The Roles of Ecology and Habitat Use in Explaining Range Shifts by Chipmunks in Yosemite National Park

Despite substantial evidence that global climates are changing, predicting organismal responses to such changes poses a vexing research challenge, in part because responses can vary dramatically, even among closely related species. Studies of chipmunks (Tamias spp.) in Yosemite National Park provide a unique opportunity to explore the reasons for variation in species-specific spatial and temporal responses to a century of environmental change. Comparisons of historic and modern distributions of these animals indicate that while the Alpine Chipmunk (T. alpinus) has experienced a marked upward elevational range contraction over the past century, the Lodgepole Chipmunk (T. speciosus) has undergone effectively no change in its elevational range during this period. The reasons for this striking difference in range response are poorly understood. I therefore chose to explore the roles of several biotic factors in shaping patterns of response by these species. Specifically, I focused on habitat specialization and dietary overlap as potential contributors to differences in range response./nI began by selecting three sites in Yosemite National Park where T. alpinus and T. speciosus co-occur, so that I could compare patterns of habitat use in areas of sympatry between the two species. I carried out live-trapping and radio-tracking of chipmunks at each site during the summers of 2011, 2012, and 2013. I integrated these data with analyses of vegetation cover (NDVI) to quantify interspecific differences in spatial overlap and habitat use. I found that considerable interspecific spatial overlap exists, creating high potential for interspecific competitive interactions to occur. I also report evidence for differences in habitat use, with T. alpinus typically found in areas with lower vegetation cover and T. speciosus in areas with relatively higher vegetation cover./nBuilding on the results of the NDVI analyses, I used field-collected microhabitat data to describe characteristics of habitats used by each species in greater detail and to assess degree of specialization in habitat use. I found evidence for interspecific differences in types of habitats used by each species, with lower tree cover and larger amounts of exposed rock in habitats occupied by T. alpinus. Interspecific differences also existed in habitat breadth, with higher variation in amount of downed wood in areas used by T. speciosus. These results are consistent with the characterization of T. alpinus as a high elevation specialist and suggest that the elevational range contraction reported for this species may reflect habitat tracking./nFinally, I took a longer-term approach by examining evidence for dietary changes and changes in cranial morphology in these species over the past century. Stable isotope analyses of hair samples from modern and historical museum specimens of each species collected at the same localities indicated that signatures of temporal dietary change were more pronounced in T. alpinus, although diet breadth did not appear to differ consistently between the study species. Morphometric analyses of crania from these specimens revealed significant temporal changes in cranial shape for T. alpinus, with less pronounced changes in shape for T. speciosus; evidence of selection on skull morphology was detected for T. alpinus but not T. speciosus. These results are consistent with growing evidence that T. alpinus is generally more responsive to environmental change than T. speciosus. However, the observation of large amounts of dietary change in T. alpinus is somewhat inconsistent with expectations based on habitat tracking— if T. alpinus has shifted range to remain in similar habitats over the past century, one would expect to see little change in diet. This in mind, our results emphasize the complex and often geographically variable nature of responses to environmental change./nIn general, my findings suggest that habitat specialization may be associated with greater response to environmental change. My data underscore the complicated ways in which habitat use and dietary breadth act as contributors to range response. Future studies will build upon my findings to explore how local environmental conditions interact with interspecific differences in ecology and habitat use to generate variation in patterns of range change over time.

Advisor: Lacey, Eileen A