Data from: 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.

气候变暖被预测将引发外温动物（ectothermic species）的大规模灭绝事件。热带与温带外温动物间存在一项显著差异：热带物种的生存临界最高温度（upper temperature limit for survival, T_max）更接近其适合度最优温度（temperature at which fitness is maximized, T_opt）。因此，即便环境温度小幅升高，也可能使热带外温动物面临极高的灭绝风险；而温带外温动物则拥有更宽的温度缓冲区间。尽管这一模式已被广泛观测到，但其背后的形成机制尚未得到充分阐释。我们构建了一套数学框架，将适合度的温度响应拆解为繁殖力、死亡率与发育速率三个组成部分，并利用昆虫相关数据对模型预测进行验证。研究发现，高温下适合度下降的原因在于繁殖力与死亡率的温度响应方向相反：当温度超出最优范围后，繁殖力随温度升高而降低，而死亡率则持续上升。T_opt与T_max的接近程度，取决于发育速率的温度响应如何介导繁殖力与死亡率之间的相互作用。当发育对温度高度敏感时，只有当繁殖力随温度升高开始下降后，死亡率才会超过繁殖量；这会导致一旦温度超出T_opt，适合度便会迅速降至零。该模型能够准确预测不同纬度地区昆虫的适合度-温度关系的实证观测结果，同时有望解释学界广泛报道的物候转变现象以及适合度响应的纬度差异。