Helfield, James. 2001. Interactions of salmon, bear and riparian vegetation in Alaska. Ph.D.
Anadromous Pacific salmon (Onchorhynchus spp.) spend most of their lives feeding and growing at sea before returning to fresh water to spawn and die in their natal streams. Returning salmon provide a seasonal food source for numerous mammal and bird species, and nutrients from decaying salmon carcasses are incorporated into freshwater biota at various trophic levels. Consequently, annual spawning migrations provide a mechanism for transporting marine-derived nutrients from the fertile northern Pacific Ocean to freshwater and terrestrial ecosystems. Riparian trees and shrubs near spawning streams derive approximately 22-26% of their foliar nitrogen (N) from spawning salmon, and growth rates of Sitka spruce (Picea sitchensis) and white spruce (P. glauca) are significantly increased as a consequence of this nutrient subsidy. Marine derived nitrogen (MDN) is less important to riparian ecosystems where symbiotic N-fixation by alder (Alnus crispa) is prevalent, although salmon carcasses may be an important source of other marine nutrients affecting productivity in these forests. Since riparian forests affect the quality of instream habitat through shading, sediment and nutrient filtration and production of large woody debris, this fertilization process serves not only to enhance riparian production, but may also act as a positive feedback mechanism by which salmon-borne nutrients improve spawning and rearing habitat for subsequent salmon generations.
Brown bear (Ursus arctos) are an important vector for transferring marine nutrients to riparian forests, through dissemination of partially-eaten salmon carcasses and salmon-enriched wastes. To the extent that this process affects productivity and species composition in riparian forests, interactions of salmon and bear may be characterized as keystone interactions controlling the long-term structure and dynamics of riparian communities. It should be recognized that marine nutrients may also be transferred to riparian systems via other terrestrial piscivores and abiotic processes, and that the relative importance of these different pathways varies spatially and temporally within and among salmon-bearing watersheds. Accordingly, it may be more meaningful to consider the interactions and processes that structure riparian communities rather than their specific component parts. These findings illustrate the complexity of interactions surrounding riparian ecosystems, the importance of linkages across ecosystem boundaries, and the interdependence of salmon populations, terrestrial wildlife and riparian vegetation.