| Anthropogenic disturbance has been described as an agent of ecological divergence, yet our understanding of the processes linking these phenomena are limited. The changes in resource availability (i.e., resource limitation) following a disturbance may favor variation in physiology, behavior, or ecology (e.g., habitat use and diet) in a species in order to minimize competition and satisfy life-history demands. Consequently, populations in disturbed environments may differ in these characteristics from populations in environments where resources are abundant. In this dissertation I address these considerations for tree lizards (Urosaurus ornatus) from grassland regions varying in prescribed burn history. In the southwestern US, burning induces environmental shifts towards structural homogenization and grass-dominance, resulting in resource-limited environments. Moreover, tree lizards are polymorphic in reproductive behavior and throat color that is maintained by socially-mediated sexual selection. In the following chapters I first introduce my study system and the role of anthropogenic disturbance in generating environmental variation (Chapter 1). I then validate the use of stable isotope analysis to describe one of the major consequences of resource limitation, trophic niche divergence, among color morphs of my focal taxon (Chapter 2). I then use field-collected isotopic data to demonstrate that color morphs differing in reproductive behavior may also diverge in ecology and morphology (Chapter 3). Thus, color polymorphic species are likely maintained in part by both divergent natural and sexual selection, and consequently, not all morphs may respond equally to environmental perturbations. Using a novel approach, I link morphological and ecological traits with environmental variation, illustrating that U. ornatus color morphs differing in morphological trait combinations also differ in the degree they 'fit' their microhabitats (Chapter 4). Specifically, dominant morphs, and those lizards that exhibit divergent (non-average) morphologies, tended to 'fit' best. I use capture-mark-recapture data to explicitly demonstrate that those lizards also exhibited a survival advantage in more-disturbed sites. Finally, I demonstrate that environmental variation alters microhabitat use and spatial segregation of U. ornatus morphs, resulting in increased spatial overlap and more-intense social interactions among male lizards, favoring both ecological and phenotypic divergence in burned habitats (Chapter 5). Altogether, my findings suggest that divergence in phenotypic and ecological traits in U. ornatus may be an adaptive response to resource limitation resulting from environmental variation. |
