Lessons from an eradication under multiple constraints of an island rat population of record density.

Invasive rats threaten island biodiversity, disrupting ecosystems and endangering native species. Although rat eradication has succeeded on many islands, tropical islands present unique management challenges. Strict regulations and financial constraints on some tropical islands further limit proven eradication methods, complicating rodent management. We applied a real-time active adaptive management approach that provided a cautious, cost-efficient, and scientifically grounded pathway to rat eradication, while adhering to strict environmental regulations, on Ilha do Meio, Brazil. The cost was US$3300 per hectare, and the management actions were grounded in close interdisciplinary collaboration. We applied rodenticide (brodifacoum), monitored the rat population, and made iterative management adjustments. The rat overpopulation was eradicated within 5 months, and population increases were observed early on in the threatened masked booby (Sula dactylatra), and the endemic Noronha elaenia (Elaenia ridleyana) and Noronha skink (Trachylepis atlantica). Despite logistical constraints, our approach proved effective and cost-efficient, marking its first application in a biological system. Our findings highlight the value of innovation, close interdisciplinary collaboration, and adaptive decision-making when the application of best-practice methods is constrained.