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Initial effects of reintroduced wolves Canis lupus on bighorn sheep Ovis canadensis dynamics in Yellowstone National Park

White, Patrick J., Lemke, Thomas O., Tyers, Daniel B., Fuller, Julie A.
Wildlife biology 2008 v.14 no.1 pp. 138-146
Canis lupus, Cervus elaphus, Ovis canadensis, adults, ecosystems, elks, ewes, keratoconjunctivitis, males, mortality, national parks, population growth, prediction, time series analysis, winter, wolves, Montana, Wyoming
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 of recruitment, and estimates of survival for bighorn sheep Ovis canadensis during 1995-2005 to evaluate the prediction that sheep numbers would decrease in the northern portion of Yellowstone National Park, Montana and Wyoming, USA, owing to lower survival and recruitment following wolf reintroduction. The number of wolves residing in 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 by 7%% annually during 1998-2005 (95%% CI: 2-11%%). Recruitment followed a similar temporal pattern, decreasing to 7-11 lambs//100 ewes during the severe winter of 1997 and the following winter, but then increasing to 21-34 lambs//100 ewes during 1998-2005. Annual estimates of survival for 14 adult females and four males 1-3 years old were high (0.94; 95%% CI: 0.89-0.97) and indicative of an increasing or constant population. Thus, the presence of wolves did not prevent the bighorn sheep population from increasing slowly during the decade following reintroduction. However, sheep counts remain low compared to the 487 sheep observed before an outbreak of keratoconjunctivitis caused 60%% mortality during 1982, suggesting that other factors limited the recovery of this relatively isolated, high-elevation, native sheep population. Increases in abundance and recruitment of bighorn sheep during 1998-2005 were concurrent with a 50%% decrease in the numbers of northern Yellowstone elk after wolf reintroduction. Thus, the potential effects of decreased competition for resources between elk and bighorn sheep on lamb recruitment and sheep population growth merit further investigation.