EMPIRICAL-EVIDENCE OF THE ROLE OF HETEROGENEITY IN RATIO-DEPENDENT CONSUMPTION

Classical models describing the number of prey consumed by predators rest on an analogy with the law of mass action and, consequently, the functional response of predators depends only on the density of prey. An alternative model is that the functional response depends on the ratio of prey and predator densities. We hypothesize that the applicability of one or the other model depends on the degree of heterogeneity of predators and prey in space, the prey-dependent model being appropriate in homogeneous situations while the ratio-dependent model is appropriate in heterogeneous situations. We have designed experiments to test this hypothesis, using cladocerans filter-feeding on algae. The design is such that the two types of dependence can be discriminated by observation of equilibrium patterns. Daphnia magna and Simocephalus vetulus, the two cladoceran species tested, differ in their spatial distributions. Daphnia has homogeneous distribution whereas Simocephalus has heterogeneous distribution. Experimental results support the hypothesis: D. magna follows the prey-dependent model and S. vetulus follows the ratio-dependent model. By artificially modifying the environment of the two species, we forced D. magna to a heterogeneous distribution and S. vetulus to a homogeneous distribution. As a consequence, each species changed its dependence, further confirming our hypothesis.