Carnivore re-colonisation: reality, possibility and a non-equilibrium century for grizzly bears in the Southern Yellowstone Ecosystem

Most large native carnivores have experienced range contractions due to conflicts with humans, although neither rates of spatial collapse nor expansion have been well characterised. In North America, the grizzly bear (Ursus arctos) once ranged from Mexico northward to Alaska, however its range in the continental USA has been reduced by 95–98%. Under the U. S. Endangered Species Act, the Yellowstone grizzly bear population has re-colonised habitats outside Yellowstone National Park. We analysed historical and current records, including data on radio-collared bears, (i) to evaluate changes in grizzly bear distribution in the southern Greater Yellowstone Ecosystem (GYE) over a 100-year period, (ii) to utilise historical rates of re-colonisation to project future expansion trends and (iii) to evaluate the reality of future expansion based on human limitations and land use. Analysis of distribution in 20-year increments reflects range reduction from south to north (1900–1940) and expansion to the south (1940–2000). Expansion was exponential and the area occupied by grizzly bears doubled approximately every 20 years. A complementary analysis of bear occurrence in Grand Teton National Park also suggests an unprecedented period of rapid expansion during the last 20–30 years. The grizzly bear population currently has re-occupied about 50 % of the southern GYE. Based on assumptions of continued protection and ecological stasis, our model suggests total occupancy in 25 years. Alternatively, extrapolatio