The path to extinction

How New Zealand’s isolation made our native species vulnerable (8 minute read)

Among the many lessons of the Covid-19 pandemic is this one – it’s good to be an island. The pandemic has reminded us that New Zealand is a long way from anywhere, and sometimes that is useful.

But New Zealand’s isolation didn’t start with the pandemic. New Zealand has been isolated from any other land mass for more than 80 million years. Our isolation means that New Zealand developed many unique species. More than 70% of our birds, more than 80% of our insects, freshwater fish and plants and 100% of our frogs and reptiles are found nowhere else. Isolation has also shaped the types of animal we have in New Zealand – apart from three species of bat, one of which is now extinct, New Zealand has never had land mammals.

Without mammals, evolution took New Zealand’s animals in some unusual directions. In most parts of the world, the largest predators are bears and big cats. In New Zealand, the largest predator was a gigantic eagle (pouakai), with a wingspan of up to 3 metres. There were other large predatory birds too – the Eyle’s harrier and the laughing owl (whēkau). Medium-sized predators included the kārearea kārearea (New Zealand falcon) and ruru (morepork).

Because New Zealand’s medium and large predators were birds, native species that were the target of these predators developed some unusual adaptations to avoid them. Some became nocturnal, like our native frogs, and birds such as the kiwi and kākāpō. Flight, which is costly in terms of energy, was no advantage in escaping predators that could fly themselves. It was much safer to hide on the ground, and so many birds became flightless or had limited flight abilities. Of New Zealand’s flightless birds, the five species of kiwi are the most famous, but New Zealand had many others, including nine species of moa – giant relatives of the kiwi. Flightlessness wasn’t confined to birds either. New Zealand has an unusually high number of flightless insects, not just the well-known wētā, but also species such as weevils and stoneflies.

Since the arrival of humans in New Zealand, though, something catastrophic has happened to our flightless species. More than twenty species of flightless bird have become extinct, and those that survive, such as our flightless parrot, the kākāpō, are almost all under threat. Birds with weak flight, such as the kōkako, have also suffered. The picture is far less clear for insects, since scientists have assessed the threatened status of only 13% of New Zealand’s native insects. But some well-known large flightless insects, such as the wētāpunga, are now confined to a few offshore islands, as are our flightless flax weevils.

Although not truly flightless, the kōkako spends most of its time hopping or running, often on the ground.

The extinctions and declines seen in native species following the arrival of humans in New Zealand is part of a larger pattern. In Europe, North and South America, and Australia, large animals, such as the woolly mammoth, almost all became extinct soon after humans colonised their continents. There has been a long debate over exactly why this happened – were humans the cause, or was it something else, such as the change in climate at the end of the last ice age?

While the debate isn’t yet settled, in the case of New Zealand, the evidence is strong – the extinctions were caused by humans. Research has shown that, prior to human arrival, species such as the moa had large and stable populations. Then, within a few centuries of human arrival, they were gone.

Although we know that many species have become extinct since humans arrived in New Zealand, that doesn’t tell us exactly why. Hunting is often blamed, both for species like the moa and more recently-extinct species such as the huia. Habitat destruction, the leading cause of extinction globally, is also blamed. There has certainly been a huge loss of forest cover in New Zealand since humans arrived, and it’s likely that forest destruction contributed to New Zealand’s many extinctions. The combination of hunting and habitat destruction probably did most of the damage to large species like the moa. But there’s another factor as well. For the smaller species, more damage was done by the mammals that humans introduced.

Introduced mammals have been a problem in other places, but, because New Zealand lacked mammals before humans arrived, the impact has been much greater. It wasn’t just one or two species either – it was an onslaught. Māori introduced the kiore (a type of rat) and kurī (a type of dog). Rats, cats and mice scuttled ashore from early European ships. Pigs and goats were gifted to Māori by early European visitors. Rabbits and deer were introduced for sport and food by European settlers. Mustelids – ferrets, stoats, and weasels – were introduced in an ill-advised and unsuccessful attempt to control rabbits. Australian brushtailed possums were introduced in the hope of establishing a fur trade.

New Zealand’s animals, adapted as they were to winged predators, didn’t stand a chance against the ground-hunting predatory mammals. Our plants struggled too. Adapted to browsing birds and insects, some didn’t cope with goats, deer and possums. The arrival and spread of mammals in New Zealand began to change the New Zealand environment, in some cases irreversibly. There were extinctions, such as the flightless birds I mentioned earlier. While relatively few plants became extinct, as far as we know, many became rare, and there were changes in forests caused by species such as deer and possums.

Recognition that the introduction of mammals was harming the native species of New Zealand came early. In both “The Voyage of the Beagle” and “On the Origin of Species”, Charles Darwin mentioned the spread of introduced species, such as the Norway rat, in New Zealand. He predicted a dire fate for New Zealand’s native plants and animals, stating that:

“From the extraordinary manner in which European productions have recently spread over New Zealand, and have seized on places which must have been previously occupied, we may believe, if all the animals and plants of Great Britain were set free in New Zealand, that in the course of time a multitude of British forms would become thoroughly naturalised there, and would exterminate many of the natives.”

Darwin was not the only one to notice. In 1865, the ornithologist Walter Buller observed that flightless birds would not survive long in the face of dogs, cats and rats. Buller also opposed the introduction of ferrets, fearing that they would mean the end of New Zealand’s flightless birds, as well as many other native species. Thomas Potts, a politician as well as a passionate conservationist, was also concerned for the fate of New Zealand’s native species, especially the birds. He proposed preserving areas of New Zealand to protect native species, suggesting that islands, such as Resolution Island or those off the north-eastern coast of the North Island, could be used for such a purpose.

In May 1891, Potts got his wish, and Resolution Island became New Zealand’s first reserve for protecting New Zealand’s plants and animals. By this time, mustelids had spread far and wide in New Zealand, and New Zealand’s flightless birds were disappearing rapidly. The island’s first caretaker, Richard Henry, collected kiwi and kākāpō and transferred them to Resolution Island. His efforts were doomed to failure, as Resolution Island was too close to the mainland, and stoats swam across. Nonetheless, the principle was established, and other islands followed Resolution. Hauturu (Little Barrier Island) near Auckland became a reserve in 1895, and the only predatory mammals to reach there were kiore and cats, so it became an important sanctuary. Kāpiti Island near Wellington became a reserve in 1897. The Manawatāwhi (Three Kings) Islands, fifty kilometres north of New Zealand, were purchased to create a reserve in 1908.

Although the islands became an important refuge for native species, even remote islands, such as Manawatāwhi, were not safe from introduced mammals. Goats, rabbits, pigs, cattle and sheep were all introduced to remote islands as food for shipwrecked sailors during the nineteenth century. These animals, particularly the goats and rabbits, soon destroyed much of the vegetation on these islands. And so, in the early 1900s, New Zealand began removing mammals from some of the islands where they had been introduced.

At first, these eradications were aimed at removing domesticated species, like rabbits, goats, pigs and cats, using trapping and shooting. Then, in the 1960s, Taukihepa (Big South Cape Island), to the west of Stewart Island, was invaded by rats. This was a desperate situation, because there were three species of bird that had been wiped out on every other part of New Zealand, and survived only there. Despite the efforts of the Wildlife Service, two of these three species, a flightless snipe and a nearly-flightless bush wren, became extinct.

The tragedy of Taukihepa brought attention to the effect of rats on New Zealand’s native species. In the 1970s, conservationists came to the depressing conclusion that without stricter controls to prevent rat invasion on remote islands, many of New Zealand’s native species were doomed. Once the rats had invaded, they knew from the example of Taukihepa that the most vulnerable species could be gone within a couple of years. Worse, once rats invaded, there was little hope of removing them.

But at the same time as these depressing conclusions were being drawn, there were some glimmers of hope. While attempting to reduce rat numbers with poison and traps, conservationists found that they had actually eradicated rats from a few small islands. Was there hope of turning the tide? Could eradication be achieved on a larger scale? And could this help New Zealand’s native species recover? These are the questions I will consider next week.

[1] On the Origin of Species, chapter 10.