The Effect of Group Size on Relative Competitive Ability

Abstract

Recent influential models of the distribution of foraging animals have recognised the importance of individual variation in competitive ability, but have assumed that this property is intrinsic to the individual. However, Tregenza et al. suggested that in continuous input situations (where resources are supplied at a steady rate and utilised immediately) the relative competitive ability of an individual may be affected by competitor density. We therefore examined the effect of group size (which was equivalent to competitor density) on relative competitive abilities within a shoal of European minnows (Phoxinus phoxinus) exploiting a single, continuous-input food patch. In partial agreement with Tregenza et al. and in contrast to Sutherland and Parker's 'Phenotype scales intercept' model of the ideal free distribution (IFD) with competitive asymmetries, we found two processes which act in opposite directions. These effects can cancel each other out to give the misleading idea that there is no effect of group size upon relative competitive ability. These effects were dependent upon the difference in body size between the two fish in a pair, but this difference itself had no direct effect on relative competitive ability. The reasons for the differences between our results and those of Tregenza et al. are discussed.