The presence of introduced pests, such as possums and stoats, has had a dramatic effect on the range of native wildlife and plants all over New Zealand.

The key to restoring our sanctuary valley back to its pre-human state is to ensure introduced mammalian pests are kept out - via our 8.6km exclusion fence and strict bio-security measures.

It is estimated that possum browsing, before the fence was built, removed 400 tonnes of vegetation from the sanctuary valley in a year, severely affecting regionally rare plant species such as northern rātā, tree fuchsia and kohekohe. Habitat loss and predation from rats, stoats, ferrets and cats saw the number of native bird species that would have been present here decline from about 20 to less than 10, all in low numbers.

World-firsts

By 2000, our sanctuary valley was free of introduced pests. This impressive feat was the achievement of a number of world-firsts:

• The first time 13 species had been targeted for eradication at once (1 or 2 being the norm)
• The first eradication operation targeting hedgehogs and hares
• The first pest-free zone in an urban environment
• The first eradication operation on the mainland targeting rodents, cats, possums, mustelids and rabbits.

A five-phase eradication programme

Date	Action
December 1998	Eradication planning completed. Necessary consents obtained.
August 1999	8.6 km perimeter fence completed. 50 km of tracks cut for bait stations.
June – September 1999	Possum trapping conducted by Wellington Regional Council. Three tonnes of possums were trapped. Two cats and three stoats fitted with radio collars to monitor the effectiveness of the poison operation.
	Distribution of poison bait over two days. 4.5 tonnes of bait spread by helicopter and by hand. The death of the three radio-tagged stoats showed that poison was effective for this species. Although cats had escaped over the fence before the poison operation, they were never detected inside the fence again, showing the effectiveness of the fence.
October 1999 – January 2000	Ground-based techniques including trapping used to detect and eradicate remaining animals. No rats or mice detected in follow-up monitoring – our sanctuary valley is free of mammalian pests!

Mice make it back in

The Karori Sanctuary Trust operations team now focus on the time-consuming and difficult task of monitoring for any signs of reinvasion. Unfortunately mice were detected in 2000 and have continued to reinvade since then, probably due to minor flaws in the fence caused by settling or objects bumping into 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.

Weasel incursions

In 2004 and 2008 weasels were detected and an eradication programme using traps began immediately. To increase the chances of successful weasel eradication, especially because females can be very difficult to trap when breeding, mice were poisoned. Mice are the preferred prey of weasels.

Ongoing monitoring

All vehicles, bags and equipment are checked prior to entry to the valley to minimise risk of accidental introductions, but mice have been able to exploit flaws or damage to the fence. We have always assumed that the fence will never be effective 100% of the time (even offshore islands are at risk from accidental introductions), so we have audit and control response procedures in place. The monitoring programme and the rapid response to any pest reinvasions will continue to be an ongoing task for Zealandia as part of its 500-year restoration strategy.

More information

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