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
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.