Consumer Responses to Resource Density Affect Resource Partitioning as a Coexistence Mechanism of Competition in Consumer-Resource Systems

Competition for shared consumable resources is an important form of competition and has been investigated using mechanistic models of consumer and resource growth. The shape of the relationship between consumption and resource density has traditionally been linear but there are many biological phenomena that can lead to nonlinear relationships between consumption and resource density. This nonlinearity has been shown to have important effects for competition between two consumers for a single limiting resource, yet our understanding of multi species competition for multiple resources has been based mostly on results from models with a linear function describing consumption. The mechanism of coexistence in these models is resource partitioning, a central concept for both competition and coexistence. I evaluate how nonlinear relationships between consumption and resource density affect expectations of coexistence by directly comparing analytical results from MacArthur style consumer-resource models with linear functional responses and nonlinear functional responses. The general concepts defining resource partitioning are the same for all models that I analyze; the ratio of per-capita resource growth must be between the ratios of proportional resource use at equilibrium for each consumer. Nonlinearity affects the way in which consumers draw down resources to equilibrium compared to the linear model. These effects are additive when consumptive rate saturates with total density and both additive and multiplicative when consumptive rate saturates independently on each resource. Interesting and unintuitive cases arise when consumers are limited at high resource densities including negative R*s that lead to coexistence. When consumptive rate saturates with total resource density, inhibition of consumption of one resource by consumption of others can buffer competitive advantages of species and may produce more regions of coexistence than when consumptive rate saturates on each resource independently. Future studies investigating resource competition will need to measure a multidimensional functional response that manipulates both total and individual resource density in a response surface design to uncover possible conditions for coexistence that before this study had previously not been considered.