Mutualisms as consumer-resource interactions.
Holland, J. Nathaniel*,1, Ness, Josh2, Boyle, Alice, Bronstein, Judith, 1 Rice University, Houston, TX2 University of Arizona, Tucson, AZ
ABSTRACT- Recognition that predator-prey interactions involve consumer-resource interactions has served as a conceptual foundation for mechanistic understanding of the influences of predation on patterns and processes in ecology and evolutionary biology. In contrast, while mutualism is increasingly recognized as important in nature, it is still often perceived as an eccentric case of interspecific interactions with little relevance to major patterns and processes in ecology. This perspective stems in part from the paucity of general principles that can unify mutualistic systems varying greatly in natural history. In this paper, we develop one such principle, which is that mutualism, like predator-prey interactions, is a consumer-resource interaction. Almost all mutualisms involve the transfer of energy and nutrients between individuals of two species; one species functions as a consumer and the other as a resource. Yet, mutualism differs from predator-prey interactions in that the outcome of the consumer-resource interaction results in a net positive effect on per-capita reproduction and/or survival of both populations. We identify three ways in which consumer-resource interactions take place within mutualisms: two-way consumer-resource, one-way consumer-resource, and indirect interactions. We examine many different mutualistic systems, identifying which of these three means of consumer-resource interaction each exemplifies. Using case examples, we discuss why the exchange of resources and services leads to mutualism, rather than to predation or competition. We further discuss how consumer-resource mutualisms are often context-dependent, varying with the supply of an extrinsic resource or with the presence or abundance of a predator or parasite of one of the mutualists. The consumer-resource interaction between individuals of mutualistic populations may often generate the mechanism underlying the effects that interactions have on population-level attributes, such as the growth or size of a population.
Key words: trophic, predation, food web
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