Rod Morris

The tale of the hungry mouse

Something is nibbling at the heart of our ecosystems. As possums, rats and stoats disappear thanks to Predator Free 2050 operations, mouse numbers are expected to climb. Are we prepared? And since mice eat our wētā, beetles, geckos and skinks—rather than our charismatic birds—do we care enough to do anything about it?

Written by       Photographed by James Reardon and Rod Morris

On Ōtamahua/Quail Island, a research assistant takes a fur sample and ear tags a wild mouse. For a long time, mouse control has been put in the too-hard basket.

It’s 4.30 on a January morning on Ōtamahua/Quail Island, and just light enough for scientist James Reardon to read the tiny ear tag on the mouse he holds cupped in his left hand. The lights of Lyttelton glow orange across the water as he quickly records the tag number, sex and weight of the wild rodent. He takes a few strands of fur, then the mouse is free to go, a brown flash disappearing into the long grass.

“Once you’ve held your tenth mouse, you get more relaxed about it,” says Reardon. “I think we’re up to 2000 captures now, so we’re all getting pretty good at it.”

Ten other fieldworkers are out in the predawn chill, walking gridlines through the dewy grass to check the 600 live-capture traps in Reardon’s study area. They record any mice they find in the traps, and ear-tag any that haven’t been tagged already.

Reardon’s project is trying to fill a gaping hole in New Zealand’s conservation knowledge: how to kill large numbers of mice on our mainland. “For everything else, we know what to do, if someone would give us the money to do it,” he says. “But with mice we don’t actually have a solution. So it’s an absolute priority.”

Reardon’s current project involves a poison called cholecalciferol, a form of highly concentrated vitamin D. He’s trialling whether putting cholecalciferol bait out in very closely spaced stations can bring mouse numbers right down and, most importantly, whether it can keep them down for an extended period of time.

“Mice fly under the radar because they’re these tiny things—they look fairly harmless,” says Reardon. “But mice are doing a massive amount of damage. They’re omnivores, so as well as eating seeds and fruit, they’re eating invertebrates, and we’re increasingly concerned that lizards are taking a major hit from them.”

Mice are wily creatures—not all will enter traps, and they can detect 1080 in baits.

Since the first mice scrambled onto New Zealand soil in 1824, they have firmly established themselves in every ecosystem in the country, and are now—scientists are increasingly certain—quietly eating away at our biodiversity from the bottom up.

“Mice are the most successful mammal on the planet,” says John Innes, a senior researcher at Manaaki Whenua–Landcare Research. “They’re explorers, and really they’ve conquered the world.”

Innes has spent most of his 40-year career studying rodent ecology, and his most recent research at Sanctuary Mountain Maungatautari, due to be published in 2022, aims to quantify the damage these tiny intruders are doing. He and his colleagues found that when mice were the only mammal species present in the forest, they killed about half of all caterpillars, spiders, wētā and beetles.

Innes is also concerned about lizards. “The lizard fauna is unbelievably extraordinary in New Zealand,” he says. “We’ve got more species of lizard than of native land birds, and I don’t think most people know that. And we’re only beginning to understand the amount of damage that mice are doing to our lizards.”

[Chapter Break]

Kneeling beside a small plastic tracking tunnel, I slide out the tracking card, careful not to get the thick black ink on my fingers. There’s a smattering of fine dots over most of the card—ground-wētā tracks. And one skittery line of long-toed footprints—a McCann’s or southern grass skink. No mouse prints.

I’m at my local sanctuary, Mokomoko Dryland Sanctuary, 14 hectares of schist-bluffed hillside behind Alexandra. It’s a classic Central Otago autumn day, crisp and blue.

I’m checking inside the perimeter fence for signs of mice, as I’ve done every week for the past four months. The pest-proof fence protects a community of threatened lizard species—Otago skinks, grand skinks and jewelled geckos. It’s a seriously engineered piece of defence, complete with a one-metre skirting under the soil, a hood on top, and specially designed culverts and entry gates.

Back in 2009, before the Central Otago Ecological Trust built the full 14-hectare sanctuary, it built a 0.3-hectare fenced area as a pilot study. The Otago skinks inside the small fence were doing well, until mice got in. “Two Dutch students watched a mouse, in broad daylight, attack an Otago skink and make it bleed. And that’s a 30-centimetre-long lizard,” explains trust chair Grant Norbury, an ecologist with Manaaki Whenua.

A mouse eats a southern grass skink on the Otago Peninsula. Aotearoa’s lizards haven’t evolved to evade mammals like mice, which hunt by scent and are small enough to fit into lizards’ retreats. “Lizards are basically just little protein popsicles for small predators like mice,” says Reardon.

Norbury and the students kept track of the Otago skinks in the small fenced area, and at the end of the study, after six months, only a third were left. It was obvious that the trust had to use the most mouse-proof design they could find for the large fence. But, despite the best engineering, mice still occasionally get in—as they do at all of New Zealand’s fenced sanctuaries.

When mice do get in, a highly effective poison called brodifacoum is used to eliminate them. But brodifacoum comes with a hitch: once it’s been used in an environment, it’s there to stay. For a long time. The word scientists use is “bioaccumulation”, meaning it builds up in organisms and ecosystems over time, rather than degrading like some other toxins. Small amounts of brodifacoum move around the food chain, accumulating in the soil, the insects, the small birds, the kārearea that occasionally swoops over me while I work. And when enough builds up in an animal’s tissues, it dies.

That’s why the Department of Conservation (DOC) recommends that brodifacoum is used only twice in the lifespan of the longest-living animal in the ecosystem—and the sanctuary’s geckos can live more than 50 years. So, while brodifacoum works well for island eradications, it can’t be used again and again in our mountains and forests, or in our fenced sanctuaries. We need another option.

[Chapter Break]

When I call Rod Morris early one morning in May, he tells me there’s frost on the ground in Dunedin. “The mice will be out, though,” he says. “The thing about mice is that, if they’ve got food, they can deal with cold. They can live in a deep freezer all their lives, eating the frozen food.”

Morris, a wildlife photographer who’s been involved in New Zealand conservation for six decades, is concerned that we’re underestimating what an enemy the mouse is. “Because they’re tiny, we think they’re fragile. But, in fact, they’re so small that they cannot afford to hibernate—they have to remain active. And they seem to be pre-adapted to deal with bitter cold.”

Fur samples are checked under a fluorescent microscope. Any mice that have crossed the boundaries of the study area will show a tell-tale pink trace of Rhodamine B, picked up from Rhodamine-laced bait around the study perimeter.

This means mice can survive at very high altitudes. Morris tells a story from his time in the Wildlife Service in Fiordland, when a food drop from a plane missed his team’s hut and landed on a ridge above them. “The weather was bad, and by the time we got to it a couple of days later, mice were already there. They’d found that food in the middle of winter in thick snow up at 1450 metres.”

The alpine zone is home to some of our rare large invertebrates, like giant wētā and giant leaf-veined slugs. It’s also where many of our most threatened geckos and skinks live, and where new species of lizard are still being discovered.

It’s thought that many of these species used to live in the lowlands as well, but are now surviving only on our mountaintops, due to what Morris describes as the “rising tide of mammalian predators” that have gradually swamped our backcountry. A tide that, with climate change, is moving higher.

[Chapter Break]

One of the things that makes mice so incredibly successful is that, when there’s lots of food available, their population explodes. Mice are breeding machines. Female mice can become pregnant at six weeks old, and it takes them just three weeks to produce a litter of six mice.

For Morris, the mouse problem was recently highlighted in his own backyard. Twenty years ago, he created a skink garden at his Otago Peninsula home by stacking rocks and planting wiry creeping shrubs. Southern grass skinks moved in and thrived. Then, several years ago, he added silver tussock, a plant skinks love, to the garden. The skinks did even better, and last summer, on his way to the clothesline, he could count 13 pregnant skinks out basking. But that summer, the tussock masted—it seeded heavily. Then the mice came.

“And I don’t believe we’ve got any skinks left,” says Morris. There have been none out basking for several weeks. “I’ve been finding little skink skeletons in the tussock—skinks that have been totally eaten away
by mice.”

Vicki Meyers and Nils Fleishman check the Ōtamahua/Quail Island traplines. Placing traps every 15 metres provides a detailed picture of mouse movements, and the map shows that mice are more mobile than we’d realised. “While most mice are moving about 30 metres, two per cent are moving really huge distances, up to 800 metres in 24 hours,” says Reardon.
When there’s plenty of food available, mice can have a litter of ten or so young every month. “If you do the sums, two mice can turn into thousands very quickly,” says Reardon. In native tussock and forests, a food boom happens every few years when these species mast, but in exotic grasses it happens at the end of every summer.

In an experiment designed to test how quickly a mouse population can grow, a pair of mice were released onto 650-metre-long Te Haupa/Saddle Island in the inner Hauraki Gulf, which had previously been cleared of mice. Within five months, there were just as many mice on the island as there had been before the eradication.

When tussock, beech or rimu forest mast—something that happens every three or four years—mice keep breeding through the winter. At a time of year when their numbers would usually drop, mice instead soar to plague proportions. Meanwhile, over winter, alpine geckos, skinks and big invertebrates are dormant. “The mice find these animals hibernating and resting, and they just gnaw away at them,” says Morris.

And, while mice can increase their populations dramatically, our lizards can’t. Perhaps because of our cool climate and our lack of mammal predators, Aotearoa’s lizards have evolved to do things more slowly than their tropical cousins, including breeding. The harlequin gecko, which lives on Rakiura/Stewart Island, has just one or two babies after what scientists think is a 24-month pregnancy—and it’s likely that they don’t start reproducing until they’re teenagers.

Our lizard populations just can’t stand up to the threat that mice pose—but do we really care enough to do something about it?

[Chapter Break]

In 2016, New Zealand announced a lofty goal to become predator free by 2050. The vision took shape as a bid to eradicate possums, stoats and rats from all of New Zealand, and it’s focusing a great deal of attention, energy and money on predator control. But scientists fear there could be some unintended consequences.

“It’s well known that when ship rats are removed, mice increase,” says Innes, the rodent ecologist. “The problem’s going to get bigger rather than smaller because of what mice do when you take all other predators away.”

This isn’t because rats have been killing large numbers of mice. Rather, it seems that when you get rid of rats, the behaviour of mice changes. “Basically, when you remove rats, mice feel much happier about going foraging in the open,” says Innes. Mice are able to spend more time hunting for a wider variety of food, and this allows their populations to soar. Mice begin to leave the forest floor and climb trees to forage in the canopy, an area that ship rats usually keep for themselves.

“To be honest, I’m just waiting until this erupts,” says Innes. “It’s a bigger priority than people are currently acknowledging.”

Dan Tompkins, Predator Free 2050’s science director, says the plan is to keep an eye on mice. “We recognise that there may be some cases where mouse numbers might go up. Where that’s a risk, as in the case of rat control, people will be looking out for mice numbers as well, and should the mouse numbers show any sign that they’re going up as a response, then they’ll know and can do something about it,” he says.

Given the current limitations of our mouse-control tools, however, there is some uncertainty about how effective this intervention would be.

“We’re at a point with lizard conservation now that we were [at] with birds back in the 1960s,” says Lynn Adams, who leads DOC’s national lizard technical advisory group. “Back then, we were just beginning to understand who the primary predators were, and we didn’t have tools or toxins to control those species. So that’s the work we need to do, building our knowledge like we have since the 60s for our bird conservation.”

But within the current scope of Predator Free 2050, this isn’t a priority.

“Our best pest-control methods, as they stand, are just not good enough to protect lizards,” says Adams. “And so the absolute overarching priority for all our work is to get effective mouse-control tools and techniques developed.”

[Chapter Break]

Back on Ōtamahua/Quail Island, Reardon has received the results from the autumn phase of his study on cholecalciferol. They’re not encouraging. “In the first two months, the toxin got mouse numbers down to around two per hectare, which is exactly what we want,” says Reardon. “But what we’ve confirmed is that the toxin doesn’t keep them there.”

By December, mouse numbers were climbing again, despite the poisoned bait, back up to more than 20 per hectare. With all the long grass seeding and the abundant insect life, it seems the mice had so much natural food that they weren’t interested in the bait. “We’re not looking at a winning formula here on the island yet,” says Reardon. “Even an incredibly intensive baiting regime through the summer hasn’t knocked the population down to anything like the level they’d have to be to allow our native species to recover.”

The 4am start means the team can clear all the traps before the they get dangerously hot for the mice inside them.

The level that Reardon is referring to comes from research by Norbury and colleague Deb Wilson at Macraes, Central Otago, also due for publication in 2022. They looked at the relationship between mouse numbers and the presence of native lizards, wētā and other invertebrates. They found that, where mouse numbers were high, numbers of native skinks, geckos and invertebrates were very low. That wasn’t surprising. Where the results got interesting was that, where mouse numbers were moderate or even moderately low, there were still hardly any lizards and invertebrates. It was only where mouse numbers were knocked back to extremely low levels—where mice footprints were found in just five per cent of tracking tunnels—that a good number of lizards and invertebrates were found.

“It means that if we want to reduce mouse impacts, we need to get them down to very low levels,” says Norbury. “Otherwise we won’t see recovery of these small native fauna.”

In the autumn of 2021, Reardon also received some bad news. What was planned to be a multi-year research project—looking at long-term methods and trialling alternative poisons if needed—would now be funded only until October 2021. “It’s disappointing, but not altogether surprising,” says Reardon. “With the pandemic, and a rearranging of priorities, there’s not the same funding available. And this kind of biodiversity threat just isn’t front and centre of the decision-making processes.”

[Chapter Break]

Reardon fell in love with lizards when he was six—to him, they looked like tiny living dinosaurs. And when he was 29, he found himself exchanging his native Wales, a land with three species of native lizard, for Aotearoa.

But New Zealand is a strange kind of lizard-lover’s heaven. As a country we’re so focused on being a “land of birds” that there’s little conservation funding left to protect lizards, and their survival is increasingly threatened.

“I’ve talked to some people who go, ‘Well, yeah, but look, they’re still there, so what are you on about? They’re not all gone, are they?’” says Reardon. “But we’re finding pockets of 30 lizards here, 40 there, when they would have been in their uncountable millions before humans arrived.”

Until I became involved with Mokomoko Dryland Sanctuary two years ago, I was as herpetologically ignorant as the average New Zealander. But I’ve learned a lot. I’ve learned that we’re discovering new native lizard species at a rate of about two a year. I’ve learned that New Zealand has the oldest living geckos on the planet, recorded in their mid-50s. That we’ve got skinks as long as a man’s forearm, and geckos that live on mountaintops. That 40 per cent of our lizards are classified as “threatened” and a further 50 per cent as “at risk”. And I’ve learned how it feels to spend hours searching native shrubs for a perfectly camouflaged jewelled gecko. How the jolt of recognition when you finally see one—luminously green and perfectly patterned—is so compelling that if you didn’t have a family or a day job, you’d spend the next month searching for another one.

A mouse invades a rock wren nest near Mount Tutoko, Fiordland. Mice will eat almost anything: seeds, leaves, invertebrates, lizards, fish eggs, berries, and the eggs of small birds such as titipounamu/rifleman and kākāriki. They’re agile climbers, and have been found as high as 11 metres up trees.

Morris has spent a lot of time photographing our lesser-known fauna. “We have beautiful alpine leaf-veined slugs which no one’s looking at at the moment,” he says. “They’re very large, six or seven inches long [15 to 18 centimetres]. In Fiordland, they’re khaki colour with a bright yellow head, and in the Seaward Kaikōuras, I’ve seen black ones with white polka dots.

“Some of these animals don’t even have names yet. They would have been much more widespread in the past, but now the only people who ever see them are mountaineers. Climbers on the Shadowland wall in Fiordland knew all about them. They’d tell me, ‘Those bloody things. You go to put your hand on a handhold and it comes away with a fistful of slimy slug.’”

Since mice eat these lesser-known invertebrates and lizards—and not our large, charismatic birds—mouse control has not been a conservation priority. It took Reardon seven years just to gain the funding to begin investigating the mouse problem. (Every year since 2012 he has presented the case to DOC for mouse-management research—with greater and greater urgency.)

“There’s a real concern with things like our lizards and invertebrates that aren’t so visible in the landscape that they’re just winking out left and right and we don’t even know about it,” he says. “We know so little about these species and their susceptibility that we could lose them and not even know why. Which really bothers me.”

Morris agrees that we need to look more closely when we set our conservation priorities. “It’s the smaller things—seedlings, invertebrates, lizards—that play huge ecological roles in decomposition, regeneration, pollination and dispersal,” he says. “Mice feed on these smaller things. And that in turn is just weakening the whole fabric of the ecosystem, destroying it from the bottom up.

“But because we’re human and we think big is special and important, we’re totally blind to what is happening. Mice are actually gnawing the ecological rug out from under our feet.”

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