Completely eradicating pests from an island is a major conservation achievement, such as the recently announced eradication of goats from 15,380 ha Aldabra atoll. However, reliably confirming the absence of a species is difficult, bringing to mind the famous mantra ‘absence of evidence is not evidence of absence’. So how do eradication managers finally confirm that an eradication has been successful, especially for particularly cryptic species like small rodents? Historically, eradication managers have simply sat back and waited long enough that any survivors would either naturally die out, or breed up to large numbers once again. This became known as the ‘two year’ rule in New Zealand, based on evidence from the first documented rat invasion on Big South Cape Island which took two year from start to finish, with ‘finish’ being the extinction of two species of bird and one species of bat. Knowing that rats took only two years to become abundant on a nearly 1,000 ha island meant that after two years of no rat sign on an island after eradication, managers could be confident that the rats had indeed been eradicated.
Unfortunately, the two year rule means a lot of waiting (precisely two years in fact), before success can be confirmed, and before island restoration projects can be ramped up, such as species reintroduction projects. This lost time, and money invested in ongoing monitoring, is not optimal, and so researchers from New Zealand and Mexico have collaborated to develop a rapid assessment method for confirming eradication, published this month in the Journal of Applied Ecology. Using statistical models of the rate of rat growth and spread, coupled with a systematic monitoring grid across the island, these researchers have calculated what percentage confidence managers can have in eradication having been successful. For example, scientists and managers might be willing to accept a minimum of 90% confidence after 12 months, since its usually not possible to be 100% confident until you are closer to the traditional ‘two year’ mark.
The researchers tested their method on Isla Isabel in Mexico, an important site to confirm rat eradication since it had failed the previous time. In the coming year we hope to test this new method on a number of islands around New Zealand from where mice are being eradicated, such as Maud Island where the Department of Conservation wants to confirm mouse eradication as soon as possible.