Bighorn Sheep Demography Following Wolf Reintroduction: Communication to Wildlife Biology

White, P.; Lemke, Thomas; Tyers, Daniel; Fuller, Julie

Wolves Canis lupus may naturally achieve densities that contribute to significant changes in prey populations and entire ecosystems. We analyzed a time series of counts, index recruitment, and estimates of survival for bighorn sheep Ovis canadensis during 1995-2005 to evaluate the prediction that sheep numbers would decrease in the nothern portion of Yellowstone National Park, Montana and Wyoming, USA, owing to lower survival and recruitment following wolf reintroduction. The number of wolves residing on the northern range increased from 21 to a maximum of 106 in response to an abundant elk Cervus elaphus population and legal protection. Counts of bighorn sheep decreased following the severe winter of 1997, but then increased and remained within 10-20% of the 213 sheep counted when wolves were reintroduced. The relative change in total population size was not significantly different from zero (r= 0.01, SE= 0.06, P= 0.86) and a random model was supported using an information-theoretic framework. Observed recruitment averaged 24 lambs/100 ewes during the severe winter of 1997 and the following winter and then increasing to 34/100 ewes in 2005. Annual estimates of survival for 14 adult females and 4 males 1-3 years old were high (0.94; 95% CI: 089 to 0.97) and indicative of an increasing or constant population. These findings suggest the demography of bighorn sheep in northern Yellowstone was not substantially influenced by wolves during the decade following reintroduction.