CAN ANIMALS ADAPT TO EXPECTED RATES OF ANTHROPOGENIC CLIMATE CHANGE? EVIDENCE FROM PALEOMIDDEN STUDIES ON NEOTOMA

The onset and intensification of high-latitude glacial cycles some 2.6 Ma ago profoundly influenced the abundance, distribution and evolution of species. Recent evidence suggests that some of the climate oscillations were rapid events, perhaps occurring over decades instead of hundreds or thousands of years as previously thought. Given the current acknowledgement that anthropogenic climate change is one of the most pressing environmental and conservation issues humans face, studies of biotic responses to past climate change have taken on new urgency. The most recent projections suggest increases of 2.8 to 10 C within the next 100 years. Yet, the magnitude, timing and nature of biotic responses to such changes remains unclear. Here, we examine the response of woodrats (Neotoma) to late Quaternary climate change at a variety of sites throughout the southwest. We measure the radiocarbon-dated fossilized pellets contained in abundance within middens, and from them estimate body size (a technique validated in earlier work). We find remarkable congruence across the geographic range in the response to climate change. Chronosequences plotted for each location demonstrate that woodrats respond as expected on the basis of Bergmann's rule: colder climatic conditions select for larger body size and warmer conditions select for smaller body size. For this genus, at least, alterations in body size appear to be the most parsimonious way to deal with changing climatic regimes. The more closely located sites are, the greater the concordance in observed patterns. Samples of the same age yield completely overlapping body mass estimates suggesting they are part of a single metapopulation, responding similarly to the climatic challenges of the late Holocene. Using a robust relationship between average population body mass and average July ambient temperature of the habitat constructed with extant populations, we estimate the evolutionary rate of change required to adapt to a warming of 5.8C over 100 years. This result is compared with the evolutionary rates of body size change calculated from the fossil record to determine if rates of this magnitude were achieved during the late Quaternary.