Life History Attributes of Black Bears (Ursus Americanus) in Northern Wisconsin
Schindler, Kathleen D.M.
University of Wisconsin-Stevens Point, College of Natural Resources
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Effective wildlife population management requires a thorough understanding of life history attributes of the species of interest. Specifically, data on reproduction and survival can allow managers to target sensitive population segments and effectively manage for desired population level outcomes. Effective management should be a priority for every wildlife species, especially for long-lived species with low reproductive rates, such as black bears. The Wisconsin Department of Natural Resources (WDNR) manages Wisconsin’s black bear population through a regulated annual harvest. The number of harvest permits issued each year is based on a black bear population projection generated from a population model, BEARPOP2. This model requires input parameters, such as reproductive rates and sex ratios. Currently, the WDNR is in need of up-to-date and accurate parameter estimates. In this study, I used data from a long-term mark-recapture study to estimate reproductive and survival rates of black bears in northern Wisconsin. To estimate reproductive parameters, I analyzed 20 years of annual den survey data (1989-2008). I estimated average age of first reproduction, mean litter size, cub sex ratio, age- and year-specific pregnancy rates, and interbirth interval. Based on knowledge of bear biology, I tested for effects of age, winter severity, average precipitation, and average temperature on reproductive rates. Overall, productivity (mean litter size, pregnancy rates) of female black bears increased with age. Annual reproductive parameters were not related to environmental variables. All estimated reproductive parameters were comparable to those estimated for female black bears in other regions. In particular, our estimated high mean litter size (2.46 cubs/litter), short interbirth interval (2 years), and relatively low average age of first reproduction matched estimates for black bears in regions with high quality habitat for the species. To estimate survival rates of female black bears in northern Wisconsin, I generated encounter histories from 19-years of mark-recapture records (1989-2007). I generated a set of a priori known-fate (Kaplan-Meier) survival models using Program MARK. Based on knowledge of bear biology, I tested for effects of age, winter severity, average precipitation, and hunting effort on survival rates. I generated a set of eleven candidate models, and using an information-theoretic approach, I then generated a final set of three models. These models (an intercept-only model, a global model, and a model incorporating the effects of senescence) all have potential use for wildlife managers. The intercept-only model generated an overall survival estimate of 0.80 for female black bears. Survival of female cubs (0.70) was calculated as a simple proportion using data from annual den processing events. Finally, I combined age-specific survival rates and fecundity to infer possible consequences to population dynamics. I used life table analysis to estimate an annual rate of increase of 3-4% for black bears in northern Wisconsin. I then conducted elasticity analysis and found that λ for the black bear population in Wisconsin is most sensitive to changes in cub and yearling survival. The information generated from this project is intended to aid the WDNR in both evaluating their progress toward management goals and updating their black bear population model.