Development of occupancy surveys for mountain ungulates

Abstract

Bighorn sheep (Ovis canadensis canadensis) and mountain goats (Oreamnos americanus) overlap in broad food and habitat requirements. In places where mountain goats are non-native there are concerns over potential competition between the two species. The southern Gallatin Mountain range, within and adjacent to the northwest boundary of Yellowstone National Park has both native bighorn sheep and non-native mountain goats. Existing observations of both species for this area vary in spatial precision and there are no records of where observers looked for animals but did not detect them. To gain a better understanding of the relationship between bighorn sheep and mountain goats and their habitat, it is necessary to understand resource selection and the extent of overlap in resource use at fine spatial and temporal scales. I used logistic regression to relate existing presence-only bighorn sheep and mountain goat data for this area to landscape features I expected would be important to both species. Using resulting coefficient estimates, I constructed a relative habitat suitability map and used it to define four survey regions within the study area. The crew of four spent 113 observer days afield and hiked 210 miles recording occupancy data for both mountain ungulates within these four survey regions. Observers surveyed 6,603 100 x 100 meter grid cells, with 15 groups of bighorn sheep and 34 groups of mountain goats observed during surveys. Because there were more mountain goat observations available, I used only mountain goat data to conduct formal occupancy analyses. Mountain goat occupancy was positively associated with ruggedness at the 100 meter scale and there was an important interaction between distance to escape terrain and tree cover at the 500 meter scale. As the distance to escape terrain increased mountain goats were less likely to occupy treed areas. The ruggedness index used in my presence-only modeling effort was based on the rate of change in slope. By using a ruggedness index which included changes in slope and aspect I improved model performance. This research demonstrates the feasibility of conducting occupancy surveys in mountainous terrain and provides interesting biological insights regarding mountain goats and their habitat.