Why is it important to study individual diet specialization? Ecologically, studying individual-level diet variation represents a more complete description of a biological system. Information on individual resource use is necessary if we want to make the transition from phenomenological models of population dynamics to mechanistic models in which the dynamics are predicted from the properties of its components (e.g., individual foraging decisions). Evolutionarily, since variation is the raw material for evolution by natural selection, intraspe-cific niche variation thus represents an important target for natural selection. Furthermore, IS, species specialization, and similarly generalization are all affected by intra-and interspecific competition, predation, and prey dynamics, so the topic of food specialization is at the interface of both ecological and evolutionary studies.
Intra- and interspecific competition, types of social interactions, and the risk of predation or parasitism are factors generally used in describing a population's ecology, all of which can depend on an individual's resource use. Risk factors connected to diet are common because foraging individuals can be vulnerable to predators and parasites associated with a particular diet. Social and competitive interactions between individuals are strongest among individuals that use the same subset of resources especially if different resources are associated to different microhabitats. As a result, populations with large between-individual variation can be divided into subgroups that may compete within themselves but with low between-group competition. Consequently, censuses of total population size will not serve as a good proxy for the level of intraspecific competition. Instead, exploitative competition will be both density and frequency dependent.
Resource-specific ecological interactions mean that individuals within the same population can be subject to different selective pressures. Habitat and resource use affect an individual's energy income, mating options, and exposure to risk. Niche variation among individuals is therefore likely to be a major source of variation in fitness and may play a central role in evolutionary diversification. In many models of evolutionary diversification, divergence is driven by disruptive selection because phenotypically average individuals (or generalists) experience disproportionately more intense competition than rare phenotypes (or specialists) with access to exclusive resources. When resource-specific fitness and IS lead to frequency-dependent interactions, IS may lead to disruptive selection that will facilitate adaptive speciation. Whether such effects lead to trait evolution and specia-tion will depend on the heritability of the traits and the temporal consistency of the interindividual variation.
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