Trophic Structure, Effective Trophic Position, and Connectivity in Food Webs

Abstract

A new measure of distance from the food source to any member of a food web is introduced. This measure is referred to as effective tropic position. Effective trophic position is defined as a function of energy ingested per unit time by a population and the production of the autotrophs necessary to maintain that population. Trophic position thus defined is a generalization of the trophic-level concept. An algorithm is developed to construct food webs with ecological constraints to examine statistical distributions of the standing crops and trophic positions in food webs. Ecological constraints are imposed on food chain length and on the range of transfer efficiencies in food webs. A Monte Carlo technique was used to generate model food webs with an arbitrary number of internal connections. The number of connections ranges from the minimum possible (corresponding to simple food chains) to the maximum possible (extremely connected food webs). The relationship between biomass and trophic position distributions of food webs are examined. The moments of these distributions are determined in part by the connectivity of food webs, and a possible application is the estimation of connectivity and trophic position distributions by measuring the biomass distributions. Regressions were found relating food-web connectivity to the moments of the biomass distributions and the distributions along the effective trophic position scale. The Monte Carlo results indicate that food web standing crops should be log Pearson Type I distributed. A similar result is found in field measures of food webs.