Allocating biosecurity resources between preventing, detecting, and eradicating island invasions

Finding efficient ways to manage the threat of invasive species helps make the most of limited resources. Different management actions reduce the impact of invasions differently: preventing invasion eliminates impacts entirely, surveillance can facilitate early detection and eradication, and removing individuals can reduce future impact. Few studies have examined the trade-off between all three facets of invasion management. Using a simple model of island invasion, we find how resources should be allocated to each action to minimise the total cost of management and impact. We use a case study of black rat (Rattus rattus) invasion on Barrow Island, Western Australia. The optimal amount to invest in each management action depends on the effectiveness of each action, and the magnitude of impact caused by different stages of invasion. If the pest is currently absent, it is more cost-effective to prevent impacts through prevention or surveillance. If the pest is already widespread, it can sometimes be cost-effective to give up rather than attempting eradication. This model of invasion can provide useful decision support by identifying the trade-offs inherent in each candidate management strategy, the thresholds that alter optimal strategies, and the parameters for which we need more information.