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Pacific salmon effects on stream ecosystems: a quantitative synthesis


Pacific salmon (Oncorhynchus spp.) disturb sediments and fertilize streams with marine-derived nutrients during their annual spawning runs, leading researchers to classify these fish as ecosystem engineers and providers of resource subsidies. While these processes strongly influence the structure and function of salmon streams, the magnitude of salmon influence varies widely across studies. Here, we use meta-analysis to evaluate potential sources of variability among studies in stream ecosystem responses to salmon. Results obtained from 37 publications that collectively included 79 streams revealed positive, but highly inconsistent, overall effects of salmon on dissolved nutrients, sediment biofilm, macroinvertebrates, resident fish, and isotopic enrichment. Variation in these response variables was commonly influenced by salmon biomass, stream discharge, sediment size, and whether studies used artificial carcass treatments or observed a natural spawning run. Dissolved nutrients were positively related to salmon biomass per unit discharge, and the slope of the relationship for natural runs was five to ten times higher than for carcass additions. Mean effects on ammonium and phosphorus were also greater for natural runs than carcass additions, an effect attributable to excretion by live salmon. In contrast, we observed larger positive effects on benthic macroinvertebrates for carcass additions than for natural runs, likely because disturbance by live salmon was absent. Furthermore, benthic macroinvertebrates and biofilm associated with small sediments (<32 mm) displayed a negative response to salmon while those associated with large sediments (>32 mm) showed a positive response. This comprehensive analysis is the first to quantitatively identify environmental and methodological variables that influence the observed effects of salmon. Identifying sources of variation in salmon–stream interactions is a critical step toward understanding why engineering and subsidy effects vary so dramatically over space and time, and toward developing management strategies that will preserve the ecological integrity of salmon streams.

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We acknowledge three anonymous reviewers for providing suggestions that greatly improved the manuscript. We also thank members of the Stream Ecology Laboratory at the University of Notre Dame, as well as Peter Levi, David Lodge, and Jason McLachlan. Mark Drew generously performed GIS analyses and generated the map of study locations, and Matthew Goslin assisted with salmon distribution meta-data. Michael Brueseke, Susanne Hebbler, Margaret O’Sullivan, and Alexander Reisinger assisted with data extraction and management. Jared Aldstadt, Jessica Gurevitch, Laurie Marczak, and Michael Rosenberg kindly provided important insights regarding methodological questions. We also thank Shannon Claeson, Larry Hanne, JoAnna Lessard, and Margaret Wilzbach for providing unpublished data. Funding was provided by the USDA-CSREES National Research Initiative Competitive Grants Program (Managed Ecosystems Program 2006-35101-16566) and the Great Lakes Fishery Trust (Project 2007.857). D.J.J. was also supported by fellowships from the Center for Aquatic Conservation at the University of Notre Dame and the Peter J. Schmitt Foundation.

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Correspondence to David J. Janetski.

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Communicated by Marc Mangel.

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Janetski, D.J., Chaloner, D.T., Tiegs, S.D. et al. Pacific salmon effects on stream ecosystems: a quantitative synthesis. Oecologia 159, 583–595 (2009).

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  • Ecosystem engineering
  • Marine-derived nutrients
  • Meta-analysis
  • Oncorhynchus
  • Resource subsidy