COMPLEMENTARY FORAGING BEHAVIORS ALLOW COEXISTENCE OF TWO CONSUMERS

Abstract

We developed a mathematical model based on the microalgal-gastropod system studied by Schmitt, in which two coexisting consumers (Tegula eisini and T. au- reotincta) feed on a common resource. The two consumers differ in their foraging behavior and their ability to remove microalgae from rock surfaces. T. eisini is a digger, moving slowly and grazing the algae down to almost bare substrate, whereas T. aureotincta is a grazer, moving more quickly and leaving behind a larger fraction of the algal layer. These complementary foraging strategies result in a size-structured algal resource, with each size class differentially accessible to each of the consumers. Our model recognized three ac- cessibility states for an algal patch: a refuge (recently grazed by the digger and currently inaccessible to either consumer), a low level (exploitable only by the digger), and a high level (exploitable by both consumers). We assumed that all interactions between consumers and resources were linear and examined the relatively short time-scale dynamics of feeding, algal renewal, and individual consumer growth at fixed densities of consumers. Thus, our model complemented related models that have focused on population dynamics rather than foraging behavior. The model revealed that coexistence of two consumers feeding on a single algal resource can be mediated by differences in the consumers' foraging modes and the resource structure that these behaviors create. We then estimated model parameters using data from Schmitt's experimental studies of Tegula. The fits to the experimental data were all very good, and the resulting parameter values placed the system very close to a narrow coexistence region, demonstrating that foraging complementarity in this system facilitates coexistence. The foraging trade-offs observed here are likely to be common in many consumer-resource systems. Indeed, mech- anisms similar to those we discuss have been suggested in many other systems in which similar consumers also coexist. This model not only demonstrates that such an argument is theoretically plausible, but also provides the first application of the model, showing that the observed conditions for the Tegula system fall very close to the appropriate parameter space. Such quantitative tests are critical if we are to rigorously test the models developed to explain patterns of coexistence.