Species distributions are shifting northwards and to higher elevations as a result of ongoing climate warming (Parmesan, 2006). These shifts appear at both long and short time scales (e.g., since 21kyBP, Guralnick, 2007; Grayson, 2005; the last one hundred years, Pounds et al., 2005; Beever et al., 2003) and are in agreement with those predicted by niche-based species distribution models (SDMs, Guisan and Zimmermann, 2000; Araújo et al., 2005). Although this pattern of shifts is likely broadly valid, individual species display idiosyncratic responses to climate change (Parmesan and Yohe, 2003). This suggests the importance of predicting the response of individual species to climate change and of understanding how

and why these responses vary among species. Two factors that likely explain this variation are the differing physiological tolerances of species and whether species populations can adapt to rapidly changing climates. Species with populations that differ in dispersal capabilities, phenology, average fi tness, or levels of heritable genetic variation also should differ in ability to adapt and migrate in response to environmental change (Holt et al., 2003). Therefore, determining the contributions of niche shifts and migration to species response to climate change is an important step toward improving predictions of biotic responses to global warming (Pearman et al., 2008b).