Consumer responses to resource density affect species coexistence in consumer-resource systems

Background/Question/Methods

Mechanistic models of competition through shared resources demonstrate that greater resource partitioning leads to greater opportunities for competitive coexistence. This theory has been used extensively to predict the outcome of competitive systems, such as plant competition and competition in chemostats. The conclusions generated in this framework, however, are based on the assumption that resource intake rate by consumers (the consumer functional response) depends linearly on resource density. While this may sometimes be the case in nature, many studies have revealed nonlinear functional responses, which reflect consumer satiation at high resource densities. Furthermore, consumer satiation may depend on total resource density or on separate resources independently. If consumer functional responses satiate at high resource densities, not only can consumers differ in their consumption at low resource density, they can differ at high resource density. This may provide more opportunities to partition resources. We investigate the role of saturating functional responses for changing the classic conclusions of resource competition in a system with two consumers and two resources. We focus on quantifying the contribution of differences in consumer satiation to resource partitioning relative to the contribution of differences in attack rate alone, which define resource partitioning in linear models.

Results/Conclusions

We analyze model equilibria to derive equations expressing conditions for coexistence in terms of the forms of the consumers’ functional responses. Differences from coexistence conditions of linear models depend on whether consumers satiate on total available resources or have independent responses to each resource. The former may occur when the mode of consumption is the same across resources (e.g. filter feeders, carnivores, herbivores) and the latter may occur when the mode of consumption differs across resources (i.e. plants obtaining water and light). When consumers satiate on total available resources, differences in resource equilibria caused by both consumer attack rates and handling times alter coexistence conditions relative to classic models. When consumers satiate on separate resources independently, resource consumption rates at equilibrium define coexistence relative to simple resource attack rates in classic models. Both the attack rate and maximum consumption rate realized at high resource densities are important and provide opportunities for coexistence. We show that the relative contribution of functional response to resource partitioning is quadratic in consumer mortality rates, whereas classic partitioning is linear. Thus, for given levels of classic resource partitioning between consumers, differences in responses to resource density can further partition resources that promote coexistence.