On the implications of predator avoidance

Abstract

The question to what extent predators regulate the populations of their food species has a long history of debate, and today top-down control of plankton dynamics still is a major issue in limnology. Over the last decade a fascinating extra dimension of this topic has been revealed. Fish, zooplankton and algae can sense their potential predators through chemical cues and adjust their ‘life style’ in a flexible way to minimize the risk of being eaten. Although much progress has been made in elucidating the mechanisms involved, the implications of such predation avoidance for the potential of predators to reduce the abundance of their food populations are still poorly understood. Model analyses suggest that predation avoidance leads to more stable food webs in which the effects of nutrient enrichment are more evenly distributed over the trophic levels. In this article it is argued that the cost which predation avoidance usually has in terms of food conditions implies a tricky caveat in interpreting observations on predation rates and nutritional status in natural populations. Prey that sense the presence of predators and aggregates at safe sites where food conditions are poor, may appear starved and actual predation losses may be low. Such observations suggest that food limitation is the dominant force limiting their population development, but although starvation may indeed be the proximate cause of poor growth and reproduction, predation is clearly the ultimate factor involved in this situation. Thus when predation avoidance occurs, standard demographic analyses will tend to underestimate the importance of predators in regulating the dynamics of their prey in the field.