Spatial predictability of resources and the ideal free distribution in convict cichlids, Cichlasoma nigrofasciatum

Abstract

Abstract. An investigation was conducted to determine whether the spatial predictability of food affects the ability of groups of juvenile convict cichlids, Cichlasoma nigrofasciatum, to achieve an ideal free distribution (IFD). Groups of six fish competed for Daphnia magna prey in one of three environments differing in the degree to which food was predictable in space. Prey were distributed among four patches, each defined by its probability of receiving the prey (0·667, 0·167, 0·083 and 0·083). Spatial predictability was manipulated by varying how often these probabilities were re-assigned to the patches over six trials. In the Predictable treatment, patch probabilities remained the same throughout the six trials. In the Intermediate treatment, probabilities were re-assigned to the patches prior to each of the six trials. In the Unpredictable treatment, probabilities were re-assigned to the patches six times within each trial. Thus, groups of fish experienced either one, six or 36 spatial configurations of the patch probabilities in the Predictable, Intermediate and Unpredictable treatments, respectively. The distributions of fish and their patterns of resource use were compared with those predicted by the IFD, ideal despotic, and the IFD with perceptual constraints models of habitat selection. In the Predictable treatment, the distribution of fish was qualitatively consistent with the predictions of the ideal despotic distribution. Dominant fish were the most successful foragers, spending more of their time and obtaining more of their food in the best patch than subordinates. In the Unpredictable treatment, both dominant and subordinate fish under-used the best patch, suggesting they were unable to track its location. However, dominant fish had higher foraging rates than subordinates, probably because they were better at detecting and capturing prey. The distribution of fish in the Intermediate treatment was closest to an IFD, despite differences between fish in competitive ability. These results suggest that the predictive power of the IFD is influenced by the spatial predictability of the environment.