Applying lessons learnt from tropical rodent eradications: a second attempt to remove invasive rats from Desecheo National Wildlife Refuge, Puerto Rico.
The introduction of invasive rats, goats, and rhesus macaques to Desecheo National Wildlife Refuge, Puerto Rico led to the extirpation of regionally significant seabird colonies and negatively impacted plant and endemic reptile species. In 2012, following the successful removal of goats and macaques from Desecheo, an attempt to remove black rats using aerially broadcast rodenticide and bait stations was unsuccessful. A review of the operation suggested that the most likely contributors to the failure were: unusually high availability of alternative foods resulting from higher than average rainfall, and insufficient bait availability. In 2016, a second, successful attempt to remove rats was conducted that incorporated best practice guidelines developed during a workshop that focused on addressing the higher failure rate observed when removing rats from tropical islands. Project partners developed a decision-making process to assess the risks to success posed by environmental conditions and established go/no-go decision points leading up to implementation. Observed environmental conditions appeared suitable, and the operation was completed using aerial broadcast of bait in two applications with a target sowing rate of 34 kg/ha separated by 22 days. Application rates achieved on the ground were stratified such that anticipated high risk areas in the cliffs and valleys received additional bait. We consider the following to be key to the success of the second attempt: (1) monitoring environmental conditions prior to the operation, and proceeding only if conditions were conducive to success, (2) reinterpretation of bait availability data using the lower 99% confidence interval to inform application rates and ensure sufficient coverage across the entire island, (3) treating the two applications as independent, (4) increasing the interval between applications, (5) seeking regulatory approval to give the operational team sufficient flexibility to ensure a minimum application rate at every point on the island, and (6) being responsive to operational monitoring and making any necessary adjustments.