Heterogeneous responses to environmental change: contrasting behavior and physiology in two California chipmunks

Biotic responses to environmental change can vary markedly, even among closely related, ecologically similar species. Such responses may be conspicuous (e.g., climate-associated range shifts) or they may be subtler and more challenging to detect. In the latter case, organisms may use individually variable mechanisms, including modifications of behavior and physiology, to cope with environmental change in situ. Further, in addition to providing mechanisms of response to environmental change, behavioral and physiological traits may be indicators of habitat suitability. Thus, to understand and, ideally, to predict how species will respond to environmental change, it is necessary to determine which traits are associated with vulnerability and to identify which factors constrain range limits for vulnerable species. My dissertation focuses on the behavior and physiology of the alpine chipmunk (Tamias alpinus) and the lodgepole chipmunk (T. speciosus), two co-occurring, closely related species that have been characterized by very distinct spatial responses to environmental change in Yosemite National Park, CA. Over the past century, T. alpinus has contracted its range upward in elevation; during the same period, T. speciosus has displayed no significant elevational range shift. To assess the role of behavioral and physiological variability in generating these responses, I explored interspecific differences in baseline stress hormone (glucocorticoid, GC) levels and behavioral activity budgets with the goal of identifying the environmental factors that are most important for determining range limits in the study species. First, I validated a non-invasive method to measure fecal GC metabolites (FGMs) in both study species. By exposing captive individuals to a series of controlled challenges, I also identified interspecific differences in stress reactivity, with T. alpinus being generally more stress-responsive. Next, I validated the use of accelerometers to remotely document the behavioral activity budgets of the study species, demonstrating that these sensors can be employed to collect behavioral data from free-living animals. I then deployed accelerometers across broader spatial and temporal scales. I used the resulting data to construct models that integrate intrinsic biological and environmental parameters to identify key predictors of activity in each species. I found that, compared to T. alpinus, activity in T. speciosus was characterized by generally greater inter-individual variance and greater variability in response to environmental parameters. Finally, I used FGM data collected over three years and at multiple sites in and around Yosemite National Park in conjunction with data regarding multiple extrinsic (environmental) and intrinsic (life history) parameters to identify the factors that best predict FGMs in the study species. These analyses revealed FGM levels are more strongly related to environmental parameters in T. alpinus than in T. speciosus. In summary, my research indicates that T. alpinus is more stress-responsive to external, environmental challenges, and potentially less flexible in responding behaviorally to environmental conditions than T. speciosus. Overall, these results indicate that behavior and physiology are likely to be important determinants of a species’ response to environmental change. These findings also suggest that individual species vary in their general sensitivity to environmental change, with some species being more change-responsive than others.

Advisor: Lacey, Eileen A