Rat plague puts native species at risk

UNDER THREAT: Bush robins, like this one spotted in the Lewis Pass National Reserve, could become easy targets for predators.
UNDER THREAT: Bush robins, like this one spotted in the Lewis Pass National Reserve, could become easy targets for predators.

New Zealand's beech forests could become like a scene from The Pied Piper of Hamelin with a population explosion of ship rats predicted this year, a senior Department of Conservation ranger says.

The population boom has the potential to wipe out local populations of vulnerable species, including mohua, long-tailed bats and kakariki.

"You'll be able to come here and walk around and you'll see rats running across the road, you'll see rats running through the forest in broad daylight, let alone in the evenings," said Department of Conservation senior ranger Brad Edwards.

"Effectively they are going to cover the whole valley. They will chew the milking cups out of the dairy sheds, they will chew the electrics out of people's cars in their carports, they'll be trying to get into the houses and they will be doing all that and more in the bush."

Climatic conditions over the past two winters - a cooler one followed by a warmer one - appear to have triggered the onset of a bumper crop of seed, or beech mast.

The seed acted as "carbohydrate bombs" for predators, provoking a population boom of rats and stoats.

Ranger Joris Tinnemans spends about six weeks annually shooting beech tree branches to check seed pod numbers.

On Monday, Tinnemans checked red beech trees in Lewis Pass National Reserve. From one sample he counted 11 seed pods - in previous years this number was usually fewer than three. "This is a lot of seed. This will definitely get the rats and mice going."

Early predictions suggest 1 million tonnes of seed will drop in autumn, triggering a plague of about 30 million rats and tens and thousands of stoats nationally.

When the seed runs out, the predators will turn on the wildlife.

DOC monitors rodent numbers with tracking tunnels. There are just 400 in the Lewis Pass National Reserve.

If the tunnels sit at 10 per cent activity then there is some predation going on, but history showed if there was a population eruption this increased very quickly.

At 30 per cent there was likely to be significant damage, particularly to hole-resting species including mohua. as ship rats climbed.

Edwards predicted there would be a massive growth curve of rodent numbers, maybe even up to 100 per cent - at which point the population would quickly eat all the seed so would turn to wildlife.

"They [wildlife] are living with a peak number of predators in their ecosystem and food is scarce and they are on the menu."

Previous predator population explosions had led to isolated extinction of mohua on Mt Stokes.

Edwards feared this could happen again to other vulnerable and threatened species, including long-tailed bats and kaka.

Nothing could be done to prevent the mast, but DOC would be using 1080 to target pest species later in the year.

"It's more effective and it provides us with more assured outcomes during these really big plague events."

Edwards believed there was a moral responsibility to protect animals and ecosystems for future generations.

He said timing of the 1080 drop was critical as too early and it was not effective enough and too late meant too many birds would have been lost already.

When 1080 would be dropped would vary between forests.

The Press