A framework for elucidating the temperature dependence of fitness

Climate warming is predicted to cause large-scale extinctions of ectothermic species. A striking difference between tropical and temperate ectotherms is that tropical species exhibit an upper temperature limit for survival (T_max) that is closer to the temperature at which fitness is maximized (T_opt). Thus, even a small increase in environmental temperature could put tropical ectotherms at high risk of extinction, whereas temperate ectotherms have a wider temperature cushion. Although this pattern is widely observed, the mechanisms that produce it are not well-understood. We develop a mathematical framework to partition the temperature response of fitness into its components (fecundity, mortality and development) and test model predictions with data for insects. We find that fitness declines at high temperatures because the temperature responses of fecundity and mortality act in opposite ways: fecundity decreases with temperature once temperatures exceed the optimal range, whereas mortality continues to increase. The proximity of T_opt to T_max depends on how the temperature response of development mediates the interaction between fecundity and mortality. When development is highly temperature-sensitive, mortality exceeds reproduction only after fecundity has started to decline with temperature, which causes fitness to decline rapidly to zero once temperatures exceed T_opt. The model correctly predicts empirically observed fitness-temperature relationships in insects from different latitudes. It potentially explains the widely-reported phenological shifts and latitudinal differences in fitness responses.