Moa's ark? Uncovering 'the Great Australianisation' of NZ

The gigantic Haast's eagle shared common ancestry with an Australian species - making it one of many examples of trans-Tasman bird pairings in our ancient history. Illustration / John Megahan

New Zealand has long have been thought of as a "moa's ark", where an abundance of species flourished and evolved in our isolated prehistoric wilderness.

Yet, in revealing why so many of our native birds have Aussie cousins, scientists have further up-ended the notion that we've always been cut off from the rest of the world's ancient life.

In a new paper, Dr Michael Knapp of Otago University's Department of Anatomy and colleagues investigated how it was that many Australian birds managed to establish themselves here between two million and 2.5 million years ago.

Earlier findings had shown New Zealand's massive, lost raptors – the Eyles' harrier and Haast's eagle – shared common ancestors with Australian species, but both evolved to reach incredible sizes over their two million years here.

Knapp began searching for other trans-Tasman bird pairs with a shared ancestor, only to find none had gone through the same super-sizing process.

"But as I was looking at the pairs I started wondering, why are there all of these New Zealand-Australian sister species with a common ancestor around two to 2.5 million years ago?" he said.

"Surely that cannot be a coincidence."

His colleague Dr Nic Rawlence, who was busy at the time discovering a bulky, near-flightless native black swan called the pouwa, had been struck by the same pattern.

"The more we looked, the more examples we found of divergences between unique New Zealand birds, and their close Australian cousins, dating to around the same time," Rawlence said.

Along with the two big raptors, they also found the ancestors of the New Zealand raven, South Island takahe and North Island takahe had arrived on our shores from Australia.

The crucial piece of the puzzle turned to be a unique factor highlighted by Canterbury Museum's Paul Scofield: most of these immigrants happened to be open land species.

Another study co-author, Matt McGlone of Manaaki Whenua-Landcare Research, helped the team understand more about what habitats there might have been at the time.

'THE GREAT AUSTRALIANISATION'

Prehistoric bird pairings with Australian species have also included the Takahe. Photo / Paul Nevin, DOC

"It turns out that before 2.7 million years ago, Aotearoa was largely forested, with forest adapted birds," Rawlence explained.

As the climate changed going into the ice ages, New Zealand became more open.

"Rather than the local forest adapted birds moving into open habitat, open habitat species from Australia colonised our shores, in what I like to call the 'Great Australianisation of New Zealand'."

By that point in time, he pointed out, Australia had been characterised by mostly open habitat for millions of years.

Knapp added that, as New Zealand's birds were generally poorly adapted to open habitats, their Aussie counterparts had a competitive advantage.

Moreover, their revelation opened the door to another.

When Polynesian settlers arrived in the country and began clearing forests, they happened to do the landscape what the ice ages had done millions of years before.

"Once again, new open land habitat was available, and the extinction of native New Zealand bird species, for example through introduced predators had once again opened niches for Australian species to settle in," Knapp said.

As had been shown by studies revealing how penguins and sea lions had moved in to replace other populations far back in our history, the study showed it wasn't only the ability to disperse to fresh habitats that allowed animals to colonise new areas.

"More importantly, it's the ability to establish," Rawlence said.

"With the current rate of habitat modification and climate change, this may have profound effects on what insects, plants and animals arrive in Aotearoa New Zealand and established, whether they are wanted or not."

Further, the fresh insights forced a re-think about our ancient habitats being geographically isolated from those across oceans.

"New Zealand was once considered a moa's ark, a land isolated and lost in time with a fauna and flora that could trace its ancestry in New Zealand back to the age of the dinosaurs," Knapp said.

"That has long been disproven, but we now see more and more how strongly influenced New Zealand's biota are by other Southern Hemisphere landmasses.

"It is fascinating to see how quickly species form Australia can respond to new habitats becoming available 2000km away, on the other side of the Tasman Sea."

REWRITING HISTORY

Rather than being an Aussie immigrant, DNA studies suggest our national bird the kiwi descended from a giant and extinct native of Madagascar, the elephant bird. Photo / Destination Rotorua

The new findings demonstrated how much our view of what characterised New Zealand's nature was merely a snapshot in time.

"Turn the clock back, and the picture changes significantly," Knapp said.

"Iconic New Zealand giants, like Haast's eagle and Eyles' harrier were tiny vagrants from Australia only two million years ago. The pukeko flew in only a few hundred years ago.

"New Zealand beeches, long thought to be a classic relic from the age of the dinosaurs, arrived long after New Zealand was isolated, eucalypts, which we think of as typically Australian today, were native to New Zealand just two million years ago.

"This illustration of how dynamic nature can be is fascinating for me."

Added Rawlence: "We like to think of our biodiversity as unique to New Zealand, but we are also a 'flypaper of the Pacific', home to many species that have arrived here from afar."

Did this mean that our national bird might also be an Aussie immigrant?

To the likely relief of many Kiwis, for now, the answer remained no.

Recent DNA studies suggested the kiwi's ancestor was, oddly, a giant and extinct native of Madagascar, the elephant bird.

The moa, meanwhile, has been found to be more closely related to South America's tinamous.

Scientists have concluded both the kiwi and moa separately evolved to become flightless after their ancestors flew here.

And although Australia might potentially be able to claim the Haast's eagle as theirs, Rawlence said that by the time humans arrived in New Zealand, "they were definitely well and truly Kiwi".

What more big surprises might turn up?

"I think there are potentially quite a lot more to come," Rawlence said.

"Only just recently was it shown that the ancestors of Adzebill were the tiny Madagascan woodrails and flufftails.

"We have only just scratched the surface of what there is to learn and are now moving onto some of the less 'sexy' New Zealand birds.

"With new sequencing technologies and ever improved DNA extraction techniques, there is a lot more to learn."