Trends in functional composition of small mammal communities across millennial time scales

Abstract

Rich fossil deposits of the late Quaternary help us understand responses of biodiversity to global change and thus predict the future of ecosystems. Studies from the late Quaternary, however, are often limited taxonomically, geographically (often one site), and by their use of largely taxon‐based metrics that do not inform about ecosystem‐level consequences of biodiversity change. Here, we compare change in functional composition of small mammal communities at El Mirón Cave (Spain) and Samwell Cave (California, USA) across the last 22 000 years, and examine their relationships with climate and vegetation. We find opposing temporal trends between the two locations. European small mammal communities occupied increasingly greater trait space, driven by increases in arboreal granivory and frugivory as ground‐dwelling herbivory declined toward the late Holocene. North American communities occupied smaller trait space as ground‐level foraging and insectivory increased and arboreal herbivory and mean body mass declined. Our results point to the importance of the interaction between climate change and vegetation shifts for explaining changes in small mammal functional diversity, through their synergistic impacts on individual traits. Specifically, increasing temperature across both continents likely led to increases in nocturnal activity and declines in assemblage mean body mass, while transition to mixed forest (El Mirón) or open woodland (Samwell) resulted in increasing structural complexity of vegetation that potentially supported more diverse community‐level dietary characteristics. These results demonstrate the ability of a trait‐based approach to identify how environmental variables correlate with changes in community functional composition through time and gain insight into the potential consequences of environmental change for ecosystem functioning.