Principal Investigators:
Jeffrey B. Stetz, Ph.D. (Sinopah Wildlife Research Associates)
Michael S. Mitchell, Ph.D. (Montana Cooperative Wildlife Research Unit)
Katherine C. Kendall, M.Sc. (United States Geological Survey, retired)

Background:
The density of animals in a population is among the most important and relied-upon measures of population performance for monitoring and managing wildlife.  For a given area and time, density reflects the outcome of all four primary demographic factors: births + immigration - deaths - emigration (BIDE).  Understanding the drivers of density is, therefore, among the most important, and most challenging, goals of ecology.  This is particularly difficult when ecologically similar species overlap and compete for similar resources.  We used noninvasive genetic sampling (NGS) with spatially-explicit capture-recapture models to explore the seasonal variation in density patterns of grizzly and black bears in and around Glacier National Park, MT.  Unlike previous studies, we considered putative effects of competition between these species as well as seasonal variation in resource quality.

Objectives:
1) to test hypotheses about how biotic and abiotic factors are related to seasonal density patterns of sympatric bear populations.
2) provide scientifically defensible insights into bear ecology to inform management efforts (e.g., population augmentation and reintroduction efforts).

Study Area:
The study included all areas within 10km of Glacier National Park (GNP), MT, south of the U.S.-Canada border.  The remaining area (44%) outside GNP consisted of Tribal, U.S. Forest Service, and relatively small parcels of state and private lands.  Location of hair traps (n=550) and landcover types (bottom left), and bear rubs (n=1,366) and habitat security (bottom right).  Figure from Stetz et al. 2018.

Results/Conclusions/Future Direction:
Our use of multiple NGS methods and the large spatial extent of our study allowed us to successfully model hypothesized relationships between seasonal bear densities and environmental factors for both grizzly and black bears.  Our results suggest grizzly bear density was lower in areas of high black bear density during spring and summer, with intraspecific densities being important during the breeding season.  Black bears had lower densities in areas of high grizzly bear density in spring; however, density of black bears in early and late summer was best explained by primary photosynthetic productivity.  Our results are consistent with the hypothesis that smaller-bodied, more abundant black bears may influence the density patterns of behaviorally dominant grizzly bears through exploitative competition, given their highly overlapping diet and dietary efficiencies.  We also found that seasonal variation in resource availability should be considered when attempting to relate environmental conditions to animal density, particularly in temperate climates.

For more details, see our paper in the journal Ecography (pdf).