Role of brown bears (Ursus arctos) in the flow of marine nitrogen into a terrestrial ecosystem

We quantified the amount, spatial distribution, and importance of salmon (O ncorhynchus spp.)-derived nitrogen (N) by brown bears (Ursus arctos) on the Kenai Peninsula, Alaska. We tested and confirmed the hypothesis that the s table isotope signature (delta(15)N) Of N in foliage of white spruce (Picea glauca) was inversely proportional to the distance from salmon-spawning st reams (r=-0.99 and P<0.05 in two separate watersheds). Locations of radio-c ollared brown bears, relative to their distance from a stream, were highly correlated with delta(15)N depletion of foliage across the same gradient (r =-0.98 and -0.96 and P<0.05 in the same two separate watersheds). Mean rate s 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/ye ar per bear) was excreted in urine, 3% (1.1+/-0.1 kg/year per bear) was exc reted in feces, and <1% (0.3+/-0.1 kg/year per bear) was retained in the bo dy. On an area basis, salmon-N redistribution rates were as high as 5.1+/-0 .7 mg/m(2) per year per bear within 500 m of the stream but dropped off gre atly 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 th at, bears had distributed 83-84%. Thus, brown bears can be an important vec tor of salmon-derived N into riparian ecosystems, but their effects are hig hly variable spatially and a function of bear density.