Assessing alternative drivers of occupancy, abundance, and elevational range retraction at the range core of a climate-sensitive mammal

Abstract

Ecological niche theory and niche conservativism suggest that rising temperatures globally will continue pressuring species to track cooler environments. Examining changes in occupancy and abundance together across bioclimatic gradients can inform forecasts of expected range shifts. Although occupancy and abundance reflect similar aspects of species-environment relationships, they are governed by different underlying processes. Abundance is thought to be more reflective of shorter-term conditions affecting vital rates, whereas site occupancy often reflects habitat suitability. By directly comparing results of both response types, as well as elevational range retraction, we offer a robust method for assessing complex species-climate relationships. In this study, we test how populations of the American pika (Ochotona princeps), a small montane lagomorph, respond to varying climatic conditions. To do so, we tested and compared the drivers of site occupancy, abundance, and upslope retraction, across 760 talus patches, nested within 64 watersheds across the Northern Rocky Mountains, USA. Using mixed-effects modeling, paired with an information-theoretic approach, we tested model suites that reflected hypothesized species-climate relationships to identify the top models of each of our response classes. Approximately one third (33.9%) of patches were found extirpated. The most important environmental predictors differed among occupancy, abundance, and amount of upslope retraction. For site occupancy, the top model included metrics of summer acute heat stress, actual evapotranspiration, and habitat availability. For abundance, acute heat stress and the preceding winter's mean temperature (i.e. chronic cold stress) was the top-ranked model, suggesting rapid responses of populations to recent climatic conditions. Furthermore, we found that a model including both chronic heat and chronic cold stress best predicted the total amount of vertical retraction across watersheds, whereas acute heat stress and summer precipitation best explained the residuals. Our results emphasize the complexity associated with evaluating species responses to environmental change and that results from occupancy analyses should be used with caution when extrapolating to predicting abundances across varied landscapes. Our method for assessing the drivers of elevational retraction across a suite of watersheds has widespread applications for evaluating species response to changing climatic conditions elsewhere.