Factors limiting a bighorn sheep population in Montana following a dieoff

Conservation and management of bighorn sheep populations is complicated by the species' susceptibility to a multitude of pathogens and the long-term i nfluence of disease upon population dynamics. Small, post-dieoff sheep herd s are often unable to fully recover to pre-dieoff densities, and researcher s have identified several factors limiting such populations including disea se, predation, competition, climate, nutritional deficiencies, and loss of genetic variability. We studied a small bighorn sheep herd in west central Montana from 1995 to 1997 to identify the mechanisms responsible for poor p opulation performance subsequent to a dieoff in 1984. In contrast to the mi gratory pre-dieoff population, the current sedentary population was restric ted to a small area of low elevation range throughout the year. Whereas we observed limited annual adult mortality due to disease and predation, ewe p roductivity and lamb recruitment rates were low in all years. Lamb producti on and survival were highly correlated with summer climatic conditions, wit h the highest rate of disease-mediated lamb mortality occurring during a su mmer drought and the lowest rate of ewe productivity occurring subsequent t o this drought. Our data suggest that disease continues to play a significa nt role this bighorn sheep population, and nutritional quality of summer fo rage influenced not only sheep immunocompetence and susceptibility to disea se but also herd productivity. Winter is generally considered the critical season for ungulate populations in the northern Rocky Mountains, yet summer climatic conditions can have important implications for performance of sed entary, low elevation sheep populations. We suggest that an understanding o f population-specific limiting factors is essential to successful managemen t of post-dieoff bighorn sheep herds. Data obtained through rigorous field studies facilitate the development and implementation of efficient, biologi cally sound strategies to improve performance of these populations.