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. aureotincta) 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 accessibility 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, mechanisms 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.

Grazers and Diggers: Exploitation Competition and Coexistence among Foragers with Different Feeding Strategies on a Single Resource

“…Given the uncertainty in the fitted parameters, the model predictions were not inconsistent with observed coexistence in the field (Schmitt 1996). In this article we have reviewed, generalized, and extended the work of Possingham (1987) and Wilson et al (1999) by investigating how the model of resource renewal affects coexistence.…”

“…Coexistence was dependent on the competitive ability of both species, which was defined as the mean net energy gained per calorie extracted from a flower, divided by the cost of using each flower. Wilson et al (1999) fitted a similar mathematical model, which described the dynamics of two benthic grazers competing for algae, to data collected by Schmitt (1996). Although the model required a number of parameters, the data were sufficient to give estimates to all parameters but one.…”

Altered complementary feeding strategies of the consumers Hydrobia ulvae and Idotea emarginata via passive selectivity

“…However, in the mixed-grazer treatments I. emarginata in general showed a higher uptake of the labelled algal material as H. ulvae. Many studies in community ecology have tried to analyse the eVects of complementary feeding and coexistence on the resource use of competitors (Rossi et al 1983;Wilson et al 1999). In our study, interspeciWc competition appeared to induce a shift in assimilation eYciency leading to a stronger uptake of the algal material by I. emarginata.…”

“…Many studies in community ecology have investigated the eVect of coexistence on the resource use of competitors and demonstrated that complementary feeding facilitates coexistence (Ricklefs and Schluter 1993;Wilson et al 1999). In our study interspeciWc competition appeared to induce a passive shift in feeding preference and in resource use by I. emarginata in the presence of H. ulvae.…”

“…Wilson et al (1999) demonstrated that complementary feeding facilitates coexistence and that this is an aspect which commonly regulates consumer-resource systems. In this context the grazers' ability to choose actively between food components (further referred to as "active selectivity") is an important feature that can be signiWcant in inXuencing grazer-microalgae interactions and is a major force in structuring benthic microalgae community composition (Jaschinski et al 2008).…”