Science of a 'mega mast' & planning wide-scale predator control

A widespread ‘mega-mast’ mass seeding event is likely to lead to a large plague of rats and stoats in New Zealand southern beech forests.

In response, the Department of Conservation (DOC) is aiming to control predators over an unprecedented one million hectares of conservation land, to protect vulnerable native species such as kaka, whio, mohua and orange-fronted parakeets.

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The summer of 2019 has been a mast year, when trees across large areas of forest synchronously flower en masse. The prolific fruits and seeds that follow have triggered good breeding seasons for many birds, from kea to orange-fronted parakeets, kaka and mohua.

The 2019 mega- mast year has been particularly striking in southern beech forests.

The five species of southern beech trees dominate 2 million hectares of forest in New Zealand, and they are a significant feature of a further 2 million hectares.

But, to mangle a metaphor, this silver cloud has a very dark lining. Because when southern beech forest has a big mast seeding year, it is both very good and very bad news for native birds.

Introduced mice and rats feed on the seeds, breed well, and quickly reach plague proportions. Rats are significant predators of eggs and chicks.

The rodents are also food for stoats, fuelling a further plague of mustelids, which go on to prey heavily on native birds once rodent numbers fall away. The main impact of rats and stoats on birds happens in the spring and summer following the mast seeding, when the seeds have either geminated or been eaten.

Previous mast events have led to the local extinctions of populations of mohua and orange-fronted parakeets.

DOC has known for about 15 months that 2019 would see large mast seeding event across a wide area. This is because these events are triggered by climatic events, such as summer temperatures and rainfall, which allows a mast to be predicted.

An ecological model called delta-T (where delta = difference, and T = temperature) has proved to be an accurate predictor of mast seeding years in Chionochloa tussocks as well as southern beech forests. A cool summer in year 1, followed by a warmer than average summer in year 2, will lead to a mast event in year 3.

Beech seeds are very small, but DOC ecologist Clayson Howell says very large numbers are produced in a mast year.

“In the order of five or six thousand [beech] seeds per square metre is what we would be expecting this year,” says Clayson. “In really big years it can be up to 15,000 seeds per square metres – and in some years it can be zero.”

There are 70 sites around New Zealand, in different beech forest types, where seed fall is measured each year. This information is put into a large database and is part of a complex ecological model to try and understand what factors are important in mast years.

“We use that data to explain the seed patterns that have happened in the past, in order to predict what happens in the future,” says Clayson.

Work by Landcare Research scientists looking at 40 years of data has shown that most mast years are localised and cover less than 10 percent of the beech forests.

A mega-mast is one that affects more than half of the beech forests, and over a 25-year period there has been an average of 5.2 mega-masts. The years 1979, 1982, 1995, 1999, 2008, 2014 and 2016 were all mega-mast years.

The West Coast of the South Island is particularly prone to mast seeding events, with one occurring on average every three years. In most other places, they occur every four to five years.

Will climate change cause more frequent mast years? Models suggests ‘no’

An apparent increase in the frequency of mast years has led to concerns that they may become more frequent or more widespread as climate change increases average temperatures.

In the original delta-T scientific paper, published in 2013, lead author Dave Kelly, a botany professor at the University of Canterbury, whose long term work on mass flowering led to the creation of the delta-T model, wrote that warming temperatures will not cause more frequent mast years. This is because the delta-T model reflects a relative temperature difference between two years, rather than absolute temperatures.

Further modelling work on the effect of increasing temperatures on the frequency of mast seeding years has found no significant effect.

In a 2016 Climate Change Impact report by Barron et al, for example, the authors modelled various ICPP climate scenarios (RCPs) and concluded that “in the long-term (to 2100) the frequency of mega-masts is not predicted to increase significantly under any RCP scenario.”

Beech trees can’t seed heavily every year

Clayson Howell says it may be that trees will not have sufficient resources such as energy stores to flower more frequently than they currently do.

“I don’t think that trees have particularly good memories. I don’t they’re assessing the [temperature] of one year against the next,” says Clayson. “I think it’s more likely to be a threshold of temperature that allows them to build up big enough reserves over a summer.”

Clayson points out that it is not just temperature that trees respond to. “There are fertility gradients and there’s different amounts of rainfall.”

Tiakina Nga Manu – Battle for our Birds

DOC began carrying out large scale predator control operations using ground control, such as bait and traps, as well as aerial 1080, in response to large mast years in 2014, 2016 and 2017.

These operations are branded Tiakina Nga Manu – Battle for our Birds, and focus on protecting populations of rare, vulnerable species such as rock wrens, orange-fronted parakeet, mohua, and long-tailed and short-tailed bats.

Previous Battle for our Birds programmes have been very effective in controlling pests and boosting bird numbers.

Peter Morton co-ordinates the Department of Conservation’s predator control programme. He is currently planning the department’s largest response to date, which will cover up to one million hectares of forest. In 2016, they treated 840,000 hectares.

“This one’s looking like a doozy,” he says. “As soon as we got through the 2018 summer we had warning bells ringing that ‘hello – it looks like we’ve got a mast coming’.”

Following the mass flowering seen this summer, DOC staff have been using helicopters to collect branches of beech trees from likely problem areas to see how heavy the seed set has been.

“So far everywhere they’ve looked they’ve found extremely high number of seeds being produced,” says Peter.

They are also using rat tracking tunnels on the forest floor to find out how many rats there are in different forests.

Peter says that evidence collected so far this year suggests that about 1.4 million hectares of vulnerable conservation land will have significant seed fall and predator plagues.

As the mast progresses, Peter and his team will begin to prioritise which areas are most in need of predator control, and when the best time is to treat each one.

More money is not the simple answer

Forest and Bird has been in the news recently saying the DOC needs more money so it can apply Tiakina Nga Manu across a larger area than one million hectares.

But Peter says the limitation he faces is not just funding.

“We are aiming to pretty much run at the limits of New Zealand’s current predator-control capacity.

“Money is one of those constraints, but we also actually run out of skilled people to do this work. There’s a pool of contractors out there who are experts in this field, but there’s only so many of them. There’s only so many skilled helicopter pilots able to do this work.”

Peter says that time is another significant constraint.

“A lot of the impact we are trying to stop occurs through the spring and summer months, when rats and stoats, in particular, are busy chewing their way through birds’ eggs and nestlings. So if we don’t control those pests prior to roughly the end of this year, it’s too late.”

Triage

Peter says that deciding where to focus aerial 1080 predator control is essentially triage. He says DOC knows where the most vulnerable populations are and has to gauge how much pressure they will be facing from rats and stoats.

“it’s pretty straightforward this year,” he says. “Basically the pressure is severe everywhere that we are trying to protect … so we target control to the populations that are most at risk of severe decline or even local extinction.”

1080 and traps

Peter says aerial 1080 is the most effective predator control tool DOC has. It kills rats and mice directly when they eat baits, while stoats die from secondary poisoning after eating rodents that have consumed baits. It is very cost-effective and can be used over large areas of remote and difficult-to-access backcountry.

Peter says that traps are effective in some places, and especially for ongoing ground control. While self resetting traps can catch multiple animals without needing to be checked, the limitation of single-set traps in a large mast year is that predators quickly overwhelm them and it is difficult to keep them clear and set ready to catch the next animal.

Find out more

Listen to the audio to hear about long-term seed collection in the Orongorongo Valley, modelling beech mast events, and DOC’s Tiakina Nga Manu predator control programme.

The science of 1080 use in conservation.