Facilitation among piscivorous predators: effects of prey habitat use

The combined effects of predators on prey may substantially differ from thatof each predator species alone because of alterations in prey behavior. Using enclosures within a pond, we examined experimentally the effects of two piscivorous predators on prey mortality and prey resource levels in two habitats. The two predators use two different foraging modes, which also allowed us to examine the behaviorally induced indirect effects of prey on predator growth and prey food resources. Both perch (Perca fluviatilis) and pike (Esox lucius) caused significant mortality of roach (Rutilus rutilus), and the combined predator mortality was higher than predicted from a multiplicative prey consumption model. Growth rates of perch were similar when enclosures contained only perch and when they contained perch combined with pike. The growth rate of pike was higher when they were together with perch compared to when alone. Growth of roach was similar among treatments. The invertebrate food resources of roach increased by a factor 10 in the open water but remained at similar levels throughout the experiment in the vegetation. Biomass of Daphnia longispina, the dominant zooplankton species in the open water, was strongly correlated with mortality of roach, indicating a density-mediated indirect effect of predators on prey resources. There was no indirect effect on D. longispina in the vegetation caused by habitat restriction of roach and only a weak relationship in the open water. There was a strong indirect effect of pike predation on macroinvertebrates induced by a habitat shift of roach. Our results suggest that there was facilitation between predators caused by conflicting antipredator behavior of roach, which resulted in density-mediated indirect effects on prey resources. The behavioral response of roach to the two predators also induced indirect effects on invertebrate prey.