Influence of thermal regime on the life histories and production of Rocky Mountain aquatic insects
McCarty, Jennifer Denise
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Life history traits of aquatic insect taxa such as metabolism, terminal body size, and fecundity vary along natural thermal gradients. Body size, in particular, is expected to respond to temperature and may have important consequences for fecundity and the production of insects. The Thermal Equilibrium Hypothesis (TEH) predicts that aquatic insect taxa are most abundant at an intermediate 'optimal' temperature where life history traits such as terminal body size and reproductive potential are maximized, i.e., the thermal 'optimum'. A competing hypothesis, the Temperature Size Rule (TSR), predicts that individuals developing at the coldest temperatures in their range will grow more slowly, but attain the largest body sizes and therefore exhibit greater fecundity than individuals growing at warmer temperatures. Implicit in both of these theories is that population-level production, a measure of population 'success', will be greatest where terminal body size and fecundity are maximized. Few studies have investigated the TEH in the field, and none have measured the relationship between production and other life history traits in the context of these theories. Our study focused on three common Rocky Mountain aquatic insect taxa: Drunella doddsii, Hydropsyche cockerelli, and Ephemeralla infrequens. We quantified the influence of thermal regime on growth rates, terminal body size, reproductive potential, and population-level biomass and production, all of which potentially limit the longitudinal distribution and success of these taxa. We found that growth varied strongly with season and site, leading to significant variation in the timing of growth and terminal body size. Reproductive potential was negatively associated with mean annual temperature as predicted by the TSR. Unexpectedly, reproductive potential was not always correlated with terminal body size. Population density, biomass, and secondary production were generally positively correlated with terminal body size for D. doddsii and H. cockerelli, as expected from both predictive models. In contrast, these relationships were not as consistent for E. infrequens. Our findings provide new insight as to how thermal variation influences the ecology of aquatic insects in the context of the TEH and TSR. Our results should be valuable for predicting population and community responses to ongoing changes in climate.