Using Experiments and Models to Untangle Direct and Indirect Effects: Is There Hope for Understanding Fishery Systems?

In multi-species communities, direct interactions among species create the potential for substantial indirect effects. The indirect effect of one species on another emerges from the densities of the interacting species along the path linking those species and the effect of species' traits on the magnitude of the per capita direct effects exerted by each species along that path. The dramatic changes in the size structure of many species that have been subjected to size-selective harvesting raises the question of whether this exchange of phenotypes—smaller fish for larger fish—will have substantial consequences for the ecosystem through changing the net indirect effects of the harvested species. We examine this question through two avenues. First, we review how body size in fishes influences the per capita direct effects exerted by a species. Second, we highlight experimental research with the Trinidadian guppy, Poecilia reticulata Peters, 1859, that separates the indirect effects of exchanging phenotypes from those of altering densities. Together, these lines of evidence indicate that exchanging phenotypes can alter the indirect effects generated by a species in ways that resonate through ecosystems. We advocate a combination of further experiments in model systems and thoughtful mathematical simulation to understand these effects in larger ecosystems. We suggest that these efforts can improve our ability to anticipate the consequences of exchanging phenotypes through size-selective harvesting, with an eye toward a better approach to managing the complex ecosystems in which fisheries are embedded.