UC Santa Cruz

Much can be learned of complex ecological communities by identifying the ecologically-important species and studying the contingencies of their interactions. Recent evidence suggests that a major mediator of species interactions is trait variation within such species. Western mosquitofish (Gambusia affinis) is a globally invasive predator with potent effects on ecosystems and substantial trait variation among populations. In this dissertation, I sought to determine the traits and environmental conditions that mediate variation in the community and ecosystem effects of mosquitofish populations. In Chapter 2 I tested for the effects of mosquitofish sex ratio variation on experimental pond ecosystems. Mosquitofish exhibit sexual dimorphism and their sex ratios vary widely across their global range from highly female-biased to highly male-biased. In experimental ponds, female-biased populations had higher rates of prey depletion and nutrient excretion which induced stronger pelagic trophic cascades, causing significant increases to important ecosystem variables such as water temperature and pH. In Chapter 3 I tested how temperature mediates the importance of mosquitofish trait variation for experimental pond ecosystems. A growing literature demonstrates that top-down effects increase under warming. Thus, I predicted that the differences in ecological role among mosquitofish populations would become larger under warming. In support of my prediction, warming increased top-down effects and increased the ecological divergence between mosquitofish populations. In Chapter 4 I tested how evolutionary adaptation to temperature affects traits and ecological effects of mosquitofish. Mosquitofish exhibited increased mortality, and evolved slower growth and earlier maturity at higher temperatures. Warm-adapted mosquitofish also consumed smaller pelagic zooplankton prey and did not show a strong ontogenetic niche shift to consuming large benthic macroinvertebrates. Because warm-adapted mosquitofish grew slower, reproduced earlier, and continued to consume zooplankton throughout life, they exacerbated warming-induced shifts towards smaller zooplankton communities. This dissertation reveals considerable effects of mosquitofish demographic variation, interpopulation variation, and contemporary evolution on community and ecosystem ecology. These results suggest that the structure and functioning of an ecosystem can depend strongly upon the traits of the ecologically important species present.