Testing ideal free distribution in the fathead minnow (Pimephales promelas): Size, learning, and the Annie Sullivan effect.

Abstract

The theory of ideal free distribution for continuous-input systems predicts that animals will perform input-matching and distribute themselves throughout their environments in proportion to resource distribution. For this to take place, animals must be ""ideal"" and ""free,"" meaning that they understand the conditions of their environment, and are uninhibited in their mobility. For the fathead minnow (Pimephales promelas), we expect that in experimental conditions fish will distribute according to the predictions of IFD theory. During our experiment, conditions are made ""free"" by standardizing fish trials according to size, and ""ideal"" by controlling for the Annie Sullivan effect. This ensoures that all fish equally understand the conditions of their environment, including food sources. Findings show that mean fish distributions during feeding are both significantly different from non-feeding distributions, and are significantly close to expected distributions according to the proportion of food provided at habitat sites. Data also shows that fish are evenly distributed during non-feeding periods, and that larger fish feed more quickly than smaller fish. This allows us to conclude that fathead minnows input-match and disperse according to the predictions of IFD theory. Although fish populations do not always meet IFD distributions in the wild, deviations from expected proportions can indicate the presence of important environmental, competitive, and predative factors. In this sense, IFD theory is significant to understanding foraging behavior by acting as a null hypothesis.