Invasive species - Rat eradication - Oceania

Rats contribute to the decline of tropical seabird populations by affecting their breeding success through direct predation of eggs and chicks. When they coexist with other predators, invasive rats may also generate indirect interactions via the changes they impose on the structure of communities and trophic interactions following invasion (‘hyperpredation process’), or when apex predators are eradicated from the ecosystem (‘mesopredator release effect’). Understanding these effects is necessary to implement restoration operations that actually benefit threatened seabird populations.

Rat eradications on tropical islands have been less successful than operations in temperate climates. This is likely due to poor understanding of the factors unique to tropical regions that rat populations respond to, such as high numbers of land crabs, aseasonal climates and habitats not found at higher latitudes. On Aldabra Atoll, southern Seychelles, black rats were monitored for one year in three habitats over three climatic seasons to investigate changes in density and breeding to inform planning for a possible rat eradication.

Invasive rats are found on most island groups of the world, and usually more than one species has invaded. On tropical islands populations of different invasive rat species can co-exist on very small islands, but the population dynamics of such co-existing rat species, their impact on each other, and the mechanisms of coexistence are not well known. This lack of knowledge is a barrier to improving the success rate of tropical island rat eradications.

Rodents remain one of the most widespread and damaging invasive alien species on islands globally. The current toolbox for insular rodent eradications is reliant on the application of sufficient anticoagulant toxicant into every potential rodent territory across an island. Despite significant advances in the use of these toxicants over recent decades, numerous situations remain where eradication is challenging or not yet feasible.

Rodent eradications undertaken on tropical islands are more likely to fail than eradications undertaken at higher latitudes. We report on 12 independent rodent eradication projects undertaken on tropical islands that utilized the results of an in situ bait availability study prior to eradication to inform, a priori, the bait application rate selected for the eradication. These projects also monitored bait availability during the eradication.