PREDATOR-MEDIATED COMPETITION IN PREY REFUGES - THE IMPORTANCE OF HABITAT DEPENDENT PREY RESOURCES

Predators may affect their prey both directly by eating them and indirectly by affecting their behaviors. The behavioral, non-numerical, effects of predator presence on the relative performances of juvenile perch (Perca fluviatilis) and juvenile roach (Rutilus rutilus) were studied in a field enclosure experiment. Each enclosure consisted of a predator-safe part with complex physical structure (artificial vegetation) and an open water with predators present or absent. In half of the enclosures, the bottom of the prey refuge was sealed with a plastic sheet which prevented the juvenile fish from bottom feeding and also reduced the densities of invertebrates attached to the artificial vegetation. In the absence of predators (piscivorous perch), juvenile roach fed mainly on Bosmina whereas they fed mainly on detritus/algae in the presence of predators. Zooplankton biomasses in the open water were higher in the presence of predators than in their absence suggesting that the predators restricted the habitat use of the juvenile fish. The predator-induced shift in roach resource use resulted in a decreased growth rate of roach. The food resource use of juvenile perch was more affected by the availability of invertebrates attached to the artificial vegetation than by piscivore presence and growth of juvenile perch was affected by piscivore presence only in the enclosures with bottom sheets where they had no access to bottom-living prey. Previous laboratory studies have suggested that the physical structure per se is of major importance for the competitive interactions between juvenile perch and roach, but this enclosure experiment indicates that the prey resources associated with physical structure may also be important. In the absence of piscivores, I suggest that the open water is the preferred habitat for the juveniles of both species, but habitats with complex physical structure may serve as a competitive refuge for juvenile perch. Interactions between perch and roach are, however, also affected by the efficiency with which the juveniles use physical structure as a predator refuge, and roach have been shown to be more efficient than perch in evading predators in the vegetation.