How the Kāpiti Island Rat eradication (almost) didn’t happen | Conservation blog

Source: Department of Conservation

By Lee Barry, DOC Ranger Biodiversity

October 2021 marked 25 years since rats were eradicated from Kāpiti Island, making it one of the first and largest predator-free island sanctuaries. To celebrate, the operational team reunited in the old Whare on Kāpiti. Biodiversity Ranger Lee Barry thinks back to the events leading up to the declaration of Kāpiti as officially Predator Free.

The deafening chatter of the birds is what hits you first.

From the musical taunting of the tīeke and the chattering kākāriki, to the screeching kākā and cackling tūī; these decibels of birdsong are the soundtrack to Kāpiti Island.

And they’re all here thanks to one intense push 25 years ago to make Kāpiti officially predator-free.

An Iconic Island

Kāpiti is, in a word, iconic; the large barrier island shelters the coast from offshore weather and gives the area a beachy climate, its name, and a classy motif. Classified as a Nature Reserve, the island sits alongside a few other precious DOC sites as the most protected land in the country. It is highly significant to Māori, being Te Rauparaha’s stronghold, the scene of tragic battles, and a Te Tiriti signing.

But these days it is most recognised for its birds, thanks to being free from all mammals for decades.

On 15 October 1996, the second poison operation to rid Kāpiti Island of rats was completed. So how did this happen?

Well, it almost didn’t.

Pioneering New Methods

The attempt to eradicate rats from Kāpiti was significant. The possum eradication a decade earlier was a long, tough, gutsy slog by a few tenacious men and their dogs, but the rat eradication was truly brave.

The island is 1965ha of dense bush and long valleys. No one, anywhere, had ever attempted to rid an island of this size of rats before. The closest in size were around 10 times smaller and had targeted only one species, while this attempt aimed to eradicate both kiore and Norway rats.

Significant, yes. Optimistic, absolutely. Foolhardy? Nah; our team was up for the challenge.

Architect of the Revolution

The architect of the mission was Raewyn Empson, a Threatened Species Officer for DOC’s Wellington Conservancy.

At the time, DOC was seeing great success from the smaller island eradications of rodents using methods not available in the past: accurate application of toxic bait from helicopters. This, back then, was revolutionary.

Raewyn recalls that the operation wasn’t supposed to be so late in the year; winter is the best time for rodent control, when rats are hungry and their numbers are lowest. By spring they start to breed again, which adds more pressure to a population control.

Unfortunately, the winter of 1996 brought an unwelcome challenge.

“The bait was still held at the factory – it was due to be delivered the day of each aerial operation, so we didn’t need to worry about storage,” says Raewyn. “The guys at the factory opened the bags of bait to check it when we confirmed the good weather window.

“They rang me to tell me that the bait – all 30 tonnes of it – was mouldy and couldn’t be used.

Having had everyone on standby for several weeks waiting for the right weather, ready and raring to go, it was a real blow. New bait had to be manufactured and cured, which would take weeks. It was too risky to spread just one lot of bait – the second was a necessary backup in case there were any gaps in distribution the first time around.”

The actual drop was the culmination of years of work – including two trial drops of non-toxic bait and on-the-ground monitoring to keep non-target species safe. Weka were transferred off the island temporarily after they were shown to be at risk from eating the pellets, while takahe and pateke were held in enclosures to keep them safe from harm.

Time for Plan B

The logistics of such a late change to the bait drop plans is mind-boggling.

Raewyn convened the advisory team to confirm that though the drop was postponed it would still go ahead. This involved renewing the necessary permissions, rebooking helicopters, boats, and dozens of personnel, re-issuing public notifications, re-planning the captive management of non-target species such as weka and much, much more.

So Kāpiti’s rats were spared a few extra months. But ultimately, they were hit by two mostly-aerial applications of brodifacoum poison, – one in September and one in October 1996.

But the work was not done. If any rats survived, there was an agonising wait ahead for Raewyn and the team. Rats would take two years to rebuild to detectable numbers, so a network of hundreds of tracking tunnels was laid across the island and checked in 1998.

Not a single print was found.

In 1999, Conservation Minister Nick Smith declared Kāpiti Island rat-free and free from all mammalian predators.

Learn more about Kāpiti Island.

MIL OSI