Lagged responses in the composition of small mammal communities to a century of climate change

Climate change has widespread effects on the distribution, abundance and behavior of species around the world, leading to the reshuffling of ecological communities. However, it remains unclear whether individual species' range shifts scale up to result in communities whose rate of change lag, lead, or track the rate of climate change. We capitalized on a century‐old dataset originally collected by Joseph Grinnell and his students, plus modern resurveys, to measure long‐term compositional responses of small mammal communities to climate change in historical and modern eras across three regions in the Sierra Nevada of California (Lassen, Yosemite, Sequoia and Kings Canyon National Parks). Across this period, mean annual temperature in each region increased and mean annual precipitation decreased. We tested whether small mammal communities have shifted their composition in favor of species more adapted to hot and dry conditions, processes known as thermophilization and negative mesophilization, respectively. We found positive thermophilization rates (communities composed of more warm‐adapted species) in one of three regions, and negative mesophilization rates (communities composed of dry‐adapted species) in one of the three regions. We show that region‐specific colonization and extinction dynamics of warm‐, cool‐, wet‐ and dry‐adapted species jointly drive thermophilization and mesophilization rates, highlighting that community change arises from both species gains and losses. Importantly, thermophilization and mesophilization rates within regions lagged behind corresponding rates of climate change on average by 0.39–1.40°C and 154–301 mm. Our results suggest that the net effects of climate change can be directional at the scale of the ecological community, despite variability in individual species responses to environmental change and the varied mechanisms that govern them. Communities, like many individual species, may already be out of equilibrium with ambient climate.