Regional

Invasive rodents have an overwhelmingly detrimental impact to native flora and fauna on islands. Rodent eradications from islands have led to valuable biodiversity conservation outcomes. Tropical islands present an additional suite of challenges for rat eradications due to unique characteristics associated with these environments. To date tropical island rat eradications have failed at a higher rate than those undertaken outside the tropics. Critical knowledge gaps exist in our understanding of what drives this outcome.

A study tour of restoration projects was conducted for seven participants from four Polynesian countries (American Samoa, Niue, Samoa and Tonga) between March 20 and 27, 2015 to Auckland, New Zealand. All participants are involved in restoration projects in their home country, most funded under the GEF-PAS "Prevention, control and management of invasive alien species in the Pacific Islands" Project. Seven restoration sites were visited, including 3 island sites and 4 mainland sites.

Wedelia, creeping oxeye, or the trailing daisy, a deceptively beautiful, bright emerald-green creeper with bright yellow daisy-like flowers, is one of the world's most aggressive weeds and is listed among these other destructive organisms as one of the worlds 1000 worst invasive alien species. IT is now firmly established in Melanesia and throughout the Pacific islands.

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.