Functional responses of optimal consumers and the potential for regulation of resource populations

Abstract

A central issue in studies of consumer-resource interactions is whether consumers regulate resource dynamics. One condition for regulation is that consumption rate of a resource increases positively with increasing resource density, that is, that the consumer's functional response must be positively density dependent. Many mammalian consumers exhibit density-independent or inversely density-dependent functional responses, suggesting that regulation will not occur. However, most studies measure functional responses for a single consumer and resource species in specific feeding trials. Many real-world consumers use more than one resource and resource choices depend on the distribution and nutritional quality of resources as well as abundances. Foragers also actively choose resources in ways that match predictions of optimal foraging theory, that is, they exhibit adaptive behaviour. This paper explores the variety of functional responses of adaptive consumers that arises from optimal choice of resources in a simple, singleconsumer- two-resource system to determine the potential for consumer regulation of resource populations. Optimal consumer behaviour can generate four types of functional responses: (1) density independent, (2) increasing, inverse density dependent, (3) increasing, positively density dependent, and (4) decreasing. A positive density-dependent functional response arises in 3 of 22 possible cases. Moreover, consumers may not exhibit the same functional response to all resources included in the diet, that is, they exhibit mixed responses to resource densities. This suggests that studies that examine the potential for consumer regulation of resources must go beyond the traditional focus (interactions between a consumer and the most dominant or abundant prey) and consider the variety of resource species selected by a consumer in a specified time period.