Daily activity timing and physiological tolerances jointly predict native bee abundance trends under climate change

Background/Question/Methods

For insects, behavioral shifts could be key determinants of responses to climate change. In bees, behavioral thermoregulation (altering activity based on ambient temperature) could buffer against thermally stressful conditions, enabling population persistence and maintenance of pollination services. But evidence is scarce of whether behavior can buffer bees with low heat or drought tolerances against climate-induced declines. We examined how physiological tolerances relate to daily activity timing in six native bee genera, and assessed whether activity timing and physiological tolerances jointly predicted abundance trends under increasing aridity and climate variability using long-term data (2002–2019; Sevilleta Long-Term Ecological Research Program, NM, USA). We measured bees’ thermal and desiccation tolerances via experimental chamber trials, and documented activity timing via digital video in the field.

Results/Conclusions

We found that bee genera differed in both maximum activity temperature and proportion of hours active per day. Genera with higher thermal tolerances were active at higher maximum temperatures and under wider temperature ranges. Importantly, physiological tolerance and activity timing interactively predicted change in abundance over time. Genera with low thermal tolerances showed abundance declines if active at hot times of day, but population stability or increases if active at cooler temperatures, while bees with high thermal tolerances consistently increased in abundance. Our results suggest that bees modulate their activity based on their physiological tolerances, and that shifts in activity timing may buffer bees with low heat tolerances against declines. Understanding the roles of both behavior and physiology could thus be critical to predicting bee abundance and pollination services trends under future climate change.