Stage-dependent predation on competitors: consequences for the outcome of a mosquito invasion.

1. Predator-mediated coexistence occurs when predation allows competitors to coexist, due to preferential consumption of a superior competitor relative to an inferior competitor. Differences between the native treehole mosquito (Aedes triseriatus) and the co-occurring Asian tiger mosquito (Aedes albopictus) in anti-predatory larval behaviours account, in part, for the greater vulnerability of this invasive species to native predatory midge (Corethrella appendiculata). We test the hypothesis that stage-dependent differences in the sizes of A. albopictus and A. triseriatus larvae, relative to the size-limited C. appendiculata, contribute to differential consumption and the likelihood of predator-mediated coexistence of these competitors. 2. In all instars, larvae of A. triseriatus were larger than A. albopictus of the same stage. Third and fourth instar C. appendiculata selectively consumed late-stage A. albopictus in preference to same-stage A. triseriatus. Small, early-stage prey larvae did not differ in vulnerability to predation, but large, late-stage larvae differed significantly in vulnerability to predation, probably owing to size-limited predation by fourth instar C. appendiculata. This effect was less pronounced for third instar C. appendiculata. 3. Prey size, in conjunction with anti-predatory behavioural responses, alters the probability of predator-mediated coexistence. A stage-structured predation model showed that equally vulnerable early stages reduce the range of environmental conditions (productivities) in which predator-mediated coexistence is possible, increasing the likelihood of both competitive exclusion of the resident species or failure of the invasive to establish. These results underscore the importance of stage-dependent interspecific differences in predator-prey interactions for determining how predators may affect community composition.