PREDATOR-PREY DYNAMICS IN AN ECOSYSTEM CONTEXT

Evaluating the role of fishes at the food web and ecosystem scales profits from an iterative process. At the community and population scales, prey selection by predators alters habitat selection behaviours of prey species as well as their abundance, size distributions, life histories and the consequent effects on their own prey. At the whole system scale, predation by fishes alters community structure and nutrient cycling. Thus, both direct and indirect predation effects are expressed in population structure, community composition and production processes at all trophic levels. These are the central tenets of the trophic cascade argument. Examples are abundant and diverse. We know that predators are size selective, that resource partitioning occurs, that functional responses link the density dependence of predator and prey populations, and that predator avoidance behaviours are common. A more significant challenge exists when attempting to use this knowledge. This presentation attempts to link theory and empiricism in forecasts of what will happen next in response to a management action or a planned experiment. Examples are drawn from whole system experiments conducted in small lakes and from large-scale manipulations of predator populations in North America's Laurentian Great Lakes. Rapid and discontinuous or non-linear responses are common. Extrapolating the lessons of mechanistic process studies proves insufficient because the context is dynamic. Inferences built from the whole ecosystem scale yield equally misleading results because the scale is too general. Resolving these problems will require a clever mix of selective applications of predator-prey theory and astute empiricism.