A mechanistic understanding of how the thermal reaction norms of ectotherms vary on a latitudinal gradient is crucial in predicting how climate warming influences the dynamics and distributions of ectotherm populations. Such geographic variation is the result of selection and constraints (trade-offs). Elucidating how warming influences reaction norm evolution is therefore crucial in predicting latitudinal variation in species abundances. We conduct a literature-based analysis of the temperature response data of 107 ectotherm species, focusing on the key life history traits: fecundity, development, and mortality. We characterize the variation in the key parameters that describe trait responses (e.g., thermal optima and response breadths) based on the thermal regime they experience (e.g., tropical vs. temperate). We focus in particular on trade-offs between life history trait responses, testing predictions from contrasting hypotheses (e.g., hotter is better, thermal generalists vs. specialists) about the nature and origin of such trade-offs.
We report three key findings. First, the physiologically optimal temperature for fecundity increases as a function of both the mean temperature and amplitude of seasonal temperature fluctuations. Interestingly, the optimum increases linearly with increasing mean temperature, but non-linearly (i.e., at a decelerating rate with increasing temperature, giving rise to a saturating response) with the amplitude of seasonal fluctuations. Second, and contrary to the predictions of the generalist-specialist hypothesis, the response breath for fecundity does not increase with increasing amplitude of seasonal fluctuations (i.e., increase in latitude). Intriguingly, we find that ectotherms at lower latitudes exhibit a disproportionately high levels of phenotypic plasticity relative to the range of thermal variation they experience compared to ectotherms at higher latitudes. Third, thermal sensitivity of mortality (i.e., the rate at which mortality increases with temperature) decreases as the optimal temperature for fecundity increases, suggesting a trade-off between the reaction norms of fecundity and mortality (i.e., an increase in maximum fecundity is possible only if the increase in mortality with temperature occurs at a lower rate).