Geographic variation in vulnerability to warming temperatures in an intertidal barnacle species

Vulnerability to warming temperatures under climate change arises when there is a gap between local climate and local physiology. Intertidal species are unique because they face 2 distinct thermal environments, and it is unclear which is the larger driver of thermal physiology and vulnerability. Here, we compare the thermal environments and physiology of 3 populations of the intertidal barnacle Balanus glandula , spanning 1460 km of its geographic range. We measured energy consumption in the laboratory across a 5 h emersion and subsequent 6 h immersion at 7 different emersion temperatures (10-38°C). We compared these results to 1 yr of emersion and immersion temperature data from each location. Our results suggest that the temperatures experienced during emersion are a greater driver of each population’s thermal physiology than those experienced during immersion. We also estimated vulnerability to future warming in 2 ways: as the total annual energy demand and as the number of days above each population’s thermal peak. These produced conflicting results. The central population spent the most days over its thermal peak, but the northernmost population had the greatest total costs over a year. This discrepancy suggests that different aspects of the thermal performance of B. glandula may be under selection from different aspects of the environment. Thus, accurate predictions of the species’ response to warming temperatures will require knowledge of both future temperature and food availability.