'Trapping just doesn't cut it': Native species under threat as rat population explodes

Sunday, 23 June 2019

Conservationists warn some species could be lost in a rat population boom. (First published 18/06/19)

A booming rat population could mean extinction for some native species - and conservationists predict the situation will only get worse.

A mega mast, or bumper crop of forest seeds, is providing a bonanza of food for rodents. In response the Department of Conservation (DOC) is planning its largest-ever predator control programme, at a cost of $38 million, to suppress rats, stoats and possums over about one million hectares.

Forest and Bird top of the South Island regional manager Debs Martin said this year twice as many rats are being recorded in trackers compared with a mast year in 2014 - and she fears for the survival of the long-tailed bats in the Pelorus area.

'We're catching hundreds of rats in our traps at the moment, but we've still got so many rats running around.

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'In a year like this, trapping just doesn't cut it.'

Two mega mast years, in 1995 and 1999, wiped out the Marlborough population of mohua (yellowhead) bird, despite years of work to increase their numbers.

Reports of a rat invasion in Auckland's Titirangi village and rats as large as cats have already been blamed on this year's hot summer, but Forest and Bird is warning conservation groups nationwide need to up their game in the coming months.

The mega mast year has created an abundance of food for native species but also rodent and stoat populations.

Martin said traps were 'overwhelmed' by the amount of rats, even with about 30 volunteers on the ground.

Forest and Bird regional manager Debs Martin says this year traps are 'overwhelmed' by the amount of rats, and even the self-resetting traps are under pressure.

In the 2014 mast year, 35 per cent of rat trackers had rat footprints. This year, it was at 66 per cent - well over the amount which could wipe out a species.

'At 15 per cent, a lot of species will be able to be OK, but when it gets over 30 per cent, that's when you find extinctions.'

Long-tailed bats, which Martin is working to protect, are considered vulnerable with decreasing populations. In a year with this many pests, species could be lost, she said.

The Pelorus Bridge Scenic Reserve is home to one of the last remaining populations of long-tailed bats.

Predator Free NZ general manager Jessi Morgan said in previous mast events there have been localised extinctions of certain species, even when there were existing predator controls.

Department of Conservation has called this year the biggest beech mast for more than 40 years.

'We're seeing conservation groups with their highest number of catches in years.'

Rat numbers are expected to increase even more in spring, Morgan said.

Forest and Bird chief conservation advisor Kevin Hackwell said the mega mast this year was significant, and with climate change, they'd keep on coming.

Hackwell said during two mega mast years, in 1995 and 1999, the Marlborough population of Mohua (yellowhead) bird were wiped out despite years of work to bring their population up from five to about 85 birds.

The 1999 event saw rats and stoats outnumber the trapping effort, he said. 'All those decades of work were for nothing.'

Hackwell urged conservation groups to check traps and re-bait more frequently in the coming months.

'The key thing we want to get across to people is their good work is potentially threatened.'

Aerial drops of 1080 were the only tool currently available to effectively knock down rodents over large areas before they reached plague levels after abeech mast, DOC said.