1. We studied the effect of size-selective predation on the outcome of competition between two differently sized prey species in a homogenous environment.

2.  Using a physiologically structured population model, we calculated equilibrium food concentrations for a range of predation scenarios defined by varying maximum predation intensity and size-selectivity, the latter being characterized by the neutral selection length (NSL), where negative selection of small prey size-classes turns into positive selection of larger prey. We parameterized the model according to the well-studied example of fish predation on two differently sized cladoceran species, Daphnia pulicaria and Daphnia galeata.

3. Although the larger D. pulicaria was principally the better competitor for food, competitive superiority shifted to the smaller D. galeata under certain predation scenarios. The lowest predation intensity needed to induce a shift from D. pulicaria to D. galeata was found at NSL values in between the sizes at maturity of both species.

4.  Analysing the per capita mortality rates as a result of the underlying parameters of the mortality model, NSL and maximum predation intensity, revealed that the tolerable mortality rate of daphnids decreased as NSL increases towards values close to the size at maturity. This effect was most pronounced in D. pulicaria, thus explaining a higher vulnerability of the larger species to size-selective predation.

5. A reduction of the size at maturity in the smaller species (D. galeata) as a phenotypic response to the presence of fish resulted not only in an increased capability to withstand predation, but also in competitive dominance shifts over the larger D. pulicaria at lower predation intensities and a much wider range of NSL as a trait-mediated indirect effect.

6.  Overall, our results demonstrate that shifts in dominance of differently sized herbivores under size-selective predation regimes may be facilitated by the demonstrated alteration in competitive capacities and thus might not be exclusively caused by direct predation effects.