PREDATOR-PREY DYNAMICS IN AN ECOSYSTEM CONTEXT

Evaluating the role of fishes at the food web and ecosystem scales pro fits from an iterative process. At the community and population scales , prey selection by predators alters habitat selection behaviours of p rey species as well as their abundance, size distributions, life histo ries and the consequent effects on their own prey. At the whole system scale, predation by fishes alters community structure and nutrient cy cling. Thus, both direct and indirect predation effects are expressed in population structure, community composition and production processe s at all trophic levels. These are the central tenets of the trophic c ascade argument. Examples are abundant and diverse. We know that preda tors are size selective, that resource partitioning occurs, that funct ional responses link the density dependence of predator and prey popul ations, and that predator avoidance behaviours are common. A more sign ificant challenge exists when attempting to use this knowledge. This p resentation attempts to link theory and empiricism in forecasts of wha t will happen next in response to a management action or a planned exp eriment. Examples are drawn from whole system experiments conducted in small lakes and from large-scale manipulations of predator population s in North America's Laurentian Great Lakes. Rapid and discontinuous o r non-linear responses are common. Extrapolating the lessons of mechan istic process studies proves insufficient because the context is dynam ic. Inferences built from the whole ecosystem scale yield equally misl eading results because the scale is too general. Resolving these probl ems will require a clever mix of selective applications of predator-pr ey theory and astute empiricism.