Selective feeding by protozoa: model and experimental behaviors and their consequences for population stability

Abstract

Selective feeding by zooplankton can have profound consequences for the stability of grazer and prey populations, as demonstrated by the behavior of plankton dynamics models. We present an analytical approach—calculation of prey ratio trajectories—that reveals unambiguously whether selective feeding behavior is stabilizing (i.e. provides a refuge for preferred prey species at low prey concentrations) or destabilizing (i.e. results in elimination of prey populations). Prey ratio trajectories were calculated for three modeled selective feeding behaviors. Constant selection was consistently destabilizing, while selection behaviors that changed inresponse to either prey ratio or prey abundance could be stabilizing. Batch culture experiments with four protozoan grazer species (three ciliates, one heterotrophic dinoftagellate) demonstrated that protozoa fed selectively in every case, weakly preferring the larger of the two algal species offered. Stabilizing selection was observed only in the experiment with Favella sp ., however, meaning that only this species altered its selection behavior in response to changing experimental conditions. Because protozoa are the major grazers of phytoplankton in many planktonic systems, our findings indicate that the use of selective feeding behaviors to stabilize plankton dynamics models needs careful evaluation. The modeling and graphical techniques presentedhere are a tool for linking further exploration of selective feeding behaviors with the development of planktondynamics models.