Spatial clumping of food increases its monopolization and defense by convict cichlids, Cichlasoma nigrofasciatum

The hypothesis that resource monopolization and defense increase as the spatial clumping of resources increases was tested using groups of three convict cichlids competing for 120 Daphnia magna prey. Spatial clumping was manipulated by varying the distance (3, 20, or 40 cm) between three tubes through which the prey appeared. As predicted, monopolization of prey (percentage eaten by the dominant fish) and frequency of aggression (chases per minute) by dominant fish increased significantly as the distance between the tubes decreased. However, there was no evidence of individual flexibility in the aggressiveness (percentage of conspecifics chased) of dominant fish across treatments. Differences among dominant fish in aggressiveness were positively correlated with their ability to monopolize prey, but the strength of the correlation decreased as the distance between the tubes increased. Aggression appears to be a more effective mechanism of interference competition when resources are clumped than when resources are dispersed.