Climatic drivers of pika population density in the Southern Rocky Mountains

Background/Question/Methods

Alpine species are among those most threatened by climatic shifts due to their physiological and geographic constraints. One such species is the American pika (Ochotona princeps), an alpine mammal found in rocky habitats throughout much of western North America. Recent evidence from the Great Basin suggests that this species is responding to climate change through local population extirpation, but it remains unclear whether these patterns of climate-related loss extend to other portions of the species’ range. Our research focuses the impact of climate change on the distribution and abundance of pikas in the Southern Rocky Mountains, a region that represents the largest, most continuous, and southern-most range of the species. In a resurvey of 69 sites historically occupied by pikas, low annual precipitation was implicated as a limiting factor for pika persistence in the region. Due to the relatively low proportion of pika populations extirpated in the Southern Rockies, in this study we investigated local population density as a more precise metric of population response. In addition, we were able to utilize microclimatic data from temperature data loggers to better understand the mechanism by which climate affects this species.

Results/Conclusions

Local pika densities were highly variable across the Southern Rockies. This result is notable because individual pikas defend territories of relatively uniform size across their range, implying that territory size (and population density) does not vary in response to local resource availability. In an analysis of climatic, landscape, and microhabitat variables, the best predictors of pika population density were climatic factors. Density was lowest at sites with highest mean summer temperature and lowest mean annual precipitation. Changes in mean annual precipitation since 1980 in the Southern Rockies have been highly heterogeneous (-32 to +202 mm). Sites at which precipitation has increased were more likely to support higher pika densities than sites becoming drier. Site aspect, elevation, latitude, and talus depth were not predictive of pika population density. Our findings indicate that hotter, drier sites do not support pikas in high densities. Direct thermal stress is implicated as a driver of lower densities at hotter sites. Data from sub-talus temperature loggers implicate a lack of snowpack at drier sites as the precipitation-based driver behind low pika densities. Reduced snow cover reduces the thermal insulation available to pikas during winter, but may also reduce water content in the pika’s forage.