Elucidating the temperature response of survivorship in insects

Summary 1. In ectotherms, survivorship is dependent on the environmental temperature. This dependence can take the form of survivorship declining sharply at low and high temperatures and being relatively constant at intermediate temperatures (i.e. an inverted U ‐shaped or ‘flat‐topped’ temperature response), or with survivorship exhibiting a maximum at an intermediate temperature (i.e. a unimodal temperature response). Data show that species differ in which response they exhibit, but there is no mechanistic explanation for why such differences exist. 2. Here we use a life‐history‐based approach to elucidate the temperature response of cumulative (egg‐to‐adult) survivorship. We focus on the fact that cumulative survivorship is a composite trait, arising from the multiplicative effects of stage‐specific survivorship. 3. We show that the temperature response of cumulative survivorship depends on whether or not different life‐history stages/age classes respond differentially to temperature. When all stages/age classes are similarly sensitive to temperature, cumulative survivorship exhibits an inverted U ‐shaped response. When different stages/age classes respond differentially to temperature variation, stages/age classes that are highly sensitive to temperature exhibit monotonically increasing/decreasing or saturating temperature responses, while stages/age classes that are relatively insensitive to temperature exhibit inverted U ‐shaped responses. Because the effects of stage/age‐specific survivorship are multiplicative, the net result is a temperature response of cumulative survivorship that is unimodal and left‐skewed (if the survivorship of the most sensitive stage/age class increases with increasing temperature) or unimodal and right‐skewed (if the survivorship of the most sensitive stage/age class decreases with increasing temperature). 4. Tests of these predictions with data from insects lead to important insights about how ectotherms with different life‐history patterns respond to temperature variation, information that is crucial in understanding how ectotherms with complex life cycles persist in the face of climate warming.