The term 'specialization' is used in a variety of ways that require some initial clarification. A species or individual may be considered 'specialized' (an adjective) if it uses a narrower diet than some reference group. For example, a specialized species might use fewer prey than are available in the environment, or than another species uses. An individual may be specialized if it uses fewer prey than its conspecifics (individually or as a group). Alternatively, the adjective has been used to indicate a species that uses a unique set of prey (not necessarily less diverse) compared to related species. Specialization can also be a verb, the evolutionary process of becoming more specialized. In this case, we are comparing the focal group to its evolutionary progenitor. Finally, a specialization can also be a noun, referring to a phenotypic trait that confers a derived ability to perform some ecological task, for example, hypertrophied pharyngeal jaws which are a specialization for molluscivory. In the context of individual food specialization, these three views correspond to asking whether or not an individual has a narrower niche than its population, describing the process of becoming more specialized, and, finally, the morphological or behavioral traits underlying food specialization among individuals.
When adopting the relativistic definition of specialization, it is possible to measure the degree of specialization (how much narrower a niche is than its reference). Consider a food resource distribution (niche distribution) that can be described by a single continuous variable such as prey size. The total niche width (TNW) of the population is simply the variance in size of all the prey captured. The TNW can be partitioned into two components: the within-individual component (WIC) and the between-individual component (BIC) so that TNW = WIC + BIC. WIC is the average variance of prey sizes found within individuals' diets, whereas BIC is the variation between individuals. Individual specialization (IS) occurs when individuals' niche widths (WICs) are much smaller than their population as a whole (TNW), or equivalently when BIC is a large proportion of TNW such that BIC/ TNW is large. An increase in BIC could occur in a number of different ways. For example, BIC can be large if individuals of different age or size classes use different diets, or if males and females use different diets. Furthermore, an increase in BIC could also be due to phenotypic variation among individuals (morphological or behavioral). Thus, individual food specialization is part of a continuum from where the WIC equals the TNW (WIC/TNW = 1), that is, no IS, to where WIC is only a small part of TNW, that is, great IS (Figure 2).
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