Variation in genetic diversity across the range of North American brown bears

Understanding the factors that influence the rate at which natural populations lose genetic diversity is a central aspect of conservation genetics because of the importance of genetic diversity in maintaining evolutionary potential and individual fitness. Concerns about loss of genetic diversity are particularly relevant to large carnivores, such as brown bears (Ursus arctos), that are distributed at low densities and are highly susceptible to human-caused population fragmentation. We used eight highly variable nuclear microsatellite markers to study current levels of genetic variation across the North American range of brown bears. The highest levels of within-population genetic diversity (H-e = 0.76) were found in northern populations in the core of the North American distribution. Diversity was significantly lower in populations at the southern fringe of the distribution, in the Northwest territories, and in southwest Alaska. Diversity was lower still in the Yellowstone Ecosystem population (H-3 = 0.55), an isolated remnant of the larger distribution that recently extended south from the Canadian border into Mexico. The insular population on the Kodiak Archipelago had very low genetic diversity (H-3 = 0.26). The Yellowstone and Kodiak data suggest that the effective population size for brown bears is much smaller than previously suspected. These results indicate that the levels of diversity in most undisturbed populations can be maintained only through connections to populations on the scale of the current North American distribution. At eh same time, the Kodiak data demonstrate that populations well under the size recommended for long-term conservation can persist and thrive for thousands of years, although the probability of such persistence remains unknown.