Invasive species such as rats pose a threat to island biodiversity. They can impact nesting seabirds and turtles as well as native plants by feeding on native seeds.

The impacts of rats have also been felt by island communities such as those on Palmerston, who are now having to compete with rats for their food crops.

The damage caused by rats from both a social and ecosystem perspective can result in hundreds of thousands of dollars in loss and damage in a small area like Palmerston.

Palmerston Atoll, with a fluctuating population of around 40 people, has for some time shown interest in eradicating rats from their island.

In a first step towards achieving this, a feasibility study is currently underway. The feasibility team involves the collaborative efforts of the New Zealand Department of Conservation (DOC), Palmerston Island Government, National Environment Service, Ministry of Agriculture and Te Ipukarea Society.

The project is funded by DOC under the Managing Invasive Species for Climate Change Adaptation in the Pacific (MISCCAP) project. MISCCAP is a partnership between the NZ Ministry of Foreign Affairs and Trade (MFAT), Secretariat of the Pacific Regional Environment Programme (SPREP), Manaaki Whenua-Landcare Research and DOC.

The three-week long project involves both field work and community meetings. The team has been consulting closely with the community so that residents fully understand what it will take to complete rat eradication on Palmerston, and what potential risks there may be, which could cause the project to fail. By identifying these risks, the project team and community can also identify mitigation methods in order for the project to be successful.

Field work studies have involved using non-toxic bait spread over a grid that simulates the baiting regime that would be used in an island wide eradication. The non-toxic bait is made up of sugary cereal that is harmless if eaten by rats or people.

The use of non-toxic bait is to determine how much bait is being consumed by rats as well as non-target species such as chickens and crabs. Understanding this will allow the team to predict the right quantity of bait needed for eradication. Too little bait can result in a failed project and too much bait would result in increased cost and logistical difficulties, and unnecessary use of excess toxin.

Since the 1930s the community has known rats to be present on Home Islet where the people reside. Over time the community then noticed rats had spread to the neighbouring motu, Cooks Islet, a popular turtle nesting site.

To confirm the presence of rats on the remaining motu, new tech equipment brought over by the DOC was deployed to detect any signs of rats. Wax tags infused with peanut butter were used to record teeth marks. Motion cameras in combination with motor lures that pump out peanut butter every few hours were used to record any movement around the peanut butter lures, to determine the presence of rats.

So far, initial monitoring surveys have indicated that rats are present only on Home and Cooks motu. They appear to be absent from the remaining motu on the atoll. However, final results will not be available until all monitoring devices are collected this coming weekend.

Rat trap tests on Home Islet however have confirmed the presence of ship rats along with Pacific rats. Ship rats are potentially more damaging to this ecosystem compared to Pacific rats, as they are more aggressive when it comes to nesting birds, and can get into prized nu mangaro coconuts. Ship rats have been reported to swim distances of over 1 kilometre between islands, meaning it is possible for the species to colonise islands within this range and establish new populations.

The team members are in the final stages of the field site visit, and the findings will be discussed with the community and summarised in a feasibility report to inform planning for the rat eradication if it is to proceed.

The eradication using toxic bait is earmarked to start mid-2023. If successful, the completion of the eradication project would make Palmerston the first community island in the Cook Islands and South Pacific to be declared rat free.