Density and genetic diversity of grizzly bears at the northern edge of their distribution

Species at the periphery of their range are typically limited in density by lower habitat quality. As a result, the Central-Marginal Hypothesis (CMH) predicts a decline in genetic diversity of populations towards the periphery of a species' range. Grizzly bears (Ursus arctos) once ranged throughout most of North America but have been extirpated from nearly half of their former range, mainly in the south. They are considered a species at risk even in Canada's remote North, where they occupy the northernmost edge of the species' continental distribution in a low-productivity tundra environment. With climate change, one of their main food items in the tundra (caribou), which has always shown yearly fluctuations, is declining, but simultaneously, grizzlies appear to be expanding their range northward, in tundra environment. Yet, a lack of population density estimates across the North is hindering effective conservation action. The CMH has implications for the viability of peripheral populations, and the links between population fluctuations, potential bottlenecks and genetic diversity need to be determined to contribute to species' conservation. Using non-invasive genetic sampling from 2012 to 2014, and autosomal DNA genotyping (via-microsatellites), we estimated bear density using a spatial capture-recapture framework and analysed genetic diversity using observed heterozygosity (H_o), Allelic Richness (AR), and expected heterozygosity (H_e). We compared our findings to other studies that used comparable methodologies on this and a related species (Black bears; Ursus americanus). We found densities of grizzly bears that were low for the species but characteristic for the region (5.9 ± 0.4 bears/1000 km²), but with high H_o (0.81 ± 0.05), AR (7 ± 0.78) and H_e (0.71 ± 0.03), despite a signal of recent bottlenecks. In both species, peripherality was not correlated with H_o but was negatively correlated with density. We suggest that the apparent growth of this expanding population of grizzlies offsets the negative impacts of recent bottlenecks on H_o. Indigenous Knowledge provides historical context (on the order of centuries – e.g., arctic large mammal fluctuations, grizzly bear bottlenecks) for the current bear population dynamics (on the order of decades – e.g., climate change, northern grizzly bear expansion).