Habitat selection and spatial responses of Bighorn sheep to forest canopy in North-Central Washington

Tiffany Lee Baker

Fire suppression has allowed conifers to encroach into historically open grasslands and shrublands across western North America. Woody encroachment may reduce habitat quantity and quality for bighorn sheep (Ovis canadensis), which rely on open escape terrain. We examined the influence of conifer canopy cover, along with topography and forage resources, on habitat selection by bighorns in north-central Washington, where thinning and prescribed fire treatments have been applied to encroaching forest to restore historic landscape conditions within and adjacent to existing bighorn habitat. To model habitat selection of bighorn sheep using Resource Selection Functions (RSFs), we estimated Utilization Distributions (UDs) from GPS (Global Positioning System) locations of 21 collared bighorns (14 females and 7 males) using the Brownian bridge movement model. After creating annual, lambing, summer, and winter 99% home ranges from UDs, we generated random points within each 99% home range to represent available habitat. We then used logistic regression to compare bighorn GPS locations (i.e., "use") to random points (i.e., "available") after linking them to habitat variables which we created in a geographic information system. As we predicted, bighorn sheep selected areas with lower tree canopy cover, even when controlling for topography and potential foraging habitat, and canopy cover was the only habitat variable that significantly predicted habitat selection by bighorn sheep in population-level models across all demographic groups and seasons. Bighorns also selected for steeper slopes; however, other topographic variables (i.e., distance to escape terrain, aspect, ruggedness, and slope x ruggedness), as well as our forage variables (i.e., distance to forage and categories of Tasseled Cap greenness) varied in their ability to predict habitat selection by bighorn sheep. Our results show that bighorn sheep select areas with lower canopy cover, thus restoring or maintaining open habitat in areas with woody encroachment may influence movements and increase the value of habitat for bighorn sheep. The RSF models we created can be used by state and federal agencies to plan forest restoration at a landscape scale to manage for bighorn sheep and other species that have adapted to similar habitat types.

Habitat selection and spatial responses of Bighorn sheep to forest canopy in North-Central Washington

Files and links (1)

Abstract

Metrics

Details