Examining Indices of Individual-level Resource Specialization

Abstract

The variety of resources that a population exploits is known as the “niche width”. A particular population has a narrow niche if only few kinds of the available resources are exploited by its members. When the individuals of a population exploit many different resources, then the population has a wide niche. From this point of view it seems that the niche is a property of the population as a whole. However, it is well known that many apparently generalist populations are in fact composed of individual specialists, that is, members that use only small subsets of the population’s niche. This approach justifies the definition of indices to measure the individual-level resource specialization. Although this kind of analysis could be applied to any niche variation: oviposition sites, habitat, etc., we focus the discussion in terms of analysis of diet data. So as to measure species niche breadth a comparison between the frequency distribution of the species’ resource use with that of all available resources is carried out. When a measure of individual specialization is considered, then one should compare the population’s total diet with the individual use. In particular, the total niche width of a population should be compared with its two components: within and between-individual variation. In this sense, in the literature several indices of intrapopulation niche variation are proposed. Our goal is to describe, compare and evaluate four of the most relevant indices applied in ecology. In this work we point out how these techniques could be developed in a compositional framework, particularly when these indices are applied to discrete diet data [e.g. frequency of different prey specimen in the diet]