Insect size responses to climate changes vary across elevations according to seasonal timing

Abstract

Widespread plastic and evolutionary responses to temperature have led to body size declines being proposed as a universal response to warming, but the high degree of variation in terrestrial ectotherms has challenged this view. We investigate whether temperature dependent development and growth rates in ectothermic organisms drive departures from universal size responses. Leveraging a long-term study of montane grasshoppers, we detect size shifts over recent decades that depend on elevation and species seasonal timing. Size shifts have been focused at low elevations with the earliest emerging species (those that overwinter as juveniles) getting bigger, and later season species getting smaller. The shifts correspond to warmer temperatures at low elevation increasing the sizes of the earliest season species but decreasing the sizes of the later season species. This is consistent with our hypothesis that the earliest season species may be able to take advantage of warmer conditions accelerating growth during early spring development, whereas warm temperatures may adversely impact later season species via mechanisms such as increased rates of energy use or thermal stress. Grasshoppers tend to capitalize on warm conditions by both getting bigger and reaching adulthood earlier. Our analysis further reinforces the need to move beyond expectations of universal responses to climate change to consider how environmental exposure and sensitivity varies across elevations and life histories.