Trophic polymorphism and genetic structure in a population of brown bears across a coastal-inland transition zone

Background/Question/Methods

The hypothesis that discrete intrapopulation differences in dietary niches can facilitate genetic differentiation relates to a fundamental debate on the drivers of genetic diversity. In the Stikine watershed in northwest British Columbia, salmon (Oncorhynchus spp.) penetrate the region along linear river systems, resource availability occurs along a coastal–inland gradient, but also across a finer-scale gradient in elevation. We hypothesized the coastal-inland and elevation gradients would create an interwoven matrix of brown bears (Ursus arctos) that specialize on salmon or terrestrial resources. Our objectives were to: (1) discern whether this population exhibits trophic polymorphisms resultant from marine and terrestrial-based dietary specializations, (2) evaluate whether there is evidence of non-random mating within the population, (3) if there is evidence of non-random mating, determine if distinct genetic clusters correspond with discrete dietary niches, and (4) identify what intrinsic and extrinsic factors best explain intrapopulation dietary niche variation. Genetic and stable isotope data were derived from guard hair from systematically distributed sampling stations. We estimated diet using a Bayesian-based mixing model and evaluated genetic population structure using programs Structure and Genepop. We used hierarchical Bayesian modeling to assess alternative models of the influence of intrinsic and extrinsic factors on bear diets.

Results/Conclusions

Our preliminary results show that discrete intrapopulation dietary niches within this bear population reflect dietary specializations on salmon or terrestrial food resources. Using program Structure, we identified two distinct genetic clusters within this grizzly bear population. Further analysis using program Genepop revealed differences (P < 0.001) in allelic frequencies between the two distinct genetic clusters, providing strong evidence against a single, random mating population. Although hierarchical analyses are ongoing, our preliminary assessment is that both intrinsic (e.g., sex-class) and extrinsic (e.g., elevation) factors are influencing intrapopulation dietary niche variation in this population of brown bears. Our work provides new insights into the ecological drivers of life history diversification and offers an alternative mechanism to the traditional concept that genetic differentiation requires physical separation of populations.