When bears sit in the woods: Brown bear (Ursus arctos) feeding on salmon alters N cycling in riparian soils

Two important themes in ecology include understanding how interactions among species control ecosystem-level processes and how habitats can be connected through transfers of materials. A spectacular example of both is the large influx of nutrients and organic matter spawning salmon supply to inland aquatic and terrestrial ecosystems and the role of predation by brown bears (Ursus arctos) in transferring these marine-derived nutrients (MDN) across habitat boundaries. However, in spite of the recognition that salmon-bear interactions are likely important in controlling the flux of nutrients from the aquatic to terrestrial environment, few studies have linked bear predation on salmon to processes such as N or C cycling. We combine landscape-level survey data and a duplicated exclosure experiments to test how brown bear foraging on salmon affects fluxes of N₂O from soils, soil respiration, and nutrient concentrations in riparian areas. Our results show bears significantly alter ecosystem-level function. Specifically, soil respiration increases with bears feeding on salmon, as does patterns of N loss from these ecosystems, while nutrient concentrations and N transformation were only marginally affected. Exclosure experiments showed that ecosystems rapidly return to background conditions when bears are removed from the system. The effects of bears are also not uniform in space. Bears concentrate there feeding activities to preferred sites and within those sites N cycling experiences more high-magnitude infrequent events than reference areas. This suggests recipient ecosystems are unlikely to show a uniform response as a result of large MDN inputs as often assumed by stable isotope studies.