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

For most species, we lack a detailed understanding of how vulnerability to warming temperatures under climate change varies across a its geographic distribution. Vulnerability arises when there is a gap between local climate and local physiology. Intertidal species are unique because they face two distinct thermal environments, and it is unclear which is the bigger driver of thermal physiology and vulnerability. Here we compare the thermal environments and physiology of three populations of the intertidal barnacle Balanus glandula , spanning 1460 km of its geographic range. We measured energy consumption in the laboratory across a 5-hour emersion and subsequent 6-hour immersion at 7 different emersion temperatures. We compared these results to one year of emersion and immersion temperature data from each location. Our results suggest that the temperatures experienced during emersion are a bigger driver of each population’s thermal physiology than those experienced during immersion. We also estimated vulnerability to future warming in two ways: by calculating the total energy consumption over a year and by calculating the number of days that temperature exceeded 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 difference may be explained in part by a strong latitudinal gradient in primary productivity that selects for higher energy demand in higher latitude populations. Thus, accurate predictions of B. glandula’s response to warming temperatures will require knowledge of both future temperature and food availability.