A DIRECT, EXPERIMENTAL TEST OF RESOURCE VS. CONSUMER DEPENDENCE: COMMENT

Abstract

Consumer-resource system dynamics hinge upon the rate at which consumers capture, consume, and convert resources into biomass. In classical functional and numerical response theory, this rate is assumed to depend on resource density but not on consumer density (reviewed in Jeschke et al. [2002]). In assuming that both densities determine this rate, consumer-dependent func tional responses (e.g., Hassell and Varley 1969, Bed dington 1975, DeAngelis et al. 1975, Arditi and Ginzburg 1989) challenge the resource-dependent tradi tion. This challenge has produced a long-standing debate (reviewed in Abrams and Ginzburg 2000). The traditional approach of fitting functional response models to time series data has yielded equivocal results (e.g., Jost and Arditi 2001) and may not be capable of resolving the debate (Lundberg and Fryxell 1995). In a recent issue of Ecology, Fussmann et al. (2005) tried to enrich the debate with empirical data. They described functional response experiments in the rotifer-algae system Brachionus calyciflorus-Monoraphidium minutum that detected consumer dependence only at unnaturally high Brachionus densities and concluded that consumer dependence plays only a minor rule for planktonic rotifers in natural environments. Here, we outline why this conclusion is an over-interpretation of their results. Their experimental approach only considers direct physical interference between Brachionus and excludes all other and more important forms of consumer dependent effects (e.g., chemically mediated interfer