Invasion success in communities with reciprocal intraguild predation depends on the stage structure of the resident population

The probability of individuals being targeted as prey often decreases as they grow in size. Such size-dependent predation risk is very common in systems with intraguild predation (IGP), i.e. when predatory species interact through predation and competition. Theory on IGP predicts that community composition depends on productivity. When recently testing this prediction using a terrestrial experimental system consisting of two phytoseiid mite species, Iphiseius degenerans as the IG-predator and Neoseiulus cucumeris as the IG-prey, and pollen (Typha latifolia) as the shared resource, we could not find the predicted community shift. Instead, we observed that IG-prey excluded IG-predators when the initial IG-prey/IG-predator ratio was high, whereas the opposite held when the initial ratio was low, which is also not predicted by theory. We therefore hypothesized that the existence of vulnerable and invulnerable stages in the two populations could be an important driver of the community composition. To test this, we first demonstrate that IG-prey adults indeed attacked IG-predator juveniles in the presence of the shared resource. Second, we show that the invasion capacity of IG-predators at high productivity levels indeed depended on the structure of resident IG-prey populations. Third, we further confirmed our hypothesis by mimicking successive invasion events of IG-predators into an established population of IG-prey at high productivity levels, which consistently failed. Our results show that the interplay between stage structure of populations and reciprocal intraguild predation is decisive at determining the species composition of communities with intraguild predation.