The invasive harlequin ladybird (Harmonia axyridis), first recorded in Britain in 2004, has subsequently spread rapidly through the country and is now locally abundant. It interacts with native aphidophagous ladybirds through both resource competition and direct predation, raising concerns that many native species may be at risk. Recently natural enemies of native ladybirds have been observed to exploit the harlequin as an alternative host. Here I extend a simple model of intraguild predation to include the dynamics of a top predator, in order to assess how the populations of the native intraguild prey and its natural enemy are affected by the arrival of a novel invader.
Results/Conclusions

Dependent on the relative magnitudes of the trophic interactions, the model predicts a wide variety of outcomes, including extinction of one or more species and stable coexistence of all three species. In some cases the shared natural enemy is able to stabilize the interaction between intraguild predator and prey, which would otherwise result in extinction of the native species. The results presented here generate testable hypotheses about the community-level impacts of invasive intraguild predators.