Testing alternative determinants of occupancy and density of a charismatic high-elevation mammal, across space and time
Population dynamics may differ across a species’ geographic range because of phenomena that result in contrasting predictions: 1) temporal variability of populations is larger near the range center than at range margins due to overcompensatory density dependence destabilizing populations; or 2) variability of populations is larger at range margins, where populations experience greater environmental fluctuations. Contemporary climate change represents a pervasive yet spatially variable disturbance for species; its effects have been predicted and often shown to be most pronounced among stenotopic (e.g., habitat-specialist), poorly dispersing, and physiologically challenged species. Using American pikas as our model organism to investigate local-extinction dynamics and patterns of abundance, we performed re-surveys at locations of historical pika records in four regions – the hydrographic Great Basin, Columbia River Gorge, northeastern California, and protected areas in southern Utah. At each site, we walked 50-m transects in talus patches using paired-observer distance sampling, to quantify detectability and density. We documented evidence of occupancy based on pika sightings, vocalizations, fresh haypiles [current occupancy]; old pellets, old haypiles, or both [past occupancy]; or none of these [unoccupied]. We considered aspects of climate and water balance, talus-habitat extent, and vegetation as potential predictors of pika occupancy and abundance, using information-theoretic approaches.
We observed lowest percent persistence of pika populations (since historic records) in the Great Basin (~55%), and highest pika persistence and density in the Columbia Gorge. Pikas have been extirpated from all historic locations within both Utah national parks we studied, but occur adjacent to Cedar Breaks National Monument. Whereas Gorge pikas exhibit adaptive capacity in their diet and habitat-use behavior, Basin pikas do not exhibit such capacity. Although 1-km-resolution DAYMET-estimated aspects of temperature, precipitation, and water balance predicted persistence in the Basin and in Utah, they did not predict them in northeastern California. Conversely, although a comprehensive re-survey of pika records in the Sierra Nevada ‘mainlands’ found local extent of talus to be a top predictor of persistence, it didn’t predict persistence in either the Basin or Utah. Pikas are at high density and nearly ubiquitous in the Cascade portion of the Gorge, but fade away with increasing distance from the Range and its microclimates. In the Basin, we are exploring relative humidity and plant chemistry as potential mediators of distributional changes. Collectively, our results illustrate the nuance and complexity with which species are responding to climate change. Understanding underlying mechanisms is critical to inform conservation and management efforts.