Island prioritization for invasive rodent eradications with an emphasis on reinvasion risk

Invasive rodents occur on over 80% of the world’s island groups, invasions are continuing, and rodent impacts on insular wildlife have been well demonstrated. The extent of this problem calls for tools to aid large-scale prioritisation among the many candidate eradication operations. As conservation funds are limited, biologists have responded with prioritisation systems based on financial cost-effectiveness. Instead, we claim that long-term conservation gain should be the primary focus when prioritising islands for invasive rodent eradication. This concept is embodied mainly by invasive rodent reinvasion risk, which we categorise as natural or anthropogenic, based on the mechanism of reinvasion and our ability to mitigate the risk. The result is a first-pass triage system that prioritises eradication programmes by their long-term conservation potential, not their immediate value for money. To construct a prioritization list, we group islands into units for simultaneous eradication, to minimize inter-island reinvasion risk, and then assign weights to levels of unit reinvasion risk and unit conservation value. The choice of parameter weights may depend on capacity for biosecurity action (i.e. reduction in reinvasion risk) and a choice of tertiary filter variables can further discriminate within priority ranks. We illustrate our prioritization framework with a case study on rodents in New Caledonia but explain how our system can be adapted to suit any invasive rodent species or island configuration.