Trophic Similarity Between Species in a Food Web

Abstract

A measure of the trophic similarity between species in a given trophic level (in general, any trophic partition) with respect to their prey is defined for the case in which quantitative indicators of the intensities of the flows are available. The measure is applied to a forbarthropod food web; the resulting grouping of species by their similarities is biologically meaningful, and provides a good resolution of the structural position of species in the food web. It is concluded that the defined coefficient provides a satisfactory quantitative way for assessing the structural role of species in natural food webs. INTRODUCTION Recent emphasis on studies of whole communities, and the increasing application of radionuclide tracers to study paths of transfer of energy and material among members of a community will produce a more detailed description of food webs than has been possible before. At the same time, sophisticated indices and measures are needed more than overall diversities attained from lists of species abundances for analysis. In a previous paper (Gallopin, in press) I defined a measure of the trophic similarity between members of the same trophic level or, in general, trophic partitions in a food web. This trophic similarity was defined using only the qualitative information about the food web, i.e., the knowledge of whether or not any two species are connected by a link (eating relation). In this paper, I define a measure of trophic similarity by making use of the information about the relative intensities of the flows among species when the flows are unequal, and I apply it to an actual situation. TROPHIC SIMILARITY WITH RESPECT TO PREY Trophic similarity when flows are equal.-A food web can be considered as composed of a set of vertices and a set of directed links joining some or all of the vertices. Each vertex represents one species (or other appropriate trophic grouping) and each link indicates a flow of matter or energy between the two species joined. A trophic partition of a food web is a subset of the species of the food web which do not have predator-prey relations among themselves. Thus, a trophic level constitutes a particular case of trophic partition. Considering a set of species pertaining to the same trophic partition or, in particular, to the same trophic level, the trophic similarity with respect to prey between any two species (Xi,Xj) of the same partition is defined (Gallopin, in press) as: S(Xi5Xj) =m +m-mj O<S(XiXj) <I1