Experimental measurements of functional response: What is the relevance for food web theory?

The functional response describes how the rate of consumption changes in response to a change in the density of the resource. Since HOLLING’s (1959) seminal study, ecologists have measured and categorized the functional response for a large number of specific consumer-resource relationships (JESCHKE et al. 2002). Functional response curves are ecologically important because they allow inferences about rates of resource depletion, predator preferences for certain prey types, and competitive hierarchies among consumers. Theoretical ecologists, however, have a more general interest in the functional response. They use functional responses as crucial link functions between consumers and their resources in generic multi-species food web models that predict community dynamics over multiple generations. Such models are particularly appropriate for planktonic communities, characterized by short generation times, and make predictions about the stability, biomass distributions, and the importance of indirect effects in these aquatic food webs. The dynamics of such model food webs, however, are extremely sensitive to the type and mathematical formulation of the functional response (WILLIAMS & MARTINEZ 2004, FUSSMANN & BLASIUS 2005). The cumulative evidence gathered from field observations and experiments can help ecologists choose functional response models representative of natural communities. I investigated whether the functional response is predatoror prey-dependent (or, more generally, consumeror resourcedependent) and how this question is best approached in experiments with aquatic microorganisms. Prey-dependence is the traditional assumption in food web models, under which the per-capita resource uptake (g) by the consumer is only a function of the resource concentration (x): g = f(x). Over the past 2 decades this view has been challenged under the banner of the predator-dependence hypothesis, which claims that per-capita food uptake should depend on both consumer concentration (y) and prey concentration: g = f(x, y). Ratio-dependence is a special form of predator-dependence; the functional response is a function of the ratio of resource and consumer concentrations: g = f(x/y). This problem is essentially unresolved, and the arguments that proponents of either view have put forward to strengthen their cases are not discussed here (see ABRAMS & GINZBURG 2000). However, all debaters agree on 2 points: (1) The question is of great importance for food web theory because models predict dramatically different dynamics depending on whether predatoror prey-dependent functional responses are chosen. (2) Targeted experimentation with real communities can bring us closer to a solution to this debate that has been characterized by an exchange of largely theoretical viewpoints. In recent years, aquatic and terrestrial ecologists have conducted experiments explicitly designed to distinguish between consumerand resource-dependence (SKALSKI & GILLIAM 2001, FUSSMANN et al. 2005, SCHENK et al. 2005, TSCHANZ et al. 2007). Although this trend is generally welcomed, some experiments have also met with criticism (JENSEN et al. 2007). It appears that a current point of debate is how such experiments ought to be designed and conducted so they provide discriminatory power and are not biased to back either hypothesis by design (FUSSMANN et al. 2007, JENSEN et al. 2007).