Deadly trinity

Ferrets, stoats and weasels, introduced to control rabbits, quickly turned their attention to native birds

In 1867, at a meeting of the Otago Acclimatisation Society, Francis Dyer Rich made a controversial suggestion. The society, he suggested, should not be supporting any further introductions of rabbits. “To give rabbits, indiscriminately, to all persons who applied for them, would create a great evil”, he said, adding that he believed seven-tenths of settlers, and all sheep farmers, would bear him out. Initially, from the account of the meeting published in the Otago Daily Times, it appears that other members were reluctant to accept his views. However, after some debate, Rich won a grudging acknowledgement that rabbits were becoming a pest, at least in some circumstances.

Within a few years, the rabbit problem could no longer be dismissed as “simply a local evil, and of rare occurrence”. By 1872, more frequent comments suggesting that the rabbit was an ill-advised introduction begin to appear in New Zealand’s newspapers, and by 1873, the “rabbit nuisance” was serious enough to prompt a public meeting at Kaikoura. Rabbits were also rapidly becoming a problem in other districts – Wairarapa, Otago and Southland – and farmers were looking for solutions.

It didn’t take long for the discussion to turn to the natural enemies of rabbits. Many shared the views of one farmer, James Mackintosh, who said “having imported the rabbits to this country, it is manifestly our duty to import in large numbers ferrets, weasels and stoats.” Their introduction, however, was not without opposition. Sir Walter Buller, an ornithologist, was an unsurprising opponent. He pointed out that it was “comparatively easy to introduce animals to a new country, and equally difficult to extirpate them when once fairly established”. He was far from alone. Sir George Grey, notorious for introducing all sorts of exotic plants and animals to Kawau Island, was also an opponent of introducing ferrets, stoats and weasels. He went so far as to propose a bill to Parliament that would have prevented their introduction, but it was defeated. By that time, though, it may have already been too late – one of the Members of Parliament opposing the bill noted that he already had weasels in his possession.

The newspapers of the era document a lively debate. One correspondent suggested that “those who advocate the introduction of the stoat and weasel very much overrate their destructive powers in the direction they wish, and underrate their capabilities for mischief”. Another pointed out that “although these vermin [stoats] prefer warm meat, they kill all they come in contact with”. Others suggested that not only would they control rabbits, but that the stoat, as a great tree climber, would “do its share towards abating the small bird nuisance”, and, furthermore, in the winter it would provide “exceedingly valuable fur”. The Wairarapa Daily Times noted that public opinion on the introduction of ferrets, stoats and weasels was “very much divided”.

Although the opposition continued, it made no difference. Ferrets were being bred in New Zealand by 1883. Francis Rich, who had raised some of the earliest concerns about the rabbit problem, imported stoats and weasels. Further introductions followed, and thousands of animals were liberated around New Zealand.

Ferrets, stoats and weasels are all members of the mustelid family, a group of mostly small carnivores which also includes the otter, skunk, sable and mink. Ferrets are the largest of the three species introduced to New Zealand, with males weighing a little over a kilo, and females about half that. Weasels are tiny, weighing only as much as a blackbird, while stoats are between the two.

Stoat: image courtesy of Ngā Manu Nature Reserve

The three species differ in habit and habitat as well as size. Ferrets are largely ground dwellers and are mostly found in pasture and riverine habitats. Stoats, on the other hand, are excellent climbers. They are found throughout New Zealand, in grassland and in forest, from the tops of mountains to the coast. Weasels, the least common of the three, are also found in grassland and in forest, although usually in areas disturbed by human activity. Although they are a predator, they are also preyed upon by the larger stoats and ferrets.

The first of the mustelids to be recognised as a problem in New Zealand was the ferret. Unlike the rabbit, which took decades to be recognised as a pest, ferrets were already causing problems by the mid-1880s. In 1886, ferrets were described as invading “every settler’s poultry yard” in the Wairarapa. Suggestions of ferrets attacking lambs followed. By the early 1890s, all three species were being described as vermin by those who cared about birds, both native and introduced. They were reported to be causing a decline in native birds, and spreading to areas far from where they had been originally released. On the other hand, their success in controlling rabbits was dubious, since rabbits were still out of control in areas where they weren’t actively being killed.

By 1892, less than a decade after their widespread introduction, the damage that mustelids were doing to native birds was recognised at the highest levels of New Zealand. The outgoing governor, William Hillier Onslow, named them as among the predators that were causing the decline in native birds, particularly the kiwi. But, from 1881, the Rabbit Nuisance Act protected any predator deemed to be a natural enemy of the rabbit, and it would continue to do so for decades.

The battle to remove the legal protection for mustelids in New Zealand, and to have them recognised as harmful, was a long one. Although some protection was removed around 1900, it was reinstated in 1923. In 1936, however, all legal protection for mustelids was removed, on the grounds of the harm they did to native and game birds, and the lack of effect they were having on rabbits.

It’s difficult to get a clear picture of exactly how damaging the three mustelids are to our native birds, as well as our reptiles, amphibians and invertebrates. Mustelids are not alone in preying on native species – rats, possums and cats are also responsible for a lot of damage. So, to understand what is important about mustelids, I’ve gone to an expert, John Bissell, who has years of experience in conservation and predator control. Based in the Wairarapa, Bissell has a particular connection with the National Wildlife Centre at Pūkaha (Mt Bruce), just north of Masterton.

Pūkaha, Bissell tells me, is the place where he has learned the most about mustelids. All three species are present in the wider area, in and around the reserve. Ferrets and weasels are mostly, but not entirely, found in the surrounding farmland and stoats occur throughout. Because of that versatility, stoats are usually thought of as the worst of the mustelids in terms of their impact around New Zealand. They are, for example, the main predator of the tiny rock wren, which lives only in alpine areas of the South Island. They are also a threat to coastal species like the yellow-eyed penguin. But is it their agility that really sets them apart. They are great tree climbers, and birds make up the largest proportion of their diet.

But their versatility isn’t the only reason that stoats are so damaging. As one of the correspondents pointed out, back when the introduction of mustelids was still a subject for debate, stoats don’t just kill when they are hungry. They kill all prey that they come into contact with, and cache what they can’t eat. This behavioural trait, which evolved to cope with fluctuating vole populations in the northern hemisphere, is equally devastating whether it’s in a henhouse or in the bush.

While stoats can take on prey much larger than themselves, one bird they can’t defeat is an adult kiwi. Once kiwi reach around 1.2 kg, they can fight off a stoat. As a result, programmes like Operation Nest Egg, where young kiwi are raised in predator-free sanctuaries then released back into the wild once they reach a suitable size, have been very successful in helping some species of kiwi increase in number.

But, Bissell says, ferrets are another matter. Adult kiwi can cope with a few stoats, but not ferrets. Even a single ferret is one too many if you are trying to protect kiwi.

In part, this is simply because ferrets are bigger so they can tackle larger prey species, but it’s more than that. Ferrets have the same behaviour that is seen with stoats – they kill everything they can get rather than just killing when they are hungry. And it gets worse. Ferrets have been found to fixate on certain types of prey – they get a taste for something and then they pursue that prey singlemindedly. If the prey that they fixate on happens to be kiwi... well, you can see what’s going to happen.

Right now, there’s a real problem, since, with an increase in rabbit numbers over the last few years, ferrets are increasing too. The time when kiwi are most at risk is the autumn, because ferrets are at their most mobile, and therefore most likely to enter the forest and encounter kiwi. Controlling them at that time is critical, which is why last year’s Covid-19 lockdown couldn’t have come at a worse time for Pūkaha’s kiwi population. While we were isolating ourselves, five of Pūkaha’s kiwi were killed by ferrets. It was a devastating loss for the kiwi population at Pūkaha, and the staff who work to protect them.

It’s a sad story for our national bird, and the kiwi is not alone. Unless we do something about predators such as mustelids, there’s no question that many of our native birds are on the path to extinction. And we shouldn’t forget that mustelids feed on skinks, geckos and invertebrates such as wētā as well. But, just as Pūkaha is a great place to learn about mustelids, it’s also a great place to learn about controlling them – and that’s what I’ll cover in the second part of this article, next week.

The Turnstone
Deadly trinity: part two
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