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.

气候变暖预计将导致变温动物（ectothermic species）大规模灭绝。热带与温带变温动物存在显著差异：热带物种的存活临界最高温度（T_max）与适配度（fitness）最大化的最适温度（T_opt）更为接近。因此，即便环境温度小幅升高，也可能使热带变温动物陷入极高的灭绝风险，而温带变温动物则拥有更宽的温度缓冲区间。尽管该模式已被广泛观测到，但其背后的形成机制仍未得到充分阐释。我们构建了一套数学框架，将适配度的温度响应拆解为繁殖力（fecundity）、死亡率（mortality）与发育（development）三个组分，并利用昆虫数据对模型预测进行验证。我们发现，高温下适配度下降的原因在于繁殖力与死亡率的温度响应呈反向作用：当温度超出最适范围后，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，仅当繁殖力随温度升高开始下降后，死亡率才会超过繁殖量，这会导致温度一旦超出T_opt，适配度便会快速降至零。该模型能够准确预测不同纬度昆虫的适配度-温度关系的观测结果，或可解释学界广泛报道的物候转变以及适配度响应的纬度差异。