Competition between a Planktivore, a Benthivore, and a Species with Ontogenetic Diet Shifts

Abstract

Along a gradient of increasing productivity in lakes, the abundance of perch (Perca fluviatilis) first increases and then decreases, whereas the abundance of ruffe (Gymnocephalus cernuus) and that of roach (Rutilus rutilus) continue to increase. Perch, unlike roach and ruffe, undergoes dramatic ontogenetic diet shifts. The decrease in the abundance of perch along the productivity gradient may be due to competition from roach at its plankton—feeding stage and from ruffe at its benthic—macroinvertebrate—feeding stage. Previous studies of these three species have focused on the effects of roach on the other species. To further examine the competitive relationship among these species, we studied the effect of increasing the density of ruffe, while maintaining a relatively high, constant density of roach, on the diet and growth of perch. Food resources were also monitored during this 2—mo experiment, which was conducted in 10 enclosures in a pond in southern Sweden. Based on prior experiments we expected roach, an efficient planktivore, to reduce the biomass of zooplankton in all enclosures. Moreover, we expected that the increased density of benthic—feeding ruffe would force perch to increase its consumption of zooplankton, resulting in reduced growth. We found, as predicted, that the biomass of cladocerans of copepods decreased over time but was not affected by ruffe density. The biomass of the commonly consumed macroinvertebrates Sialis (a megalopteran), trichopterans, and ephemeropterans was lower in enclosures with ruffe than in enclosures without ruffe. Moreover, the biomass of Sialis decreased and that of chironomids and the clam Pisidium increased over time. High ruffe densities were related to an increased proportion of zooplankton in the diet of perch in August, but not in September. The diet of ruffe changed little along the ruffe gradient. Only the proportion of trichopterans (in August) and Pisidium (in September) in the diet of ruffe increased along the ruffe gradient. The growth of both perch and ruffe decreased with increasing ruffe density, whereas the growth of roach was constant. These data suggest that one reason perch decrease in abundance along a productivity gradient may be because they are competitively sandwiched between plantivore and benthivore specialists.