Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 4410
Title: Mediation functions in Ecopath with Ecosim: handle with care
Author: C. J. Harvey
Publication Year: 2014
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Keywords: Ecopath with Ecosim,habitat,ecosystem-based management,mediation,ecological baselines,marine ecology,Puget Sound,eelgrass,

Ecosystem models are important tools for addressing complex issues such as the role of habitat in marine resource management. The Ecopath with Ecosim (EwE) software can represent several ecological processes via mediation functions, where the  abundance of one group influences trophic interactions between two other groups. I ran a series of temporal simulations in EwE, in which eelgrass (Zostera marina) was refuge habitat for juvenile Pacific salmon (Oncorhynchus spp.), and the abundance of eelgrass mediated (reduced) the vulnerability of juvenile salmon to their predators. I compared the effects of eelgrass on salmon biomass across three shapes of mediation curve (linear, hyperbolic, sigmoid) and six different initial states along each curve. Salmon responded strongest to sigmoid mediation and least to hyperbolic mediation. Salmon responses were sensitive to initial conditions, particularly along sigmoid curves. As the lower limit of the mediation curve (Mmin) approached 0, model results became nonintuitive, particularly for sigmoid curves. Because these functions are difficult to quantify or scale from empirical or experimental studies, modelers must carefully account for uncertainty when using mediation relationships in EwE. Hyperbolic mediation curves may be the most conservative when empirical or theoretical knowledge is unavailable.

Theme: Habitats to Support Sustainable Fisheries and Recovered Populations
Foci: Characterize habitat effects on ecosystem processes, ecological interactions and the health of organisms.