Life History Attributes of Black Bears (Ursus Americanus) in Northern Wisconsin
File(s)
Date
2008-05Author
Schindler, Kathleen D.M.
Publisher
University of Wisconsin-Stevens Point, College of Natural Resources
Metadata
Show full item recordAbstract
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.