The Role of Brown Bears (Ursus Arctos) in Nutrient Transport into Forests near a Salmon Stream in Coastal British Columbia, Canada

• Main Author: Morris, Arthur
• Format: Text
• Language: unknown
• Published: Utah State University 2002
• Subjects: Canada; British Columbia; Salmon Stream; Ursus arctos
• Online Access: https://dx.doi.org/10.26076/ca65-eff8; https://digitalcommons.usu.edu/etd/8275/

Nutrients from spawned salmon contribute to the fertility of rivers and riparian areas. Adjacent forests, even far from rivers, could receive substantial amounts of nitrogen and other nutrients from salmon. Since brown bears feed heavily on spawning salmon, bears probably influence the movement of nutrients from salmon into surrounding forests. Because salmon-derived nitrogen is high in 15N, increased isotopic enrichment is expected in forest soils and vegetation if this transport is occurring. Based on relative 15N enrichment of spawning areas, a quantitative estimate of marine-derived nitrogen (MDN) can be obtained using a linear two-source mixing model. To evaluate the reliability of MDN estimates based on such a two-source mixing model, we evaluated some assumptions used in mixing model calculations. We determined isotopic changes as nitrogen moved from salmon tissue into brown bear feces and soil where the bears were feeding on salmon near Knight Inlet, British Columbia. We also used a simulation model to evaluate fractionation's effect on MDN estimates. To evaluate dissemination of MDN by grizzly bears, we determined 15N of vegetation and soil from transects across bear trails and beds along the Koeye River, British Columbia. We expected to find the highest isotopic enrichment closest to bear trails and beds. We found little difference (about 2%o) between δ15N of salmon tissue and δ15N of salmon-derived N in soil. However, δ15N in other areas was high, even exceeding δ15N of salmon tissue. Using a simulation model we found that fractionation of N losses from the soil caused gross (more than 70% in some cases) overestimates of MDN. It appeared that 15N fractionation could be large enough under natural conditions to prevent accurate quantification of MDN with a two-source mixing model. Delta 15 N at bear trails and beds exceeded δ15N from several meters away on both sides (by an average of 1.5%0), and δ15N of a reference transect, supporting the assertion that bears move substantial amounts of MDN upslope. We calculated 5% to 56% MDN in soil within 10 m of bear trails and beds using δ15N data, compared to 14% MDN based on the 15N difference between reference and spawning sites.