Rabbits killing birds: modelling the hyperpredation process

1. Introduced rabbits are known to have catastrophic effects on oceanic islands, either by direct destruction of the vegetative cover, or by the resulting disturbance of indigenous vertebrates. 2. Another dramatic effect, less well known, but potentially of major importance, is the hyperpredation process. This process, related to apparent competition, predicts that an introduced prey species, well adapted to high predation pressure, could induce the extinction of an indigenous prey, through the sudden increased population size of an introduced predator. In many island ecosystems, the simultaneous presence of introduced feral cats and rabbits is thus potentially a further threat for small vertebrates endemic of these islands. 3. Through a mathematical model, we tested this hypothesis, using a tri-trophic system comprising an indigenous prey (birds), an introduced prey (rabbits) and an introduced predator (cats), and we demonstrated the theoretical existence of the hyperpredation process. 4. In addition, the numerical analysis of the model allowed a quantification of this process. It shows that the conditions required for an indigenous species to cope with the hyperpredation process imply very high intrinsic growth rates and/or carrying capacity, as well as behavioural anti-predator response to the introduced predator. Since these conditions are unlikely to be met, this process is a further potential threat to most indigenous vertebrate prey. 5. Finally, our model shows that, although it can be induced by both types of adaptation together or alone, behavioural adaptations alone are more powerful in generating the hyperpredation process, than are life history traits adaptations.