Measuring Individual-Level Resource Specialization

Abstract

Many apparently generalized species are in fact composed of individual specialists that use a small subset of the population's resource distribution. Niche variation is usually established by testing the null hypothesis that individuals draw from a common resource distribution. This approach encourages a publication bias in which negative results are rarely reported, and obscures variation in the degree of individual specialization, limiting our ability to carry out comparative studies of the causes or consequences of niche variation. To facilitate studies of the degree of individual specialization, this paper outlines four quantitative indices of intrapopulation variation in resource use. Traditionally, such variation has been measured by partitioning the population's total niche width into within-and between-individual, sex, or phenotype components. We suggest two alternative measures that quantify the mean resource overlap between an individual and its population, and we discuss the advantages and disadvantages of all four measures. The utility of all indices depends on the quality of the empirical data. If resources are measured in a coarse-grained manner, individuals may falsely appear generalized. Alternatively, specialization may be overestimated by cross-sectional sampling schemes where diet variation can reflect a patchy environment. Isotope ratios, parasites, or diet-morphology correlations can complement cross-sectional data to establish temporal consistency of individual specialization.

Covarying variances: more morphologically variable populations also exhibit more diet variation

“…The proportional similarity (PS ij ) between two individuals reflects the degree to which their stomach contents contain the same prey in the same proportions (Bolnick et al 2002). We used the standardized measure of diet variation, E = 1 − mean(PS ij ) (Araújo et al 2008).…”

Upscaling the niche variation hypothesis from the intra- to the inter-specific level

“…in which p ij is the frequency of resource j in its individual's i diet, and q j is the frequency of resource j in the population (Bolnick et al 2002). The PS i values of all individuals in the population can be calculated and summarized as a population-wide measure of inter-individual variation:…”

Diet specialization in a generalist population: the case of breeding great tits Parus major in the Mediterranean area

“…The former is a measure of the overlap between the diets of an individual and the population; the latter is the mean PSi between all pairwise combinations of individuals, or-in our case-pairs (Bolnick et al 2002). Both indices were obtained through IndSpec, version 4.0 (Bolnick et al 2002). These indices provide information on the use of trophic resources by individuals properly framed in the context of a concrete population, and range from 0 to 1, where 1 indicates that the individual's diet is equal to the mean diet of the population and lower values indicate that the individual specializes with respect to the population mean.…”

“…The former is a measure of the overlap between the diets of an individual and the population; the latter is the mean PSi between all pairwise combinations of individuals, or-in our case-pairs (Bolnick et al 2002). Both indices were obtained through IndSpec, version 4.0 (Bolnick et al 2002).…”

“…We assessed between-pair trophic specialization in relation to population niche breadth by means of the proportional similarity index, PSi = 1 − 0.5∑ j │p ij − q ij │ = ∑ j min(p ij , q ij ), and the mean pairwise overlap (see Bolnick et al 2002 for more details). The former is a measure of the overlap between the diets of an individual and the population; the latter is the mean PSi between all pairwise combinations of individuals, or-in our case-pairs (Bolnick et al 2002).…”