the theoretical work on these interactions, there is a paucity of empirical studies testing the assumptions and predictions of this body of theory.

The aim of this dissertation is to fill this gap. Chapter 1 reviews the theoretical literature related to how density-dependent population regulation has been incorporated into models of life history evolution. Additionally, I review the empirical literature on density-dependence with aim of finding general patterns in how populations are regulated. In chapter two I test whether density could potentially be important in the evolution of the life history of Trinidadian guppies (Poecilia reticulata). First, I conducted density manipulations of natural populations of guppies and found that guppy populations are regulated via density-dependent compensation of the demographic rates. Second, using factorial mesocosms experiments, crossing guppy phenotype with density, I found that the phenotypic fitness (measured as phenotypic growth rates) were dependent on density and that high predation guppies have a significant fitness advantage at high, but not low, densities. In chapter 3 I tested whether divergent guppy phenotypes had alternative affects on the structure and function of ecosystems. Each phenotype did produce significantly different phenotypes and the effects were often much larger than density--a traditionally important ecological trait. In chapter 4, I used a combination of experimental and mathematical modeling to separate the effects of divergent prey selectivity and nutrient recycling among the different phenotypes. The results of this analysis show that the indirect effects of the phenotypes are larger than the direct effects and that the interplay between the two are important in determining the evolution of foraging traits and potentially the rest of the life history.