Niche separation amongst sympatric ursids relative to salmon use

Jennifer Kay Fortin

Historically, there were sympatric brown and black bears in much of North America prior to European development. However, the two species did not evolve long-term together and dietary overlap may be extensive. The re-establishment of brown bears in Idaho concurrent with salmon recovery would benefit brown bears but may harm salmon if the total consumption by the two ursids is additive. We hypothesized that there would be minimal dietary overlap between sympatric brown bears (Ursus arctos) and American black bears (U. americanus) relative to salmon utilization when alternative foods (e.g., berries or fruits) are available. To maximize sensitivity of the test, we examined the diets of brown bears and black bears known to have visited salmon streams on the Kenai Peninsula, Alaska. Species, sex, and individual identification were determined by DNA analysis of hair and feces collected along salmon streams. Assimilated diets were estimated from stable isotope analyses of hair. Assimilated diets of brown bears were 66.0 ± 16.7% salmon, 13.9 ± 7.5% terrestrial animal matter, and 20.1 ± 17.2% plant matter. Assimilated diets of black bears were 8.0 ± 5.4% salmon, 8.4 ± 9.7% terrestrial animal matter, and 83.6 ± 7.7% plant matter. Male and female brown bears did not differ in either the proportion of dietary salmon (62.1 ± 16.9% and 68.4 ± 16.5%; F = 0.35, P = 0.5607), terrestrial animal matter (16.7 ± 9.9% and 11.8 ± 4.4%, F = 1.38, P = 0.2504), or plant matter (21.1 ± 17.4 % and 19.8 ± 17.4 %, F = 0.01, P = 0.9367). Both sexes of brown bears visited salmon streams and consumed significant amounts of salmon, but only male black bears visited streams and then consumed minimal amounts of salmon. Thus, brown bears were largely carnivorous (79.9 ± 17.2% dietary animal matter). Black bears were largely herbivorous/frugivorous (83.6 ± 7.7% dietary plant matter) and, in the case of female black bears, did not approach salmon streams. This lack of dietary overlap relative to salmon is understandable in light of the dominance of brown bears over black bears, differing energy requirements, and therefore their differing ability to efficiently exploit different food resources.

Niche separation amongst sympatric ursids relative to salmon use

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