Black Bear Population Dynamics in the North Cascades

Lindsay Welfelt

For decades, black bear populations in Washington have been managed under the assumption that availability of natural foods is the prominent driver of population dynamics. These assumptions have led to the prediction that black bear populations in western Washington are much more productive and have twice the density of bears compared to eastern Washington due to higher levels of precipitation. However, formal population abundance and growth estimates have not been conducted in a rigorous manner or over large scales; instead, statistics generated from age and sex of hunter harvested bears are used to infer population abundance and trend. Additionally, throughout much of Washington, and western Washington in particular, human populations are expanding, with the potential to influence black bear population dynamics. Due to the lack of data to assess the status of black bear populations and uncertainty regarding the effects of natural habitat and human factors on population dynamics, we conducted a research project from 2013-2017 in the western and eastern North Cascades using a combination of non-invasive and physical capture techniques. In Chapter 1 we determined that black bears in the eastern North Cascades occur at higher average densities (32.77 bears/100 km2) than on our western study area (23.20 bears/100 km2), whereby bear density was negatively associated with human activity. In Chapter 2, we estimated that observed growth rates in each area are stable (west: λo = 1.093; east: λo = 1.103) despite stochastic growth rates from survival and reproduction data that indicated the western Washington population that should be experiencing a strong population decline, the difference suggesting high immigration into the area. In Chapter 3, we used stable isotope analysis estimate the use of anthropogenic food and the relationship between diet and demographic factors. We found that consumption of anthropogenic food by black bears is common and widespread in western Washington, which produces larger bears with the potential for increased reproduction. Our results suggest that despite the potential for larger black bear populations with increased growth capacity, black bears in western Washington appear to be limited by human impacts and may not be as robust as previously thought.

Black Bear Population Dynamics in the North Cascades

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