The functional response of Brachionus calyciflorus: resource or consumer dependence?

G. Fussmann, G. Weithoff, T. Yoshida-2003

Abstract:

The uptake of resources from the environment is a vital process of all organisms. Many experimental studies have revealed that the rate at which this process occurs depends critically on the resource concentration, a relationship called “functional response”. However; whether the concentration of the consumer normally affects the functional response has been the subject of a longstanding, predominantly theoretical debate in ecology. Here we present an experimental test between the alternative hypoteses that, among planktonic organisms, food uptake depends either only on the resource concentration or on both the resource and the consumer concentration. We measured the uptake of radioactively labeled, unicellular green algae (Monoraphidium minutum =  resource) by the rotifer Brachionus calyciflorus (= consumer) for varying combinations of resource and consumer concentrations. We found the food uptake by Brachionus to depend on the algal concentration; the relationship was best described by a “Holling type 33 functional response. We detected significant consumer effects on the functional response only at extraordinarily high Brachionus densities (>100/ml), which by far exceed concentrations normally encountered in the field. We conclude that consumer dependent food uptake by planktonic invertebrates is a phenomenon that can occur under extreme laboratory conditions but probably plays a minor role in natural environments.

(Institut für Biochemie und Biologie, Universität Potsdam, Maulbeerallee 2, D-14469 Potsdam, Germany, e-mail: fussmann@rz.uni-potsdam.de)  

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