Until now, it’s been too cold for turtles to breed here but global warming could change that, putting our native species further at risk.
Hidden in some of our wet places, tucked into reeds or submersed underwater, are turtles. Banish any vision of plodding tortoises, beating the hare by determination alone: these turtles are swift. They race away before you can grab them, occasionally dart out of the water to kill small birds and wait underwater for months without breathing. They have been outwitting biosecurity efforts overseas for years. Now they’re on the loose in New Zealand – and in some places, they’re breeding.
These are red-eared slider turtles and they are ranked among the top 100 most invasive species globally. Hailing from the midwestern United States, they have dispersed internationally thanks to their endearing trait of being incredibly cute as babies. They are the most commonly traded reptiles in New Zealand.
In their first years, they’re ideal children’s pets, easily contained in an aquarium. But if you can’t imagine your toddler as an adult, try conceiving that the turtle will still be your responsibility when the child’s hair is greying and crow’s feet are deepening. Little turtles Cuff and Link, for example, featured in the 1976 Rocky movie and returned to the screen in 2018 in Creed 2. During their lifespan of many decades – in captivity, at least – red-eared sliders grow to the size of a dinner plate. With their toothless beaks, they can bite hard.
It’s no surprise, then, that turtles get dumped. “People buy little turtles but they outgrow their environment so the owners quietly release them into waterways,” says Auckland Council biosecurity principal adviser Imogen Bassett. “They are regularly spotted at Western Springs.”
They’ve been reported in drainage ditches and stormwater ponds across Auckland, as well as more pristine wetlands, such as Te Henga (Bethells Beach) on the west coast.
Turtles are freshwater creatures and scientists are most worried about their impacts on native species in freshwater ecosystems, which are already in serious trouble. They’re omnivorous, opportunistic feeders, eating animals like invertebrates, snails, small fish and tadpoles, as well as vegetation. And there’s the risk of competition for dry, sunny spots around freshwater habitats. “We’re worried about turtles competing for basking spots where birds are nesting, which would shoo birds away from their nest,” says Bassett. “Birds need to go about their breeding business in peace.”
Auckland Council last year banned the sale and breeding of the turtles because of their pest potential, but allowed turtle owners to keep their existing pets. “We know they’ve got a track record around the world as being invasive, so we had a level of concern, but that was solidified by evidence of wild hatching in the Coromandel,” says Bassett. “Prevention is better than cure. And we’ve seen people wanting to do the right thing – people are often unaware of the potential impacts these animals can have.”
The council doubts it would be possible to get rid of wild turtles once they become widely established and breed, although to date, there’s no evidence of nesting in the Auckland region, perhaps only because no one is actively looking. Bassett says there aren’t many control tools for turtles. “We want to avoid repeating the mistakes we made for other pests, such as possums or feral cats. We don’t want to wait around until they’re yet another widely established exotic pest species before we act.”
Pest control inevitably also has welfare impacts, she says. “Nobody in pest management wants to be doing all that killing. By far the most humane way is not to allow the sale when we know some will end up in the wild. It’s win-win.”
Hatching needs heat
Red-eared slider turtles have been in New Zealand since the 1950s, but historically, it’s been too cold for eggs to hatch in the wild. Female turtles bury their eggs in the ground, choosing savvy nesting spots but providing no further care. The eggs’ incubation, which lasts at least two months, therefore depends on the environment surrounding it. The 10 or so eggs are soft-shelled and need warm, humid ground to hatch. Like many reptiles, turtle hatchlings’ sex depends on their nest temperature: eggs surrounded by temperatures of about 22- 27°C hatch into males, from 28-31°C both sexes emerge, and at warmer temperatures all hatchlings are female.
But the planet is warming. Near the tropics, the added heat is skewing sex ratios in similar species. Green sea turtles hatching in northern parts of the Great Barrier Reef are now primarily female, and there are concerns the already endangered population will soon be completely female.
Here, male turtles seems to be hatching in some places. Jayne Nightingale, a retired resident of the Coromandel Peninsula’s Hot Water Beach, has been voluntarily trapping turtles for the past three years in nearby Cooks Beach, where there’s a population of red-eared sliders. Their breeding success is revealed by a large number of baby turtles. Ground zero is two stormwater ponds adjacent to a new subdivision and backed by a restored wetland. Nightingale and her collaborators have caught about 70 turtles.
“They’re shy and elusive, and go underwater as soon as you see them,” says Nightingale. “I know a breeding pair was deliberately released about 10 years ago at Cooks Beach.” Females have been released there, and one was X-rayed and found to be holding eggs. “We just need people to not release females. Then we wait 40 years for the males to die out,” she says ruefully.
All the captured Cooks Beach turtles have been sent to Nick Ling, an associate professor in ecology, biodiversity and animal behaviour at the University of Waikato. Ling worries that in 40 years, females too will hatch. Two of the Cooks Beach babies he received were female, and although he cautions it is possible they were released rather than hatched, he says it is unusual for people to release such small turtles.
He has recorded summer ground temperatures at a number of sites, including at nests. “In some places, we’re only about two degrees below the sweet spot where both males and females hatch. In 40 years’ time, with global warming, what will the situation be?” he wonders. “Also, people release females, and one female can have 400 offspring in her lifetime, so you only need one female to be released and the population keeps expanding.” He also points out that at the right stage of incubation, eggs can become female with just a week of high temperatures, perhaps during a heatwave. And nesting mothers excel at finding the right microclimate for their eggs.
Ling’s concern about the turtles grew when, six years ago, he contacted all the regional councils and learnt of 100 reports of the turtles, from Northland to Invercargill. “They’re not breeding in the south, but there are reports of nests from Whanganui northwards,” he says.
When the Listener visited, Ling’s lab housed about 70 turtles ranging from babies to 20-year-olds. He can tell from the sizes of the Cooks Beach youngsters that they have been hatching there since 2015. The turtles’ armour was impressive. They have a hard shell on top and, underneath that, all the creatures’ soft bits tuck into it tidily when they are picked up – apart from the snapping beak of an older adult that might feel threatened. That defensive design makes it hard for predators to kill them, although perhaps less so as hatchlings, and they have no natural predators here.
Ling explains that outside over winter, the turtles go underwater for months in a reptile version of hibernation. They’re actually air breathers, but,”they can absorb a bit of oxygen from water through their bums,” he adds. As cold-blooded reptiles, they rely on warmth to increase their metabolic rate, hence their need to bask.
He is working on a test to figure out what wild turtles eat here, by feeding them different diets and measuring the carbon (indicative of plant-eating) and nitrogen (which reveals a diet rich in animal protein) in scrapings of turtle shell. Ling saw a gruesome display of animal-eating in his backyard pond when a red-eared slider grasped the leg of a drinking sparrow and dragged it underwater. Two other times, he found clusters of floating sparrow feathers. The turtles are also known to take ducklings.
Ling is helping develop an eDNA test with Canberra University in Australia, where red-ears are not an established pest, but the sale and ownership have been pre-emptively banned. The test should mean turtles can be detected by simply taking a water sample. The sample theoretically holds DNA of all the life forms that have sloughed body cells into it, but unfortunately, says Ling, red-eared sliders don’t shed many cells into water. He can easily detect their DNA in his lab tanks where their density is high, but not in nearby lakes at Hamilton Gardens, where turtles are regularly sighted at lower densities. They’re breeding at the gardens’ main lake, which was created and named Turtle Lake in 1985 after a turtle was seen there. They are also in the nearby Waikato River, which is much closer than the 3-4km limit that turtles can traverse over a few days.
National strategy
Some of the known breeding spots – Cooks Beach and Hamilton – are within the jurisdiction of Waikato Regional Council, for which Danielle Kruger is a senior biosecurity officer. Waikato hasn’t formally categorised them as a pest, partly because it is not sure how effective that would be if all neighbouring regions didn’t do the same. “It’s a difficult animal to regulate at a regional level. You really want a national strategy,” says Kruger.
Auckland Council also wants national regulation. “It would make enforcement a lot simpler,” Bassett says. She points out that a National Pest Pet Biosecurity Accord established several years ago would regulate sales at a national level, but no species have been added to it.
At Waikato’s request, several councils and the Department of Conservation have this year begun regular meetings with Biosecurity New Zealand, a division of the Ministry for Primary Industries, to discuss the way forward. Adding red-eared slider turtles to the biosecurity accord, which would be the most likely route to national restrictions on breeding and distribution, doesn’t seem imminent. “We’re working with everyone to consider what to do,” says Megan Verry, who manages MPI’s pest management strategy and planning team. “But it’s also a matter of prioritising where our limited resources go.”
Verry says MPI co-ordinates with other agencies, maintains a rigorous regime at the border so new turtles don’t arrive, has a pest and disease hotline for reporting, and provides online information about the pest-pet risk. “Pet ownership is the primary way a pet becomes a pest,” she says. Biosecurity New Zealand is working with regional councils and DoC on a campaign to promote responsible pet ownership and raising awareness about pets becoming pests. For now, it wants people to report, and preferably photograph, sightings of red-eared slider turtles in the wild.
Verry points out that each regional council has powers to individually enact a ban. But doing so is a heavy burden: Auckland Council went through years of consultation, submissions and eventually an Environment Court challenge that ended after mediation. A court challenge was taken by Mark Feldman, a retired physician, who went to court to preserve people’s ability to breed, sell and enjoy turtles.
He has kept turtles since he was seven and has been researching turtle breeding for more than 40 years, mostly in Louisiana. There, he says, predators include foxes, coyotes, possums, feral cats and crows. Turtle breeding is a big business in the US, and was even bigger when vast numbers were exported to Korea, Japan and China, where red-ears are highly desired.
Feldman doesn’t think there’s a risk of turtles going feral here or even surviving long in the wild. “There’s a world of difference between animals surviving in a park or stormwater pond and surviving in New Zealand’s natural cold water wetlands. They’ve been here 70 years and they haven’t been a problem. We have many more cats, Mallard ducks and Canada geese on the loose than turtles. We have to keep a perspective.”
He says it’s not warm enough even for males to hatch on the rare occasion females find somewhere sufficiently warm and humid for their nests to be successful. Even adult survival is difficult because to digest all the plant matter they eat, they have to warm their bodies to 30°C. Without proper nourishment they eventually starve and are prone to a bacterial shell-rot disease that is eventually fatal, he says. (Ling says he sees very little shell disease in the turtles he receives, but he can’t tell how long they have been living in the wild.)
Indigenous species on islands such as our own are particularly vulnerable to invasive species. That’s because they tend to evolve without the full suite of other life forms. In our case, there were no predatory land mammals – or turtles. When overseas species arrive, some of them overwhelm the island dwellers’ defences and competitive abilities.
We’re failing to protect our indigenous species. New Zealand hosts at least 18 of what the International Union for Conservation of Nature says are the world’s 100 worst invasive species. At last count, 93.5% of our reptiles were threatened with extinction or at risk of becoming threatened, as were three-quarters of our terrestrial birds and three-quarters of freshwater fish. Many have already gone extinct.
That’s despite a huge conservation movement, much of it running on the passion of volunteers such as Jayne Nightingale at Cooks Beach. Ling says agencies such as DoC and regional councils don’t have the funding or the time to do more.
His own turtle work is currently unfunded. “It’s the same old story of conservation in New Zealand. That’s why prevention is so important.”
Winging it
Various species of pet have formed wild populations in Aotearoa, including six parrot species, koi carp, goldfish, European alpine newts and cats. The risk of pets becoming feral is most serious in areas of high population, because in these areas there are more releases or escapes.
Professor Margaret Stanley, an ecologist at the University of Auckland, says we know this from our acclimatisation societies, which in the 1800s intentionally released species such as possums and deer to establish populations. “The more individuals you release, the more likely they are to successfully establish.”
Even today, some releases are intentional. During the 1990s, a North Shore resident released rainbow lorikeets. “Presumably, he wanted more beautiful birds in our landscape,” says Stanley. The lorikeets were eradicated, but other problematic parrots have settled in, including about 200 sulphur-crested cockatoos in the Waitākere Ranges and Indian ringneck parakeet populations in Auckland and Havelock North. Biosecurity NZ has handed long-term control of the parakeets to regional councils.
Most releases are accidental. Stanley co-authored a study published last year compiling reports from two lost-pet websites showing that in any given month there are about 491 escaped birds in Auckland, most of them parrots. She describes such escapes as a “high-risk invasion pathway”.
“A big problem is that we have our own parrots that are unique – some are the world’s only examples of whole families of parrots,” says Stanley. “They’re very susceptible to new diseases. Released parrots are likely to interact with parrots like the kākā and kākāriki, which are coming back into some of our cities.
“We know there’s competition for nests and food between pest and native birds, and bats. For example, the eastern rosella and kākāriki choose the same tree species and height to make their nest holes. These parrots are also horticultural crop pests.”
Stanley wants the most problematic parrots put on the National Pest Pet Biosecurity Accord. “Unfortunately, no species have been put on it, so we’re stuck with a region-by-region approach. Many people have asked for this to happen. It’s the animal equivalent of the National Pest Plant Accord, which works well to stop pest plants being propagated or sold.”
She says that even though Auckland Council prohibited the breeding and sale of some parrot species – a process that involved a High Court challenge by the Parrot Society of New Zealand – they’re obtainable nearby. “I go on Trade Me and see some of these species on sale in Northland and Waikato.”
She says parrots are opportunistic and clever. “Prevention is key because it’s almost impossible to remove a population of parrots after it’s established. It was incredibly difficult to eradicate the rainbow lorikeets. People will be opposed to it happening. In terms of cost effectiveness and humaneness, it’s better to stop them getting established.”