Two size-dependent processes, metabolic requirements and foraging capacity, heavily influence the competitive ability of organisms. We studied the size-dependent competitive ability of roach (Rutilus rutilus) in a laboratory experiment to determine the attack rate of roach as a function of roach and zooplankton sizes. The estimated size-dependent attack rates, size-dependent metabolic demands and handling capacities were subsequently used to interpret the outcome of a competition experiment between two size classes of roach. Furthermore, size-dependent attack rates were implemented in an optimal foraging model to predict consumption rates and zooplankton selection to reveal the mechanisms behind competition.
The attack rate first increased with roach size to a maximum around 160 mm to thereafter decrease. Based on this result, we predicted that, small (150 mm) roach had a double advantage in competition for zooplankton in the pond experiment due to their higher attack rate and their lower metabolism compared to large (230 mm) roach. As expected, small roach depressed total zooplankton biomass to a higher extent than large roach, included more zooplankton in their diet and consumed smaller zooplankton. Predicted smallest size class of zooplankton in the diet was close to the observed. Also as expected, large roach fed more on the benthic resource and depressed the benthic resource to a larger extent than small roach. Large roach affected large zooplankton to a higher extent during the first part of the experiment, which could be related to their overall higher handling capacity. The higher impact of large roach on large zooplankton during the first part of the experiment, in turn, resulted in a lower estimated mass intake of zooplankton by small roach in the mixed treatment compared to small roach only treatments. Both small and large roach had a lower growth rate in the mixed treatment compared to single size class treatments. We relate the lower growth rate of small roach in the mixed treatment to large roach's higher efficiency on benthic resources and on large zooplankton during the first part of the experiment. In correspondence with diet data, large roach preferred the shore area of the pond with more benthic invertebrates and were also found more often close to the bottom. Although our results are explainable by exploitative interactions, the fact that the presence of large roach affected the feeding position of small roach points to that social interactions were also involved. Overall, our study implies that a mechanistic understanding is crucial for the interpretation of competition experiments, especially in systems with size-structured interactions.