Investigating occupancy and density of American pikas (Ochotona princeps) across precipitation gradients in the Intermountain West, USA.

Abstract

Anthropogenic climate change is altering species distributions and abundances of diverse taxa across the globe. Understanding the mechanisms underlying these shifts in distributions and abundances may provide a more complete understanding of species responses to climate change. Using an indicator species for climate change (the American pika, Ochotona princeps), we surveyed 200 talus patches in 2015-2016 across four study areas that spanned a precipitation gradient in the Intermountain West, USA. Our results yielded unexpected insights into the climatic associations underlying pika distributions and abundances. Contrary to our expectation, warmer average winter temperatures were negatively associated with pika occupancy and relative density, and further investigation revealed that winter temperatures rarely reached low enough thresholds to be considered thermally stressful. While the winter of 2015-2016 was the warmest winter on record for the contiguous United States, these results may foreshadow how montane species may respond to future climate conditions. Climate water deficit was another top-predictor of pika occupancy and relative density across spatial and temporal scales. Increasing values of climate water deficit were negatively associated with pika occupancy and relative density. This measure of soil drought has gained little attention for predicting animal distributions and abundances. Unexpectedly, we found increasing (rather than decreasing, as found by other research) cover of graminoid species to correlate positively with relative pika density, indicating that increasing cover of graminoids were associated with increasing relative density. Pikas have been suggested to prefer plants higher in secondary compounds (forbs, trees, and shrubs); however, we did not find this. Pikas have also been suggested to gain most of their water via metabolic water. Given the diuretic effects of many of these secondary compounds, and the negative relationship observed with soil drought, these results could suggest an interaction between climate water deficit and forage preference; in which pikas prefer plants lower in secondary compounds at drier sites. Our results emphasize the need to clarify mechanisms underlying species responses to recent climate change, to better inform management decisions and conservation planning.