How temperature variation drives latitudinal differences in ectotherm sensitivity to heat extremes

Abstract

Climate change is increasing the frequency of heat extremes, and therefore the overheating risk of wildlife. Latitudinal patterns of overheating risk are expected, and many analyses have concluded that tropical organisms are at greater risk because they live in environments closer to their thermal limits (i.e., have lower warming tolerance). Overheating risk should be dictated by interactions between mean habitat temperature, habitat temperature variation, organismal heat tolerance limits, and tolerance plasticity. How these factors collectively dictate overheating risk and responses to warming requires greater scrutiny, however. To address this issue, we combined data on physiological heat tolerance, heat tolerance plasticity, and temporally fine-scaled operative thermal environments for terrestrial ectotherm populations across a 120° latitudinal range. Consistent with previous research, we find that tropical organisms have lower warming tolerance than temperate organisms based on mean thermal conditions. However, temperate taxa currently have greater overheating risk than tropical taxa and will experience greater increases in overheating risk with warming because of differences in thermal variation between the regions. These results show that characterizing thermal variation and incorporating it into predictions of global change responses is critical, as doing so can alter predictions about global patterns of vulnerability to global change.