Simultaneous invasive alien predator eradication delivers the best outcomes for protected island species

Invasive species on islands rarely occur in isolation, and their removal will affect other species, both natives and invasives. Hence, conservation interventions must proceed carefully to avoid unintended consequences. A common invaded ecosystem motif consists of an invasive apex predator, an invasive mesopredator, and a native prey species—for example feral cats, rats and native seabirds. Eradication programs that specifically target apex predators can lead to mesopredator release, which may (and paradoxically) increase negative impacts on native species. We seek to develop management strategies that can remove both invasive predators, while allowing for the best recovery scenario for the native species. Specifically, we use systems of differential equations to model interacting species, and we seek to understand whether the two invasive species should be eradicated sequentially or simultaneously, when the latter option means that scarce management resources must be shared between the species. We find that (1) simultaneous eradication of both invasive species provides the greatest benefit for the native species, but (2) a sequential approach can be cheaper. Importantly, cheaper strategies incur the risk of poor native species recovery. Whether the cheaper option is to remove the apex predator first or the mesopredator first depends primarily on whether the mesopredator prefers the native species or alternative food sources. Hence, with limited knowledge of prey preferences, the simulations predator eradication strategy has the best chance of minimising unintended negative effects for native prey species.