Ecosystem interdependence as exemplified by salmon nutrient subsidies to terrestrial invertebrate food webs

Morgan D. Hocking
Department of Biology, University of Victoria
PO Box 3020, Victoria, British Columbia, Canada, V8W 3N5
Current Address: Dept of Biological Sciences, SFU, BC, Canada, email: mhocking@sfu.ca

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Cross-habitat nutrient subsidies can initiate trophic cascades that structure communities. Throughout the Pacific Rim, anadromous salmon (Oncorhynchus spp.) are an essential seasonal food source for multiple terrestrial animals and constitute a dominant nutrient subsidy to riparian forest habitats. Bears (Ursus spp.) and other terrestrial predators of spawning salmon actively transfer salmon carcasses into the forest, providing a nutrient source for terrestrial vegetation, soils, and biota, including many insects. Using stable isotope analysis of d15N and d13C, I investigate the direct and indirect pathways in which salmon nutrients enter terrestrial invertebrate food chains on the central coast of British Columbia. I compare invertebrate isotope signatures and litter community structure and diversity across waterfall barriers to salmon migration.

Many salmon carcass breeding flies and beetles demonstrate very high signatures of d15N and d13C, which suggests a diet of almost entirely marine origin. Litter-based invertebrates are highly enriched in d15N but not d13C below the falls compared with above the falls in all trophic groups. Across watersheds, d15N values in invertebrates are positively correlated to spawning density. This suggests that enrichment in ?15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than by direct consumption of salmon tissue. Salmon nutrient subsidies to terrestrial habitats result in shifts in invertebrate community structure and diversity with subsequent implications for higher vertebrate consumers, particularly the passerines. Link to photo gallery.

Publications: (see lab publications with pdfs)