Hudson Dodd’s vision for the Brook Waimarama Sanctuary is to create ‘‘a slice of what New Zealand used to be like’’.
He dreams of a Garden of Eden in Nelson’s Brook Valley, a primeval wildlife environment teeming with native birds, including rare kiwi, kaka and mohua.
For this to happen, Dodd, the sanctuary’s general manager, has to ensure the birds are safe from predators. The same predators that drove many native species to local extinction.
A 14-kilometre pest-proof fence was completed last year to prevent rats, possums and stoats from getting in. But the fence can’t control the pest populations living, feeding and killing inside the sanctuary, and manual trapping can only achieve so much.
In late 2015, the sanctuary applied for resource consent to drop 26.5 tonnes of bait laced with 450 grams of poison. The tool of choice is brodifacoum, a common rodent poison that can be bought in most supermarkets and hardware stores.
The plan was immediately met with opposition. There have been protests, submissions to council, an online petition, a Facebook group, and a trailer with a ‘Stop the Drop’ sign parked on arterial roads in the city.
The use of poison as pest control has long attracted controversy. It’s as much an ethical conundrum as it is a scientific one.
It all comes down to how you answer this question: How does one weigh up the risks of using poison against the benefits of protecting New Zealand’s native biodiversity? Whybrodifacoum? Brook Waimarama Sanctuary Trust chairman Dr Dave Butler, a zoologist and threatened species expert, says he would rather not resort to using poison.
However, to achieve the vision for the sanctuary and bring back the lost native wildlife, it’s a necessary evil.
He says there’s no other tool available that can do what brodifacoum does. ‘‘It’s the one thing that removes the full range of pests in one go.’’
Brodifacoum has been used in other mainland sanctuaries, including Zealandia in Wellington and Maungatautari in Waikato. However, the Department of Conservation only uses it on offshore islands.
The poison is contained in small, cereal baits coloured bluegreen similar to 1080. It will be deployed in three helicopter drops done two-four weeks apart in fine weather. The first drop is aimed at taking out the dominant predators, such as larger rats. The subsequent drops target less dominant predators, such as mice and the young that were previously burrowed underground. The toxin is an anticoagulant which works by increasing or decreasing the clotting time of blood, leading to death from haemorrhaging.
Butler says that the three drops will eliminate almost all of the predators within the fence. The survivors will be taken out by intensive monitoring and trapping. He concedes that there are risks involved – ‘‘You have to break some eggs to make an omelet’’ – but says they are outweighed by the benefits.
It’s likely that the poison drop will result in native birds, particularly scavengers like weka and morepork, being killed. There’s the risk of poison entering the Brook Stream which runs through Nelson. The poison takes many months to break down in the environment. It’s possible that a the aerial drops will result in poison landing outside the fence, meaning dogs and other animals could eat it. Then there’s the issue of the integrity of the fence in storms and slips. And the poison is also said to bring about a slow and painful death.
Butler says much of the opposition to brodifacoum is based on ‘‘alternative facts’’ — a term coined by United States president Donald Trump’s advisor.
There will be by-kill, he says, but the overall effect of eliminating predators will result in many more native birds. He says brodicacoum has low solubility in water and the stream will be monitored for pellets. And while he can’t guarantee pellets wouldn’t fall outside the fence, he says the technology on-board the helicopter makes the drops highly accurate.
Birdlife on Grampians coordinator Bryce Buckland, a conservationist not opposed to using poison to control predators, says he has concerns about brodifacoum.
‘‘It’s a contaminant and has a far longer half-life and stays in the environment a lot longer than 1080 does,’’ he says. ‘‘You can’t say a contaminating poison is truly safe in the environment.’’
Buckland says he’s surprised the sanctuary has opted for poison at all given the success it’s been having with trapping.
‘‘To spread it [brodifacoum] aerially, indiscriminately and without any thought about the target species, it’s very shortsighted.’’
Landcare Research wildlife toxicologist Penny Fisher says brodifacoum has the ‘‘best track record’’ of achieving pest control in fenced areas quickly. What makes it so successful is its delayed toxic effect.
‘‘It takes a few days after an animal eats a bait for it to start feeling the effects of poisoning,’’ Fisher says.
‘‘This is important especially for rodents because they are very clever in recognising food that is harmful, they can quickly associate feeling not-so-good with something they recently ate and then avoid that type of food.’’
Fisher says the main risk of brodifacoum is that it persists in animals, mainly liver tissue, for months, which can result in secondary poisoning. For example, a harrier could be poisoned by eating several contaminated rats.
However, she says one-off applications in a fenced sanctuary presented relatively low risk of secondary poisoning.
Butler says the the environmental impacts have already been thrashed out during the resource consent process and a public hearing before an independent commissioner.
‘‘I fully respect a number of people that don’t like the use of toxins and we don’t like the use of toxins. We’re just doing a one-off job. If we don’t do this then we really don’t have a sanctuary.’’
Nelson-based writer and conservationist Dave Hansford, author of the book Protecting Paradise, says the introduction of pest species to New Zealand has been an ‘‘abject disaster’’ and using poisons was one of the most effective tools available to safeguard native species.
He says the one-off use of brodifacoum in the sanctuary would have minimal negative effects.
‘‘When the science is telling us that this is going to be okay if we do this once, we mustn’t pass up another opportunity to try and rescue what’s left of our native wildlife. ‘‘We are still going to lose more species yet if we don’t do something urgently.’’