Summer range occupancy modeling of non-native mountain goats in the greater Yellowstone area

Abstract

Non-native species can have adverse impacts on native species. Predicting the potential extent of distributional expansion and abundance of an invading non-native species can inform appropriate conservation and management actions. Non-native mountain goats (Oreamnos americanus) in the greater Yellowstone area (GYA) have substantial potential to occupy similar habitats to native Rocky Mountain bighorn sheep (Ovis canadensis canadensis). To understand the potential for expansion of mountain goats in the GYA, this study evaluated detection-nondetection data derived from ground-based occupancy surveys of viewsheds partitioned into a grid of 100 × 100 m sampling units. Surveys were conducted over three summer seasons (2011–2013) in two study areas with well-established mountain goat populations. Relationships between scale-specific habitat covariates and mountain goat selection were evaluated to model occupancy and detection probabilities based on mountain goat detections in 505 of the 53,098 sampling units surveyed. Habitat selection was most strongly associated with terrain covariates, including mean slope and slope variance, at a spatial scale of 500 × 500 m, and canopy cover, heat load, and normalized difference vegetation index at a spatial scale of 100 × 100 m. These results provide new insight into multi-scale patterns of mountain goat habitat selection, as well as evidence that mean slope and slope variance are more informative terrain covariates than distance to escape terrain, which has been commonly used in published mountain goat habitat models. The model predicted 9,035 km2 of suitable habitat within the GYA, of which 57% is currently un-colonized. Seventy-five percent of all bighorn observations recorded in the GYA fall within predicted suitable mountain goat habitat. We also estimated that the GYA might have the potential to support 5,331–8,854 mountain goats when all predicted habitat is occupied, or approximately 2.5–4.2 times the most recent abundance estimate of 2,354.