Forest and Bird is warning of a plague of bird-killing pests in forests this year, fuelled by what it's described as the biggest seed-spreading "megamast" in nearly half a century.

The group has just called on the Government to urgently pump an extra $20m into the Department of Conservation (DOC), to help stop species being wiped out in parts of the country.

Forest and Bird based its prediction on climate data which it said indicated masting across 90 per cent of New Zealand's beech forests, which provided remote havens for native birds.

DOC, which was gearing up for its biggest-ever predator control operation, couldn't confirm the projection but said it was expecting a "widespread mast".

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In a mast years, trees produced an extremely heavy flowering and seeding.

Historically, this would trigger an abundance of food for native wildlife to make up for lean years.

But now mast events boosted rodent numbers - and in turn stoat numbers.

When the seed was gone, the plague of predators turned to our native birds, bats, lizards and insects.

Each year, rats, stoats and possums already killed an estimated 25 million native birds – many of which were threatened or endangered.

In beech trees, mast seeding kicked off when the average summer temperature was more than a degree higher than the average temperature of the preceding summer.

Using the Niwa temperature data sets going back four decades, Forest and Bird said it was possible to estimate the intensity and extent of previous mast events.

These showed that there had been nine significant mast events since 1974.

This year's event was dubbed a "megamast" because both beech and podocarp forests – which included kahikatea, rimu, tōtara, matai and miro - were masting at the same time across most of the country.

Forest and Bird has predicted masting across 90 per cent of New Zealand's beech forests, which provide remote havens for native birds. Photo / File
Forest and Bird has predicted masting across 90 per cent of New Zealand's beech forests, which provide remote havens for native birds. Photo / File

Forest and Bird's chief executive Kevin Hague said DOC urgently needed extra money in this year's Budget to deal with what was "a particularly severe event".

"It is significantly bigger than the recent mast events that DOC has responded to," he said.

"Most of our conservation forests will get hammered without predator control. Their birds, bats, lizards, and insects will be decimated. There will be nowhere that will be safe.

"In many places, years of hard work by community trapping groups working to save local species and allow the reintroduction of previously lost vulnerable species will be set back."

Hague said it was known that timely 1080 operations could control introduced predators and protect vast, remote, and rugged areas from localised extinctions.

"But DOC doesn't currently have enough funding in its budget to protect even its 'top priority' list of sites, let alone the majority of the country's conservation forests, from the impacts of a mast event of this scale," he said.

"We calculate that DOC needs at least double the existing $20m in this year's predator control budget to protect our most vulnerable native species."

He pointed out that in the mast of 2001, some populations of critically endangered species were wiped out.

The only known population of mohua, or yellowhead, north of Canterbury disappeared at Mt Stokes, in the Marlborough Sounds.

In a 2001 mast, the only known population of mohua, or yellowhead, north of Canterbury disappeared at Mt Stokes, in the Marlborough Sounds. Photo / DOC
In a 2001 mast, the only known population of mohua, or yellowhead, north of Canterbury disappeared at Mt Stokes, in the Marlborough Sounds. Photo / DOC

"DOC deployed its best practice ground-based trapping regime to protect these mohua, but they were overwhelmed by the plague of rats and stoats following that year's mast event," he said.

"If DOC doesn't receive extra funding to increase its capacity to respond, we will see more of these localised extinctions."

'Looking like a big mast'

In previous mast events in 2014 and 2016, DOC received emergency funding of $20m as part of the ongoing Battle for our Birds programme.

This funding was "baselined" in the 2018/19 Budget, with DOC receiving an extra $81m over four years for predator control.

This year's mast event was expected to be significantly more widespread than 2014 and 2016.

The Herald has approached Conservation Minister Eugenie Sage for comment.

DOC threats director Amber Bill confirmed a widespread mast was on the cards.

"Results from our beech and rimu seed sampling work are still being analysed, but it's looking like a big mast, especially in South Island beech forests," she said.

This year, DOC planned to carry out predator control over an area of around a million hectares – the previous largest programme was 840,000ha in 2016.

This year, DOC planned to carry out predator control over an area of around a million hectares - the previous largest programme was 840,000ha in 2016. Photo / File
This year, DOC planned to carry out predator control over an area of around a million hectares - the previous largest programme was 840,000ha in 2016. Photo / File

In 2014, 2016 and 2017, DOC undertook predator control over 600,000ha or more, due to significant beech masts, while in 2015 and 2018 the programmes were much smaller, covering about 200,000ha.

Its predator control programme for this year was estimated to cost about $38 million.

That went toward controlling pests, as well as monitoring the benefits for threatened species populations and research to improve tools and techniques.

Bill said the extra funding in last year's Budget had already enabled DOC to expand its programme.

"Our four-year plan proposes a ramp up in areas under sustained predator control— where predators are kept to low levels on an ongoing basis—from the current 800,000ha to 1.85 million ha, or 20 per cent of conservation land, by 2022."