title: Role of brown bears (Ursus arctos) in the flow of marine nitrogen into a terrestrial ecosystem.
abstract: We quantified the amount, spatial distribution, and importance of salmon (Oncorhynchus spp.)-derived nitrogen (N) by brown bears (Ursus arctos) on the Kenai Peninsula, Alaska. We tested and confirmed the hypothesis that the stable isotope signature (δ15N) of N in foliage of white spruce (Picea glauca) was inversely proportional to the distance from salmon-spawning streams (r=-0.99 and P<0.05 in two separate watersheds). Locations of radio-collared brown bears, relative to their distance from a stream, were highly correlated with δ15N depletion of foliage across the same gradient (r=-0.98 and -0.96 and P<0.05 in the same two separate watersheds). Mean rates of redistribution of salmon-derived N by adult female brown bears were 37.2±2.9 kg/year per bear (range 23.1-56.3), of which 96% (35.7±2.7 kg/year per bear) was excreted in urine, 3% (1.1±0.1 kg/year per bear) was excreted in feces, and <1% (0.3± 0.1 kg/year per bear) was retained in the body. On an area basis, salmon-N redistribution rates were as high as 5.1±0.7 mg/m2per year per bear within 500 m of the stream but dropped off greatly with increasing distance. We estimated that 15.5-17.8% of the total N in spruce foliage within 500 m of the stream was derived from salmon. Of that, bears had distributed 83-84%. Thus, brown bears can be an important vector of salmon-derived N into riparian ecosystems, but their effects are highly variable spatially and a function of bear density.
authors.affiliation: Alaska Department of Fish and Game, Division of Wildlife Conservation, 333 Raspberry Road, Anchorage, AK 99518, USA e-mail: grant_hilderbrand@fishgame.state.ak.us Tel.: +1-907-2672182, Fax: +1-907-2672433, , , , , , US.
authors.forename: G V
authors.initials: GV
authors.lastname: Hilderbrand
authors.affiliation: United States Forest Service, Pacific Northwest Research Station, 2770 Sherwood Lane, Suite 2A, Juneau, AK 99801, USA, , , , , , US.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Hanley
authors.affiliation: Departments of Natural Resource Sciences and Zoology, Washington State University, Pullman, WA 99164, USA, , , , , , US.
authors.forename: Charles T
authors.initials: CT
authors.lastname: Robbins
authors.affiliation: Alaska Department of Fish and Game, Division of Wildlife Conservation, 333 Raspberry Road, Anchorage, AK 99518, USA e-mail: grant_hilderbrand@fishgame.state.ak.us Tel.: +1-907-2672182, Fax: +1-907-2672433, , , , , , US.
authors.forename: C C
authors.initials: CC
authors.lastname: Schwartz
keywords: Key words Bear; Nitrogen; Nutrient flow; Salmon; Spruce