Prior heat accumulation reduces survival during subsequent experimental heat waves

Abstract

Heat waves, i.e., periods of extremely hot weather, are expected to increase in frequency and duration under climate change. Repeated exposure to thermal stress events such as heat waves can affect population dynamics and even population persistence. Understanding whether recent thermal history bolsters or hinders demographic responses such as survival or growth during heat waves is crucial for predicting population persistence in the face of climate change. We tested for differential mortality following an experimental heat wave in populations of the splash pool copepod, Tigriopus californicus, inhabiting pools that differ in their recent thermal history. We observed differences in survivorship that were correlated with differences in thermal history. Among multiple metrics of thermal history, daily degree-hours (a measure of heat accumulation) best explained heat wave survival such that a higher number of degree-hours resulted in reduced heat wave survival. This finding is consistent with the hypothesis that repeated exposure to sublethal temperatures reduces thermal tolerance. Increasing heat wave intensity and duration, or warmer conditions that increase heat accumulation followed by heat waves may negatively impact population persistence by exacerbating the demographic effects of heat waves. The results of this study emphasize the need to integrate mechanistic physiology within the realm of population ecology to bolster the ability to predict population-level responses to climate change.