Rat-aclysm tests forest defences

The hills are alive with the sound of squeaking. Inside the Conservation Department’s battle against a record number of rodents

When Department of Conservation ranger Heath Sinclair and his team checked traps in the Landsborough Valley, in South Westland, in August, they were so alarmed by what they saw that, on their return, a flurry of emails was sent to see what more could be done.

Out of 280 traps, the trappers found 201 rats and 15 stoats. “That was a big number,” Sinclair says. A year earlier they’d found only 29 rats. The previous record was 109 rats, in about 2010.

“It’s concerning because it’s a lot of rats, and it makes the days a lot longer,” the DoC ranger says. The traps have to be scraped out, with carcasses flung into the forest, and reset with new bait such as eggs or a rabbit-based paste, called Erayz. “Every extra minute at a trap adds up over a day.”

Funding was secured for an extra trapping run last month, which netted 187 rats – still 78 more than the previous record – and three stoats. Another check is being made this week.

The Landsborough is arguably DoC’s highest profile predator control site, a 21-year study of intensive pest control to protect the native mohua, or yellowhead bird.

This year the valley’s one of the many fronts in DoC’s fight against introduced predators, which have exploded in numbers because of the so-called mega mast – a massive seeding and fruiting of various tree and plant species that provides a food bonanza for native creatures and predators alike.

Other battle areas include Arthur’s Pass, inland from Christchurch, which is experiencing a mouse infestation, and the Makarora Valley, near Wanaka, where record rat numbers are being reported. (Because mice are such prolific breeders, and their home range is so big, you couldn’t drop enough 1080 to put a dent in a mouse plague.)

Rodent numbers have also skyrocketed in the Manawatu’s Ruahine Range. Native species usually hit trouble when predator footprints in tracking tunnels hit about 20 percent. In August, overnight tracking reached 74 percent.

In response to the widespread explosion of rodent numbers, the department’s embarking on its biggest ever predator control programme, Tiakina Ngā Manu, which is expected to cost up to $41 million and aerial 1080 drops covering some 900,000 hectares, as well as ground trapping.

“Predator numbers are exceptional, pretty much across the board,” says Peter Morton, DoC’s predator control boss. Rodent numbers are so high that the area covered by 1080 drops has shrunk and the cereal bait application rate increased, to knock the numbers down.

Crunch time to stop munch time

Rats are significant predators of New Zealand’s wildlife. (With stoats and possums, they round out the so-called big three of predators.)

Morton says rats eat chicks, eggs, and in some cases even the adults, particularly when they’re on the nest. “Which is why this spring period that we’re in now is the crunch time for most of the sites. This is when we need to be protecting the nesting birds in particular.”

It gets worse. Rats are also the main food of stoats, which kill a lot of adult wildlife species, particularly females. If you have a whole lot of rodents – rats and mice – in the forest, that inevitably boosts stoat numbers.

“They come a little later,” Morton says of stoats. “They’ll start showing up from December onwards.

“It’s sort of a double hit, with rats causing damage right now and stoats about to rise and cause damage over the first half of next year.”

At 30 key sites DoC monitors for rodents, numbers have gone through the roof at all but two. At those two spots – Wet Jacket, on the West Coast of Fiordland National Park, and Blue Mountains, in Southland – the numbers are still bad, they just haven’t increased as rapidly as expected. Wet Jacket’s operation has been parked until next year, Morton says, while rodent numbers at Blue Mountains are just now reaching dangerous levels. “We’re actually triggering an operation there, hopefully this side of Christmas.”

The place Morton’s most worried about is Arthur’s Pass National Park, which has very high mouse numbers and concerningly high rat numbers. Aerial 1080 will be dropped over almost 100,000 hectares for two overlapping reasons – to protect the nationally critical orange-fronted parakeet (kākāriki karaka), and great spotted kiwi (roroa, the tallest of the five kiwi species).

Morton says DoC is “throwing the kitchen sink” at predators in areas still frequented by the few hundred remaining orange-fronted parakeet. Those places are the Hawdon, Andrews and Poulter Valleys, as well as the south branch of the Hurunui Valley in Lake Sumner Forest Park. From the air 1080 bait is being dropped, and ground control includes both traps and toxins. “That’s literally everything that we’ve got,” Morton says.

It has been a stuttering start for helicopter operators, contractors, and DoC staff, who have been ready to go since May. Morton: “First it was snow cover, and then just a succession of wind and rain and hail and fog – you name it. So it’s been really frustrating for them, but they’re in business now. They’ve covered 30,000ha of the total treatment area.”

Public (conservation) enemy number one, a rat. Photo: DoC

Early in the predator control programme, 1.4 million hectares of forest was identified as high priority for threatened species. The gradual whittling down of the treatment area means about half a million hectares – 6 percent of the conservation estate – has been left exposed.

Morton’s reluctant to name particular areas where native species are being overrun. “The places where no predator control is occurring, by and large there are high numbers of rodents, and there will be high numbers of stoats, and we will see damage.”

The programme is focused on the most vulnerable and most threatened species populations, he says. “We are running the biggest programme of predator control that we’ve ever taken on. We’re covering as much ground as we can and we’re targeting that towards the populations that are at greatest risk of disappearing.”

The mast response is the most challenging in DoC’s history. But thankfully they were prepared – the mega mast and the commensurate rise in rodents was predicted.

“It’s been a challenge but we’re actually going really well,” Morton says. “We are now getting good results, since we changed our methodology and with some breaks in the weather, particularly right now, we’re able to cover some ground.

“We’re more than halfway and we’re happy with the results that we’re seeing now from the places that we are able to protect.”

DoC ranger Sinclair, who’s trap checking in the Landsborough this week, said last week he was expecting to find similarly high rat numbers. That could mean some long days. He recalls checking the trap lines downstream of the Upper Gates Gorge in October. “Almost every trap in that gorge had something in it.”

So, does he reckon there’ll be well over 100 rats this time? “I’d think so,” Sinclair says, “but we’ll have to wait and see.”

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