Pest Eradication

Planning for a pest free Sanctuary

The key to restoring the Karori Wildlife Sanctuary back to its pre-human state is to permanently remove introduced pests. The presence of introduced mammalian pests such as possums and stoats has had a dramatic effect on the range of native wildlife and plants in the valley.

Possum browsing was responsible for removing an estimated 400 tonnes of vegetation (leaves, fruits and flowers) from the valley a year, and was severely affecting regionally rare plant species such as northern rata, tree fuchsia and kohekohe.

Possum		Feral cat

Habitat loss and predation from rats, stoats, ferrets and cats saw the number of native bird species that would have been present in the valley decline from about 20 to less than 10, all in low numbers.

Restoration of the forest, and the return of locally extinct wildlife species such as kiwi, kaka, tuatara and native bats, would not be possible without the eradication of these pests.

Conservation manager appointed

In December 1998, Department of Conservation pest ecologist, Raewyn Empson, was employed as Conservation Scientist. She began detailed planning for the Sanctuary pest eradication programme.

The objective - to create the most pest-free square mile of urban habitat in the world.

The objective of the eradication programme was to eradicate all introduced mammals in the Sanctuary and thereby allow the recovery and restoration of self-sustaining populations of native flora and fauna.

A world first

The successful completion of the eradication created a number of world firsts.

• First time 14 species have been targeted for eradication at once.
• The first eradication operation targeting hedgehogs and hares.
• The first pest-free zone in an urban environment.
• The first eradication operation targeting rodents, cats, possums, mustelids and rabbits at a mainland site.

Five phase eradication programme

1. Planning
Completed the eradication plan and obtained the necessary consents.

2. Preparation
Fence construction and 50km of tracks cut for bait stations.

3. Possum knockdown
Wellington Regional Council trapping operation.

4. Aerial operation
Distribution of bait mainly by helicopter.

5. Followup operation
Ground-based techniques including trapping used to detect and eradicate remaining animals.

Post-eradication - ongoing monitoring programme and contingency plan to respond to any reinvasion developed and implemented.

A feral cat is fitted with a radio collar before the eradication operation.

Helicopters were used for the main poison drop.

A one hit operation

The eradication teams moved into the Sanctuary in June 1999 and by September had trapped over 3 tonnes of possums in the valley. Two cats and three stoats were also caught and fitted with radio collars to monitor the effectiveness of the poison operation.

The rodent poison was distributed over two days on September 23rd and 30th 1999. 120 volunteers and staff were involved in the two separate operations in which 4.5 tonnes of bait were delivered by helicopter and spread by hand.

The result

The effect of the two poison drops was immediate. The death of the radio-tagged stoats showed that poison was effective in targeting this species. Although cats had escaped from the Sanctuary before the poison operation, they were never detected inside the Sanctuary again, showing the effectiveness of the fence.

No sign of rats and mice were detected in follow-up monitoring beginning at the end of October 1999.

Monitoring for re-invasion

Fence maintenance is an ongoing task.

With the eradication completed, the Sanctuary operation teams began the time-consuming and difficult task of monitoring for any signs of re-invasion.

Unfortunately, mice were detected in February 2000, next to the fence, and have continued to reinvade since then, probably due to minor flaws in the fence. While modifications were made to the fence, we have been unable to prevent reinvasion and have been trialling further design modifications to be undertaken in the future.

No reinvasion of pests such as possums, rats, stoats and cats has been detected since the eradication. However, in 2004 weasels were detected and an eradication programme using traps began immediately (more information...). Eradication of weasels in 2004 was successful with no sign seen since then.

To increase the chance of a successful weasel eradication, especially because females get very difficult to trap during the breeding season, mice will be poisoned, because they are the preferred prey of weasels.

The monitoring programme and the rapid response to any pest reinvasions will continue to be an ongoing task for the Sanctuary operations team as part of the Sanctuary’s 500-year restoration strategy.

Further information

• Summary of Rodent Control Measures at Karori Wildlife Sanctuary.
• Eradication of Pests from Karori Wildlife Sanctuary: A Brief Summary (Paper presented at Mainland Island Hui, St Arnaud 10-13 July 2000) (PDF file, 112kb).