Marine islands make up 2% of the earth’s land area, yet are home to 64% of recently extinct species and 45% of critically endangered species. Invasive animals are a leading threat to island species, and their eradication is a powerful conservation tool. Because funding is insufficient to remove invaders from all islands, there is a pressing need to determine where in the world future eradications should take place in order to maximize conservation gains. Systematic conservation planning, incorporating both the potential biodiversity benefits and the costs of conservation actions, results in more efficient use of limited conservation funding. We applied a return-on-investment approach, integrating a wide range of geographic, biological, economic, and sociopolitical data, to prioritize island regions globally for invasive animal eradications. Geographic and biological parameters used in our model include the number of islands and total island area in all countries, distribution of WWF Ecoregions on islands, predicted species richness on islands for plants and birds, and IUCN-listed bird species threatened by invasive mammals. We extended traditional cost estimates to include NGO operating costs, hidden costs such as permitting to meet regulatory requirements, and potential costs arising from project failures due to corruption or political instability. 
Results/Conclusions

We found that developed countries with high island biodiversity were consistently lower priority due to costly permitting and high labor costs, while many other biodiverse countries were lower priority as a result of corruption and poor business environment. The top priority countries were those with moderate to high levels of island biodiversity, and low to moderate labor costs, corruption, and business climate, such as India, Brazil, Thailand, and Mexico. Overall, when costs and risk factors were taken into account, 50-70% of the most biodiverse countries became low priorities, while many countries with somewhat less biodiversity turn out to be highly cost-efficient places to invest in eradication projects. Our results demonstrate that NGOs conducting invasive mammal eradications can ultimately protect more species by evaluating both the potential biodiversity benefits and the costs of working in different locations. This approach is applicable to a variety of conservation problems and includes a novel application of species-area curves, WWF Ecoregion designations, and widely available international economic and political data.