Back from the brink: part one

How an obsession with killing things is saving New Zealand’s wildlife (8 minute read)

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As we are walking back from the park, my dog Donna begins pulling towards the right of the track. She’s sniffed something out, and after a few moments I realise what it is. There, tucked into the bushes, is a wooden box with mesh over both ends. It’s a trap tunnel, the kind used to keep curious dogs, as well as cats, birds and people, away from a trap designed to kill rats and mustelids such as stoats.

I’m not entirely surprised to see it. I know that there’s a Predator Free Khandallah group, and I’ve seen traps in the bush on Tarikākā. But seeing one so close to home, a five minute walk from my front door, is a reminder of how far New Zealand conservation has changed in the last few decades. When I was a child, wildlife conservation was something that happened far from the public eye, much of it on remote islands. Now, wildlife conservation means predator control in suburban backyards and reserves, and native birds are thriving as a result.

Last week I talked about the depressing conclusion drawn by New Zealand’s conservationists in the 1970s – that without stricter controls, rodents would continue to invade remote islands, wiping out more of New Zealand’s unique species. Islands were critically important for New Zealand’s native wildlife, because introduced mammals had wiped out many species on the mainland. But even remote islands had been invaded by mammals and, unless something could be done, even more of New Zealand’s native species faced extinction.

The tīeke (North Island saddleback), which was once confined to a single island (photo credit: Imogen Warren).

At the start of the 20th century, conservationists were already facing the ugly reality that the only way to save New Zealand’s native species was to start killing introduced mammals. They started with browsing mammals such as goats and rabbits on remote islands, by shooting or trapping them. But predators such as rats were another matter entirely. They couldn’t be hunted with guns, and trapping posed challenges in remote locations and difficult terrain.

The breakthrough in controlling predators came with the development of more effective rat poisons. Rats are difficult to control with poison, because they are suspicious of anything new, including new foods or a new object holding food. Faced with a new food, they will sample a small amount only. Then, if it makes them sick they will avoid that food in future. But the poisons that were developed in the late 1970s had a delayed action. Rats could eat a lethal dose before they got sick.

The first successful predator eradications were not intended to be eradications. Poison was used on some small islands with the intention of reducing rat numbers, and it was later found that there were no more rats. In the late 1970s and early 1980s, conservationists began experimenting with different poisons and ways of distributing the poisons. They found methods that worked on islands of 20 hectares or less, and then began to tackle larger islands. By the end of the 1980s, rats had been eradicated from a 170 hectare island. By the mid 1990s, rats had been eradicated from a 1900 hectare island. And by 2001, rats had been eradicated from an island larger than 11,000 hectares.

The use of poison is not without risks, of course. Although New Zealand has no native land mammals apart from insect-eating bats, the poisoning of other mammals, such as rabbits and deer is still considered a problem. Partly, the problem is that people value wild deer, despite the damage that they cause to the forest. But having other species like deer and rabbits eat the bait also leaves less for the rats, increasing the chances that an eradication might fail.

The biggest concern, though, is that poison can harm the endangered species it’s supposed to protect, especially birds, but also reptiles like the tuatara. These endangered species are at risk if they eat the bait directly, but also if they eat rats that have been poisoned. To protect the most vulnerable animals, some of them, including tuatara, weka and robins may be taken from the island before the eradication and then returned once the risk of poisoning has passed. In other cases, conservationists accept the risk of losing a small number of native animals, because experience has shown that the populations recover rapidly once predators like rats have gone.

Islands became increasingly valuable for protecting endangered species as predators were eliminated, but they didn’t completely solve New Zealand’s conservation problems. There are many types of habitat that simply don’t occur on offshore islands, such as sub-alpine and alpine shrub and grassland. Relying on islands alone would condemn species that depended on these habitats, such as the whio (blue duck) and the great spotted kiwi, to extinction. Conservation needed to come back to mainland New Zealand.

Controlling predators on the mainland posed different challenges. No matter how effective the control measures, the predators would always reinvade. This meant that methods that killed native species were much less acceptable, because control needed to be repeated again and again. Although poison remained a critical part of mainland predator control, much of the control was done using traps.

By the mid-2000s, advances in predator control on islands and the mainland meant that some endangered species could be categorised as “recovering”. It also meant that more people could see the recovery of native species when the predators were removed. By the late 1990s, community groups had picked up the idea that predator control could bring birds back to their local forest, and they wanted to get involved. But it was a different kind of project that truly galvanised local communities, and brought predator control into urban areas. That project was the Zealandia sanctuary, which brought rare birds right into people’s backyards.

The Zealandia sanctuary, just a few minutes by car or bus from Wellington’s central city, was once an area of scrub and bush set aside as a water supply area. In the early 1990s, Wellington conservationist Jim Lynch proposed fencing the area to keep out predators and to create a sanctuary for rare birds. But any old fence wouldn’t do the job – to keep out species like rats and cats, it needed to be a completely new type of fence. It needed to be high, it needed to go below the ground to prevent digging, and it needed some way of preventing agile animals like rats from climbing over it. And, because the fence was in Wellington, the fence needed to stand up to high winds.

Coming up with a design for the fence took several years of testing, using five different introduced mammals: cats, Norway rats, Australian brushtailed possums, stoats and mice. Cats could jump the highest, rats burrowed, possums climbed, stoats jumped and climbed, and mice squeezed through the tiniest of gaps. In 1998, the testing and design was completed, in 1999 the 8.6 kilometre fence was built and the following year all mammals inside the fence were eradicated.

Zealandia’s world-first predator fence

Zealandia’s fence isn’t a perfect barrier – mice wriggle through regularly and two stoats have made it over in the last 22 years – but water isn’t a perfect barrier either. Stoats have managed to reach islands more than 5 kilometres from mainland New Zealand. And even the remotest of islands have predator monitoring, in case a rat gets there somehow, perhaps swimming from a boat moored nearby.

While not perfect, Zealandia’s fence achieved something that sustained predator control could not. Without a fence, no matter how effective predator control is, there are always some rats, possums and mustelids. But a fenced sanctuary is more like an island where predators have been eradicated, rather than a mainland site. That’s important, because it has allowed the sanctuary to bring back species once wiped out on the mainland.

Birds such as the kiwi and kākā have some tolerance of predators. Although they continue to decline without intensive predator control, and do much better in sanctuaries such as Zealandia, they’ve managed to survive on mainland New Zealand until now. But some species of New Zealand’s native animals are so vulnerable that they survived only on predator-free offshore islands, or in captivity. Among these species are the tieke (saddleback), the tuatara and even flightless insects such as the giant wētā. Now, Zealandia has brought these species back to the mainland – and New Zealand’s capital city.

With a predator-free sanctuary in the heart of their city, Wellington’s human residents started to notice something. Birds weren’t just thriving inside the sanctuary fence – they were increasing in numbers outside the fence too. It wasn’t just common birds such as the tui either. Rare birds such as tīeke and kākā, which had been introduced to Zealandia, were turning up outside, in people’s backyards.

The increase in birds outside the sanctuary was called the “halo effect” and, although it was delightful, it was also a problem. The birds could be killed by predators if they went outside the sanctuary boundary. Wellington needed more than just a fence to protect its birds. But that is for next week, when I look at the role of local councils and communities in predator control in Wellington City.

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