Our Changing World: Going for eradication

Source: Radio New Zealand

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It sounds like an impossible task – eradicating all stoats, rats and possums from more than 110,000 hectares of South Westland and keeping them out.

But that’s the aim for Predator Free South Westland, a collaborative project with these three pests in its crosshairs.

Its ambitious goal is now nearing completion, and it could provide a template on how to approach large pest removal projects in Aotearoa.

Backcountry beginnings

The project area stretches from the Tasman Sea to the crest of the Southern Alps and is bounded by the Whataroa and Waiau rivers. It covers shoreline, nature reserves, farmland, townships, and lots of rugged, remote backcountry.

But there’s only one way to eat an elephant, and the first ‘bite’ was the Perth Valley. It was here that the project began in 2018.

Tackling its target pests in this rugged backcountry terrain involved 1080 toxin aerial drops. Once that was done, mopping up stragglers and continuous monitoring for reinvasion became key.

It is remote and challenging terrain, further complicated by the West Coast weather, but it became like a second home for field rangers such as Chad Cottle.

“There’s a lot of ephemeral creeks that can come up out of nowhere if you’re not aware of them,” he says.

“So we got used to the ones that came up and weren’t crossable after some rain and ones that went down really quickly. So we know where our boundaries were if we were going out during a rain day we’d know we better not cross that one because we won’t be able to get back across it if it keeps raining.”

Those early days involved chew cards and trail cameras, with rangers then classifying camera images in the hut at the end the day, but along the way the project has embraced new technology.

Now a network of 1200 AI cameras are spread across the project area. These were developed by Zero Invasive Predators, one of the project partners, alongside the Department of Conservation (DOC) and the Next Foundation.

The thermal-sensing cameras are trained to identify the three target species and alert the team by email when one is spotted. Pests are lured to the area using mayonnaise, dispensed automatically from a system that creates its own hydrogen gas to put periodic pressure on a plunger. In practical terms, these technological advances mean fewer trips to the backcountry, and therefore lower costs.

Once a pest is spotted, the team jumps into action. Decision-making on how to respond depends on what has been sighted, and where. A breeding population of rats in the backcountry would likely trigger an aerial drop. A possum could be hunted down by a species dog and dispatched with a rifle, or a live capture cage could be set for it. A stoat might be targeted with toxic rat bait.

But the backcountry is only one part of the project area. Around the small towns of Whataroa, Ōkārito and Franz Joseph the team is also working with private landowners, with a very different approach.

The end in sight

Following its early work in the Perth Valley the project area was divided into large distinct blocks of land to tackle one by one. Now it is down to the last section – around 10,000 hectares of farmland around Whataroa – which it aims to complete this year.

Here aerial toxin drops are not an option, so bait stations and trapping become key tools.

Pouri Rakete-Stones, the rural elimination team lead, has been working with farmers and landowners in the community, answering questions about what the project operating on their land would mean for them. And though many of the conversations often start with scepticism, Pouri says in general people are on board with what it’s trying to achieve.

“Most people can’t believe, they don’t believe that we can do it. First thing they say, you’re never going to get rid of rats. You know, there’s too many rats. You might be able to do it with possums, you’re never going to do it with stoats,” Pouri says.

“So trying to have that conversation about what tools we use, what techniques we do, how we go about work.

“We are elimination, we’re not suppression. So we are looking at targeting the last one. Having that conversation, telling those stories, getting them comfortable around what we can do. And then once we get on the ground and start doing the work, they can see the results pretty quickly.”

Maintenance

The project area was carefully chosen for several reasons. A high proportion of conservation land, a region home to several threatened native species, and geographical features that would help the mission.

While pests like possums have at times been spotted at surprising altitudes, the peaks of the Southern Alps seem to be forming an effective barrier against reinvasion.

The Whataroa and Waiau rivers also allow some protection, but roads and bridges across these are weaker points. Pest-proof gates have been installed on the swing bridges in the back country, but low river flow, or perhaps rat stowaways in vehicles mean that reinvasions continue to occur.

The area is thought of as having a ‘core’ that is free of targeted pests, with a buffer zone around it, into which rats, stoats and possums will stray.

Rapid AI camera recognition and response are how it deals with such incursions, but in addition, having identified the rivers as the weak point, the team is now running targeted operations on the other side of the rivers, to limit pest numbers there.

The goal now is to make this maintenance phase as affordable as possible. Nate St Hill, operation coordinator for Predator Free South Westland, says it is close to $30 a hectare, so an annual bill of $3 million (it has cost $50m to do the eradication). The agreement is that the maintenance will be supported by DOC through the Tomorrow Accord. It will continue to work to get that cost down, says Nate, by further embracing technology, thereby reducing labour time, and helicopter costs.

Seeing changes

While rats, stoats and possums are not the only introduced pest mammals in the project area, getting rid of those targeted three is leading to positive changes. Those working on the project for several years, as well as local eco-tourism operators, are reporting increases in both bird and plant life.

More kākāriki and kea have been sighted in the back country, there are reports of large flocks of kererū, and the only natural population of rowi, New Zealand’s rarest kiwi, are now starting to move beyond the Ōkārito Kiwi Sanctuary.

Dion Arnold is managing director of White Heron Sanctuary Tours, which operates in Waitangiroto Nature Reserve, near Whataroa.

The reserve is home to country’s only white heron kōtuku breeding ground, and Dion has been working here for almost three decades.

In recent years, Dion says, native species have been flourishing in the absence of pests.

“Just seeing those regenerating plants on the forest floor, areas that would have looked like a lawnmower had been through the forest in the past, in the last few years has come back with growth all across it,” he says.

“And the same with the bird life all around us. We’re hearing tūī and bellbird, grey warblers, the fantails and tomtits are around. And to see the number of those birds about having flocks of 30 or 40 of them in the trees above us is just incredible.”

The family-run business has been trapping in the area for a long time, but he says the intensive work to get rid of pests in the surrounding region means its traps are mostly for education now.

“Now we can go months and not have a catch in any of our traps out around here. It’s fantastic.”

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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand