Physiological thermal limits predict differential responses of bees to urban heat-island effects.

Changes in community composition are an important, but hard to predict, effect of climate change. Here, we use a wild-bee study system to test the ability of critical thermal maxima (CTmax , a measure of heat tolerance) to predict community responses to urban heat-island effects in Raleigh, NC, USA. Among 15 focal species, CTmax ranged from 44.6 to 51.3°C, and was strongly predictive of population responses to urban warming across 18 study sites ( r 2 = 0.44). Species with low CTmax declined the most. After phylogenetic correction, solitary species and cavity-nesting species (bumblebees) had the lowest CTmax , suggesting that these groups may be most sensitive to climate change. Community responses to urban and global warming will likely retain strong physiological signal, even after decades of warming during which time lags and interspecific interactions could modulate direct effects of temperature.