Minimum viable population and reserve sizes for naturally regulated grizzly bears in British Columbia

Abstract

Estimating minimum viable population and reserve size is a fundamental cornerstone of conservation biology—but these estimates require representative demographic parameters. For example, “Benchmark” Grizzly Bear (Ursus arctos) Management Units in British Columbia (BC) are defined as unhunted and naturally regulated populations that can serve as population sources to surrounding hunted areas and provide information on natural population processes. Such benchmarks should have a very small probability of becoming threatened (N<100 animals). The British Columbia Ministry of Environment (BCMOE) recently implemented a plan to establish one benchmark population in each of six different biogeoclimatic zones in the province. How many bears and how large an area for each benchmark remains in question. In this paper I estimate the minimum number of bears and the areal size required to accommodate environmental and demographic stochastic effects that can result in quasi-extinction thresholds of N<100 animals. Demographic data were compiled from six different grizzly bear studies in North America and mean values and standard deviations were entered into a Leslie matrix to conduct a Population Viability Analysis (PVA). I varied initial population sizes from 100, 150, 200, and 250 animals and carrying capacities from 12K, 34K, and K. Results indicated that 200–250 bears were required for a sufficiently small probability (P<0.05) of decline to a quasi-extinction threshold of 27 adult female bears (N<100 animals total) within 20 years and a mean time to extinction >20 years. Reserve sizes varied from 8556 km2 to 17,843 km2 depending on population density in each benchmark. These minimum viable populations and reserve sizes would protect approximately 12% of the estimated provincial grizzly bear population and would cover approximately 5% of the landmass of BC.