Changes in Elk Distribution and Group Sizes after Wolf Restoration

Abstract

Changes in ungulate distribution can alter competitive interactions, plant communities, risks of zoonotic disease transmission, and availability of animals for harvest. We used annual aerial survey data for northern Yellowstone elk in Montana and Wyoming, USA to evaluate factors influencing distribution and group sizes during 1987–2009 in four sectors of elk winter range corresponding to river watersheds with different minimum elevations and snowpacks. Our best logistic regression model suggested the proportion of elk occupying the upper elevation sector decreased following wolf restoration and increased snowpack. The proportion of elk occupying the lower elevation sector increased following wolf restoration and as snowpack increased at higher elevations. Linear regression suggested group sizes increased in the lower elevation sector after wolves were restored. Concurrent demographic and movement studies suggest these changes resulted primarily from the attrition of elk from high snow areas in Yellowstone National Park due to predation, and increased survival and recruitment of elk in lower snow areas outside the Park in Montana following a substantial reduction in hunter harvest. Fitness trade-offs between foraging conditions and the risks of predation (or harvest) as constrained by snow vary considerably among elk populations in the Yellowstone ecosystem.