A synthesis of bear population dynamics in Michigan
Michigan State University, (2019)Abstract
The Michigan Department of Natural Resources (DNR) manages the black bear (Ursus
americanus) population of Michigan primarily through manipulating the harvest by adjusting
the quota of hunting licenses. Estimates of the bear population play a key role in determining
the appropriate license quota to achieve the DNR’s bear population objectives. Past population
estimates were unreliable or infrequent, so we developed a statistical catch-at-age analysis
(SCAA) of the Upper Peninsula (UP) and northern Lower Peninsula (NLP) bear populations of
Michigan from 1992-2015 to inform the bear management program and to allow the DNR to
more closely monitor changes in the bear population over time.
To support the models, we estimated the aging and sexing error rates of harvested
Michigan black bears. We estimated aging error by comparing the true age of known-age bears
to the age determined through cementum annuli aging techniques. Aging error rates were low
(2%) at the lowest age categories and increased as bear age increased. We also estimated the
sexing error rates by comparing the genetic sex of harvested bears to the sex hunters reported
at the time of harvest. Sexing error was higher for females than males and differed by region.
The error rates we estimated, however, were unlikely to have major effects on analyses based
on sex and age data unless those data were heavily skewed toward one sex and the sexes
exhibited large differences in age distribution.We also described the reproductive patterns of Michigan black bears and looked for
reproductive trends over age, time and geographic region. Results showed differences
between the UP and NLP. Adult females in the UP began breeding at an older age than those in
the NLP and had smaller litters. By 4 years old, however, the proportion of females that bred
each year was approximately 50% in each region, with only occasional major fluctuations, likely
due to catastrophic mast failures. Overall, the Michigan black bear populations are as or more
productive than other black bear populations in eastern North America.
We developed the SCAA models for both the UP and NLP regions of Michigan to monitor
the black bear population and to support the DNR’s need to assess effects of past management
and support decisions on future management actions. Within each region, the final selected
models showed a stable to slightly increasing bear population in the UP and an increasing
population in the NLP from 1992 to 2015. Model evaluation raised no major concerns of model
behavior. Sensitivity analysis showed the models to be insensitive to simplifying assumptions
and available data, except for the availability of mark-recapture population estimates. We
recommend the DNR continue to estimate the bear populations through independent mark-
recapture surveys or other means every 5 years. The SCAA models provide evidence of a stable
bear population in the UP and an increasing population in the NLP. The bear populations’ high
annual mortality rates are offset by high fecundity.