Disease, Population viability, and recovery of endangered Sierra Nevada bighorn sheep

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Journal of Wildlife Management







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Sierra Nevada bighorn sheep (Ovis canadensis sierrae) experienced a severe population declineafter European settlement from which they have never recovered; this subspecies was listed as endangeredunder the United States Endangered Species Act (ESA) in 1999. Recovery of a listed species is accomplishedvia federally mandated recovery plan s with specific population goals. Our main objective was to evaluate thepotential impact of diseas e on the probability of meeting specific population size and persistence goals, asoutlined in the Sierra Nevada bighorn sheep recovery plan. We also sought to heuristically evaluate theefficacy of management strategies aimed at reducing disease risk to or impact on modeled bighornpopulations. To do this, we constructed a stochastic population projection model incorporating diseasedynamics for 3 populations (Langley, Mono, Wheeler) based on data collected from 1980 to 2007. Wemodeled the dynamics of female bighorns in 4 age classes (lamb, yearling, adult, senescent) under 2 diseasescenarios: 5% lower survival across the latter 3 age classes and persi stent 6 5% lower lamb survival (i.e., mild) or65% reduced survival across all age classes followed by persistent 65% lower lamb survival (i.e., severe). Wesimulated management strategies designed to mitigate disease risk: reducing the probability of a diseaseoutbreak (to represent a strategy like domestic sheep grazing management) and reducing mortality rate (torepresent a strategy that improved survival in the fa ce of introduced disease). Results from our pro jectionmodel indicated that management strategies need to be population specific. The population with the highestgrowth rate (^l; Langley;^l ¼ 1.13) wa s more robust to the effects of disease. By contrast, the population withthe lowest growth rate (Mono;^l ¼ 1.00) would require management intervention beyond disease manage-ment alone, and the population with a moderate growth rate (Wheeler;^l ¼ 1.07) would require manage-ment sufficient to prevent severe disease outbreaks. Because severe outbreaks increased adult mortality,disease can directly reduce the probability of meeting recovery plan goals. Although mild disease outbreakshad minimal direct effects on the popula tions, they reduced recruitment and the number of individualsavailable for translocation to other populations, which can indirectly reduce the probability of meetingoverall, range-wide minimum population size goals. Based on simulation results, we recom mend reducing theprobability of outbreak by conti nuing efforts to manage high-risk (i.e., spatially close) allotments throughrestricted grazing regimes and stray management to ensure recovery for Wheeler and Mono. Managingbighorn and domestic sheep for geographic separation until Sierra Nevada bighorn sheep achieve recoveryobjectives would enhance the likelihood of population recovery