Individual foraging specialization: niche width versus niche overlap

A recent review of individual specialization suggests it is widespread and has important implications for conservation, ecology and evolution (Bolnick et al. 2003). Dozens of studies have identified ‘‘individual foraging specialization’’ in a variety of species, yet upon closer examination these studies differ markedly in how they define and quantify specialization. Given such diverse approaches, characteristics of niches that provide information about individual differences and quantify individual foraging specialization need to be clearly identified. Individual foraging specialization has been defined in many ways, although all definitions involve the use of different dietary items or foraging tactics by individuals within a single population. A direct corollary is that at least some individuals must have narrower niches than the population niche overall. Following Bolnick and colleagues’ (2002) interpretation of Roughgarden’s (1972) niche width concept, population-level niche width (‘‘total niche width’’, TNW) can be subdivided into a ‘‘within-individual component’’ (WIC) and a ‘‘between-individual component’’ (BIC). Thus, WIC/ TNW provides a measure of how much variation in TNW can be explained by variation within or between individuals. As WIC/TNW increases, most of the variation in TNW results from variation within individuals, but as it decreases, variation in TNW results from variation between individuals. Thus, unless all individuals have identical niches, at least some niches will be narrower than that of the population and individuals will differ from one another. Individual differences clearly differentiate individual foraging specialization from ecological specialization in species or populations (Levins 1968, Fox and Morrow 1981, Futuyma and Moreno 1988) and predict individual differences in niches. But further distinctions about how specialization is defined and therefore who is most specialized vary substantially between studies. While it may initially seem that a strict definition could be applied (e.g. a specialist uses 50% of resources used by the population), biologically meaningful patterns of niche use are likely to depend on what resource and species are being considered. For example, Futuyma and Moreno (1988) recommended against a strict, rigid definition of resource specialization, viewing specialization and generalization as extremes of a continuum based on Hutchinson’s (1957) n-dimensional hypervolume of niche space. Therefore, similar to traditional approaches to generalization and specialization, individual specialization should describe individual niche use only in terms relative to the population’s niche use. So what, exactly, is individual foraging specialization? Generally, two different conceptual approaches have emerged, although both compare an individual’s niche to the population’s niche. In the first, which I will refer to as the niche width specialization concept, individual foraging specialists are individuals with narrower foraging niches than the population’s niche (Roughgarden 1972, West 1986, Werner and Sherry 1987, Holbrook and Schmitt 1992, reviewed by Bolnick et al. 2003). This should include definitions that describe ‘‘the consistent use’’ of resources or tactics in which alternative resources or tactics are used by the population (Werner et al. 1981, Partridge and Green 1985, Schindler et al. 1997), as consistency should result in a narrower niche by reducing richness or evenness (below). In the second approach, which I will refer to as the niche overlap concept, specialists have niches that exhibit little overlap with the population Oikos 116: 1431 1437, 2007 doi: 10.1111/j.2007.0030-1299.15833.x, Copyright # Oikos 2007, ISSN 0030-1299