The salmon-forest




Hocking, Morgan David

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Cross-habitat subsidies of nutrients and prey can structure community processes in receiving ecosystems. Every autumn throughout the northern Pacific region, anadromous salmon (Oncorhynchus spp.) return to their natal watersheds to spawn and die. a landward migration that imports marine protein to estuarine, freshwater and terrestrial habitats. Spawning salmon regulate the population dynamics of many wildlife species and fertilize aquatic and terrestrial systems with their nutrients and carcasses. When salmon spawn at high densities, vertebrate predators such as bears (Ursus spp.) selectively forage on energy-rich components of the salmon and transfer partially-consumed carcasses to adjacent forests. Herein. I examine the ecosystem-level consequences of the salmon subsidy in forest food webs, with a primary focus on the role of terrestrial invertebrates in the consumption, distribution and cycling of salmon nutrients and energy in terrestrial habitats. Study watersheds occur throughout coastal British Columbia, with most comparisons from two productive systems on the central coast. Both have high-density salmon spawning and 5-10m waterfalls 1-2km from the estuary that block further upstream salmon migration. Using stable isotope analysis of 615N and 613C I find that there are two principal pathways in which salmon nutrients enter forest food webs: 1) direct consumption of salmon tissue and/or salmon consumers (enrichment in 615N and 613C); or 2) marine-nitrogen fertilization of soil and vegetation N pools (enrichment in 615N hut not &3C). There is a diverse community of terrestrial invertebrates (>60 species) directly associated with the decay of salmon carrion, although from a biomass perspective this is dominated by the terrestrial Diptera. I estimate that from 134-360g of Calliphora (Calliphoridae) maggots per meter of spawning length (or 3.5-9 million individuals in whole watershed) were generated from carcass transfer on the Clatse and Neekas, with the majority produced from the large-bodied chum (O. keta) relative to pink (O. gorbuscha). Processes of competition and predation on salmon-carrion produce temporal dietary shifts in higher-level consumers, and species-specific adaptations, including communal breeding in burying beetles (Nicrophorus spp.). Overall, the insect niche generated by the input of salmon carcasses directly increases the diversity, carrying capacity and food-chain length of riparian food webs. The bottom-up effects of salmon nutrient fertilization favours plants that are competitive in nutrient-rich soils and structures a community of macro-detritivores as the primary decomposers. Salmon-nutrient fertilization is detectable in multiple trophic guilds in riparian foodwebs, with estimates of %marine-derived nitrogen (%MDN) ranging as high as 78%. Across multiple watersheds, the pattern of %MDN enrichment is most strongly predicted by chum spawning density (kg/m), including the legacy of spawning density from the past. High a'5N and %MDN values can be used as indicators for intact ecosystem processes including the presence of wildlife transfer vectors, sufficient spawning densities to facilitate selective foraging, and the further distribution of salmon nutrients and energy by terrestrial insects. Ongoing declines in salmon escapement throughout the Pacific Rim have far greater ecological context in riparian food webs that previously recognized. The salmon-forest interaction highlights the evolutionary interdependence between marine and terrestrial ecosystems in the North Pacific, and the need for ecosystem-level conservation that includes salmon, their riparian habitats, and their vertebrate and invertebrate scavengers.



salmon, spawning, British Columbia, Pacific