Habitat comparisons of historically stable and less stable Bighorn sheep populations

Abstract

Limited research has examined how habitat differences between stable and less stable bighorn populations may influence their success. Understanding these habitat differences may help explain how habitat contributes to bighorn sheep population stability. The objective of the study was to identify potential limiting habitat factors for the Tendoy Mountains bighorn sheep (Ovis canadensis) population in western Montana. Habitat variables that were evaluated are unlikely to be influenced by fine-scale weather or disturbance patterns. Land cover, slope, aspect, elevation, landscape ruggedness, solar radiation index (SRI), and distance to escape terrain were measured using GIS. Shrub canopy cover, graminoid and forb frequency, and horizontal visibility were measured in the field. Logistic regression was used to identify habitat differences between the stable and less stable sheep populations in summer and winter. Odds ratios from the logistic regressions were used to identify potential limiting habitat variables for the less stable population. Results from this study indicate that landscape ruggedness (P <0.01) and aspect (P <0.01) in summer ranges, and landscape ruggedness (P =0.01), aspect (P <0.01), and SRI (P <0.01) in winter ranges were the habitat characteristics most likely influencing population stability. Landscape ruggedness and SRI are relatively new habitat metrics that require more research to determine threshold values for bighorn sheep habitat. Results from this study provide initial insights into potential threshold values for landscape ruggedness and SRI for Rocky Mountain bighorns.