SPATIAL PREDICTABILITY OF FOOD INFLUENCES ITS MONOPOLIZATION AND DEFENSE BY JUVENILE CONVICT CICHLIDS

The hypothesis that spatially predictable resources are more easily monopolized and defended than spatially unpredictable resources was tested by allowing groups of six juvenile convict cichlids, Cichlasoma nigrofasciatum, to compete for Daphnia magna prey. One prey appeared every 15 s in one of four patches defined by their probability (0·667, 0·167, 0·083, 0·083) of receiving the prey. Spatial predictability was manipulated by varying how often these probabilities were randomly assigned to the patches over a 3-day experimental period: Once (Predictable), six times (Intermediate), or 36 times (Unpredictable). With increasing resource predictability, dominant fish became significantly more aggressive, more sedentary, and monopolized a greater share of the food. The number of prey eaten by individual fish was positively correlated with body mass and aggressiveness (the proport on of encountered conspecifics that were chased) in the Predictable treatment, with body mass in the Intermediate treatment, and with body mass and mobility (the frequency of patch switching) in the Unpredictable treatment. These results suggest that interference competition, via resource defence, is effective in spatially predictable environments, whereas exploitative competition, via scrambling, is effective in spatially unpredictable environments. © 1994 The Association for the Study of Animal Behaviour.