A graphic model is proposed relating the balance between predation and competition to size strategies and species richness in zooplankton communities. As decreasing survivorship, due to predation, and increasing food—collecting ability are arranged along the single dimension of body size, both may be specified clearly. The model indicates that not all size strategies are equally advantageous. Rather, it reveals an adaptive peak in the smaller size ranges and, less commonly, an adaptive peak at very large sizes. Moderate size strategies are feasible only if the species are flexible in size at first reproduction or cyclomorphosis. Under certain combinations of the predation and competition functions, an increase in species richness may also be expected.