Conservation managers are bracing for the biggest seeding event in New Zealand's forests for more than 40 years. Forest seeding, or masting, provides a bonanza of food for native species but also fuels rodent and stoat plagues. The Department of Conservation (DoC) is now planning its biggest ever predator control programme, at a cost of $38 million. It will target rats, stoats and possums over about one million hectares or 12 per cent of conservation land. Priority sites include Kahurangi, Abel Tasman, Arthur's Pass, Westland, Mt Aspiring and Fiordland national parks, the Catlins and Whirinaki. More than 66,000ha will be covered with trapping – and the rest with aerial 1080 poison drops. Science reporter Jamie Morton spoke to DoC principal science advisor Dr Graeme Elliott about the challenge.

This event is being described as a mega-mast. How do you define one?

A mast is when trees like beech trees seed, and a mega-mast is one where this seeding is widespread across the country.

The climate drivers behind these events seem to be the difference between two summers.

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In this case, the difference was much larger than normal and this sends a bigger signal to the trees to seed.

Because we haven't got any records of how large mast events are, other than what's been collected in the last few years, all we can do is look at the climate data and extrapolate what we think happened over a period going all the way back to the mid-70s.

And we've never had a signal as big as this, so I think it will be the most significant and widespread mast in 45 years.

In these events, down in Fiordland you can get parakeets feeding during the winter when they wouldn't normally, which means you get a lot of them.

It would have been the same once for a whole lot of species that eat seeds, kaka and kea among them, as well as insectivorous birds like robins and tomtits which benefit from all of the insects that are also turning up.

So in prehistoric times, it would have been a boon for everybody.

And now that blessing has become a curse.

Yes, exactly. But the picture is kind of complicated.

While lowland forests aren't masting so much any more, and so tend to have a reasonable supply of seed all year round, the forests in upland areas mast and have these great spikes of seed.

In a funny kind of way, a lot of species have fared better in those areas in the uplands, because the numbers of rats and stoats aren't continuously high – they're just high during these spikes.

So how has DoC been preparing for this one?

We've had a kind of staged approach over the last few years.

With the climate triggers that we could see, going back to last March, we knew then that it was likely to happen, so we started planning for it.

But we had to wait for the next real stage before we could be sure – and that was when the beech trees began flowering.

We were getting reports from people all over New Zealand last spring, which really told us it was going to be a bad year.

We had another kind of heads-up in February, when we flew around the country to take samples of the seeds just as they started falling.

By doing that we can actually get a measure of the intensity of the seed fall; in this case, the climate predictions were all true, and yes, it was clear it was a whopper.

Beech forest seeding, or masting, provides a bonanza of food for native species but also fuels rodent and stoat plagues. Photo / File
Beech forest seeding, or masting, provides a bonanza of food for native species but also fuels rodent and stoat plagues. Photo / File

This must be quite worrying to conservation managers who are personally invested in these ecosystems about to be hit.

For anyone with any kind of interest in forest wildlife, this is a worry, yeah.

We can look after some of the wildlife in our forests but we're not going to be able to do the whole lot – and we accept that we're going to take a bit of a hit this year.

Does that mean some of the gains we've made over the last decade or so will be undone?

Well, yes and no.

We've had this notion that we can't look after everything, so we look at some representative spots around the country – and these will be up for getting some predator control this summer.

But we imagine that most sites we have been looking after for a while, even with a lot of pest control, aren't going to be having a good year - although they shouldn't slide too far backwards.

At sites where we haven't been doing anything – and that's most of the country – they will take a pasting.

So we can expect to see some local extinctions of some things at some places because of the mast.

What particular species are we talking about here?

The most obvious species that will be affected are orange fronted parakeet, which live in the valleys in north Canterbury, and are incredibly rare and susceptible to this type of thing.

They are a top priority for all of the pest control that is going on over the next few months and they will be well looked after.

Next down the rank are mohua, or yellowheads, in the South Island.

They are looked after at intensively managed sites and most of them will be okay, but there are some areas they're in that haven't been prioritised highly enough and won't be getting any special protection this year.

It's a similar story for long and short-tailed bats and rock wrens.

Some mohua (yellowhead) sites are particularly vulnerable this year. Photo / DoC
Some mohua (yellowhead) sites are particularly vulnerable this year. Photo / DoC

Then there's a second tier that's largely made up of larger birds that aren't quite as at risk because they are a bit more resilient – that's kaka, kea, whio and kiwi.

There isn't any great risk of local extinctions for them this year, but they'll still take a hit.

Can we expect to see more of these big mast years under climate change?

Actually, climate change doesn't change the story much because the inter-annual increase in temperature is very small and nowhere near enough to trigger these events.

You've got to have quite a big temperature difference between summers to trigger one of these and the increase you are getting because of global warming isn't that big and doesn't do it.

But there's another story around climate change and that is that it's making the climate more variable.

That might mean an increased likelihood of one summer being a lot warmer than the previous one - but the climate scientists aren't telling us that this is likely.

So, at the moment there is no strong evidence suggesting these things are going to become more common as a result of global warming.

Yet climate change is obviously still bad for our threatened species.

Yes. We have a lot of species that are kind of refugees in the high country, because they can't live in the low country because of all of the rats.

Under climate change, these safer, higher areas will become fewer and fewer, because the rats and stoats will be able to move uphill.

So perhaps the biggest effect of climate change will be that high altitude refuges will begin to disappear.

DoC expects larger birds such as whio (pictured), kaka, kea and kiwi will be affected, but not as severely as other at-risk species. Photo / DoC
DoC expects larger birds such as whio (pictured), kaka, kea and kiwi will be affected, but not as severely as other at-risk species. Photo / DoC

You can imagine a poor little thing like a rock wren that lives at the top of the mountain is going to have to be living actually above the top of the mountain – and that's not going to work.

We saw a big spike in anti-1080 sentiment last year. With the 1080 response that this mega-mast is going to require, does that mean DoC is going to have to be proactive about getting out there and countering this stuff with public awareness and education campaigns?

Yes, with an operation like this, I'm sure there will be a big need for public awareness, so there will be a campaign to make sure everyone knows about it and to hopefully make people think that it's a good idea.