Thermal history drives trait divergence and alters the ecological role of a freshwater consumer (Gambusia affinis)

Abstract

Increasing temperature, as a result of climate change, is predicted to have numerous effects on species, including altered geographic distributions, shifts in phenologies, and decreased body size at maturity. Theory predicts that reduced body size and rising temperature, particularly from a metabolic perspective, should lead to changes in trophic interactions and ultimately ecosystem function. However, contemporary adaptation may influence the outcomes of warming, an issue that is not commonly considered in climate change research. In this thesis, I used populations of the globally invasive mosquitofish (Gambusia affinis) from a wide geothermal temperature gradient as a model system to examine how thermal history influences individual traits and, ultimately, ecological function. I measured mosquitofish metabolic rates in-situ and after acclimation in a laboratory to understand how adaptation may lead to deviation in the rates predicted by metabolic theory. I found evidence of counter-gradient variation in metabolic traits of wild populations that offset predicted energetic demand of warming. My data show that, across populations, allometric slopes increased predictably with temperature and that size-corrected metabolic rates were unrelated to temperature. My laboratory data show that the temperature sensitivity of metabolism was reduced in warm-source populations, leading to a convergence in aerobic scope between acclimation treatments. Moreover, there was a relationship between metabolism and behaviour, this was only apparent when measured in certain contexts. I further analysed dietary variation and body elemental composition across a wide temperature range to determine if diet and body stoichiometry varied with temperature rise. Finally, I used a mesocosm experiment to examine the ecological role of body size of thermally divergent populations. Mosquitofish diet changed strongly with temperature, which was reflected by gut morphology and body elemental composition. Finally, my experimental data show that ecological responses to different body size distributions were often dependent on source population. Overall, my research suggests considerable physiological adaptive flexibility to temperature and suggests that thermal history may mediate the ecological outcome of future body size declines.