Small mammal diversity loss in response to late-Pleistocene climatic change

Abstract

The worldwide extinctions of large mammals (megafauna) at the end of the Pleistocene around 10,000 years ago are the stuff of headlines. Did they fall victim to human slaughter or climate change? But what of smaller mammals — rodents, insectivores and the like — which often provide much more comprehensive fossil records than megafauna, and which are much less likely to be the targets of hunting? A study of a rich small-mammal fauna from northern California shows that small mammals were much less likely to respond to the Pleistocene–Holocene transition by becoming extinct. Instead, diversity and evenness suffered, so that less abundant species became rarer, with more generalist 'weed-like' species becoming more common. Many large mammals became extinct worldwide at the end of the Pleistocene epoch, around 12,000 years ago. Here, it is shown that smaller mammals, which often provide much more comprehensive fossil records than large mammals, were much less likely to respond to the Pleistocene–Holocene transition by becoming extinct. Instead, diversity and evenness suffered, so that less abundant species became rarer, with more generalist 'weedy' species becoming more common. Communities have been shaped in numerous ways by past climatic change; this process continues today1. At the end of the Pleistocene epoch about 11,700 years ago, North American communities were substantially altered by the interplay of two events. The climate shifted from the cold, arid Last Glacial Maximum to the warm, mesic Holocene interglacial, causing many mammal species to shift their geographic distributions substantially2,3. Populations were further stressed as humans arrived on the continent4. The resulting megafaunal extinction event, in which 70 of the roughly 220 largest mammals in North America (32%) became extinct5, has received much attention. However, responses of small mammals to events at the end of the Pleistocene have been much less studied, despite the sensitivity of these animals to current and future environmental change. Here we examine community changes in small mammals in northern California during the last ‘natural’ global warming event at the Pleistocene–Holocene transition and show that even though no small mammals in the local community became extinct, species losses and gains, combined with changes in abundance, caused declines in both the evenness and richness of communities. Modern mammalian communities are thus depauperate not only as a result of megafaunal extinctions at the end of the Pleistocene but also because of diversity loss among small mammals. Our results suggest that across future landscapes there will be some unanticipated effects of global change on diversity: restructuring of small mammal communities, significant loss of richness, and perhaps the rising dominance of native ‘weedy’ species.