Techniques for assessing population size of low-density wildlife populations

Estimating population size and density of a low-density population of black bears in Rocky Mountain National Park, Colorado

Roger A. Baldwin and Louis C. Bender

EUROPEAN JOURNAL OF WILDLIFE RESEARCH, online 19 January 2012

This paper deals with techniques for assessing population size of low-density wildlife populations.

The area of study was Rocky Mountain National Park (RMN) and the subject black bear (Ursus americanus)

Rocky Mountain National Park is home to a low-density black bear (Ursus americanus) population. A previous study (1984–1991) found bear densities among the lowest reported (1.37–1.52 bears/100 km2).

The researchers used three approaches to estimate population size and density

 (1) Minimum number known

 (2) Occupancy modelling

 (3) Catch per unit effort (CPUE)

The researchers used information from capture and remote-triggered cameras, plus visitor information, to derive a minimum known population estimate of 20–24 individuals and a median density estimate of 1.35 bears/100 km2.

Bear occupancy was estimated at 0.46 (SE = 0.11). The occupancy was positively influenced by lodgepole pine stands, non-vegetated areas, and patches density but negatively influenced by mixed conifer stands. They combined the occupancy estimate with mean home-range size and overlap for bears in RMNP to derive a density estimate of 1.44 bears/100 km2.

The researchers related CPUE to density estimates for eight low-density black bear populations to estimate density in RMNP. The estimate (1.03 bears/100 km2) was comparable to the occupancy estimate. The researchers say this approach may be appropriate for future population monitoring.

The researchers sign off with the following words “The use of corroborative techniques for assessing population size of a low-density black bear population was effective and should be considered for similar low-density wildlife populations.”