The Ecology of Individuals: Incidence and Implications of Individual Specialization Daniel I. Bolnick, Richard Svanbäck, James A. Fordyce, Louie H. Yang, Jeremy M. Davis, C. Darrin Hulsey, and Matthew L. Forister Abstract: Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual-level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species distributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between-individual variation can sometimes comprise the majority of the population's niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, behavioral, and ecological mechanisms that can generate intrapopulation variation. Finally, individual specialization has potentially important ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency-dependent interactions that can profoundly affect the population's stability, the amount of intraspecific competition, fitness-function shapes, and the population's capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions. Keywords: individual specialization, adaptive variation, niche width, resource partitioning, frequency dependence, niche variation hypothesis, individual ecology. Ecologists have long used niche theory to describe the ecology of a species as a whole, treating conspecific individuals as ecologically equivalent. For example, most models of intraspecific competition, predator-prey dynamics, and food web structure assume that conspecific individuals are identical (but see Lomnicki 1988; DeAngelis and Gross 1992). Similarly, the majority of articles on measuring species' niche width make no mention of the fact that individuals of the same species may use different resources (e.g., Hutchinson 1957; Colwell and Futuya 1971; Pielou 1972; Abrams 1980; Feinsinger et al. 1981; Linton et al. 1981). This omission persisted despite a well-developed literature on niche width variation, originating with Van Valen's (1965) niche variation hypothesis. On the basis of his observations of island and mainland bird populations, Van Valen proposed that niche expansion in the absence of interspecific competition was achieved by increased between-individual variation in resource use. The role of between-individual niche variation in niche evolution was further supported by theoretical work by Roughgarden (1972, 19The Ecology of Individuals: Incidence and Implications of Individual Specialization Daniel I. Bolnick, Richard Svanbäck, James A. Fordyce, Louie H. Yang, Jeremy M. Davis, C. Darrin Hulsey, and Matthew L. Forister Abstract: Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual-level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species distributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between-individual variation can sometimes comprise the majority of the population's niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, behavioral, and ecological mechanisms that can generate intrapopulation variation. Finally, individual specialization has potentially important ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency-dependent interactions that can profoundly affect the population's stability, the amount of intraspecific competition, fitness-function shapes, and the population's capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions. Keywords: individual specialization, adaptive variation, niche width, resource partitioning, frequency dependence, niche variation hypothesis, individual ecology. Ecologists have long used niche theory to describe the ecology of a species as a whole, treating conspecific individuals as ecologically equivalent. For example, most models of intraspecific competition, predator-prey dynamics, and food web structure assume that conspecific individuals are identical (but see Lomnicki 1988; DeAngelis and Gross 1992). Similarly, the majority of articles on measuring species' niche width make no mention of the fact that individuals of the same species may use different resources (e.g., Hutchinson 1957; Colwell and Futuya 1971; Pielou 1972; Abrams 1980; Feinsinger et al. 1981; Linton et al. 1981). This omission persisted despite a well-developed literature on niche width variation, originating with Van Valen's (1965) niche variation hypothesis. On the basis of his observations of island and mainland bird populations, Van Valen proposed that niche expansion in the absence of interspecific competition was achieved by increased between-individual variation in resource use. The role of between-individual niche variation in niche evolution was further supported by theoretical work by Roughgarden (1972, 19