Meeting Abstract
The extreme cold and hot temperatures that organisms can tolerate – critical thermal minimum (CTmin) and maximum (CTmax) – are often tightly correlated to geographic distributions of diverse species. Critical thermal limits can therefore provide key mechanistic links between organism physiology and ecology, facilitating predictions of range and phenology shifts in response to climate change. Shifts in distributions of bumblebees (genus Bombus) are likely driven by changing climate, but their critical thermal limits have rarely been measured. Bumblebee species are broadly distributed across latitudinal and altitudinal gradients; local populations must therefore contend with strongly divergent climates, and may do so by having similarly divergent critical thermal limits. We tested this prediction using the widely-distributed Western North American bumblebee, Bombus vosnesenskii. We initiated nests from queens collected at four sites distributed across altitude (sea level to 2100m) and latitude (southern CA to Oregon), reared nests in common garden conditions and measured critical thermal limits of worker bumblebees. CTmin decreased strongly with altitude, with bees reared from high altitude queens tolerant of temperatures ~5 C colder than those from low altitude, despite these populations being separated by less than 100 km and no obvious barriers to gene flow. CTmin decreased less with latitude (as expected given climatic differences between these sites). Similarly, CTmax varied little across latitude, but was ~2 C hotter for bees reared from high altitude queens. This strong evidence for local adaptation in bumblebee thermal tolerance provides a key mechanistic link between thermal physiology and geographic distribution of these critical pollinators.
