A range-wide occupancy estimate and habitat model for the endangered Point Arena mountain beaver (Aplodontia rufa nigra)

The mountain beaver (Aplodontia rufa) is notably the most primitive North American rodent with a restricted distribution in the Pacific Northwest based on its physiological limits to heat stress and water needs. The Point Arena subspecies (A. r. nigra) is federally listed as endangered and is 1 of 2 subspecies that have extremely small and disjunct distributions, putting the taxon at risk from habitat loss and warming global temperatures. We sought to understand their range-wide distribution, to predict the environmental features that best describe their occurrence, and to establish a foundation for monitoring their distribution. We randomly sampled the occurrence of Point Arena mountain beavers (PAMB) from the portion of their geographic range that was accessible (public lands plus private lands where permission was granted). We surveyed 127, 25-ha sample units for their distinctive burrows and estimated the probability of detecting burrows, if they were present, at > 90% per visit. Using this information, we estimated occupancy across the accessible portion of the range to be 26.2%. Range-wide estimates of occupancy, combined with strategically selected locations where abundance and survival can be estimated noninvasively, may comprise a realistic monitoring program for this taxon. We also used the detection and nondetection locations to develop a habitat suitability model by relating these locations to remotely sensed predictors. We evaluated 53 a priori candidate habitat suitability models and the bestfitting model included gentle slopes, low terrain roughness indices, and the high density of rivers and streams. Selecting the probability value that best separated the sample units into suitable and nonsuitable habitat, we estimated a total of 70.5 km2 of suitable habit, or approximately 40.4% of the original geographic range. New detections significantly expanded the known geographic range, moderating concerns about habitat loss, including that predicted by climate change. A substantial number of suitable areas were predicted to occur outside the current range. The identification of high suitability areas allows management agencies to prioritize areas for PAMB conservation planning, evaluate human impacts on habitat, and evaluate how a changing climate may affect distribution.