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

Abstract

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 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 (10-38{degrees}C). 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 populations emersion thermal physiology than those experienced during immersion. We also estimated vulnerability to future warming in two ways: as the total annual energy demand and as the number of days above each populations 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. The higher energetic costs in the northernmost population may be explained in part by a strong latitudinal gradient in primary productivity that is selecting for higher energy demand in higher latitude populations. Thus, accurate predictions of B. glandulas response to warming temperatures will require knowledge of both future temperature and food availability.