1 Sep 2021

The Lonely Islands: How NZ evolved in isolation

From Nine To Noon, 11:30 am on 1 September 2021

One million years ago, Aotearoa was a land with nine species of moa, many tiny frogs and flightless wrens rustling around the forest floor. 

In his new book The Lonely Islands, nature writer Terry Thomsen explores New Zealand's pre-human backstory and how our plants and animals evolved in isolation.

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 Terry Thomsen Photo: Supplied

Thomsen begins The Lonely Islands with a paradox about New Zealand that left 19th-century scientists mystified.

"New Zealand has this unusual combination of relics of [flora and fauna] that exist back from when New Zealand was joined to Pangea and Gondwana.

"But at the same time, it also has organisms that are just typical of remote ... islands which developed traits like flightlessness and so on."

We now know how Aotearoa managed to retain relics of Pangean times and avoid visitation by placental mammals and marsupials thanks to plate tectonic theory, Thomsen says.

"It was established that Zealandia, of which New Zealand is the southern part of, via a process called seafloor spreading, it split away from what was then the Australian and Antarctic parts of Gondwana.

"Before that process even began ... ocean waters were invading that zone between New Zealand and Australia and Antarctica before the physical separation."

The physical separation of New Zealand was probably complete 80 million years ago, even though seafloor spreading was still underway, Thomsen says.

"That's kind of the point from which anything that got to New Zealand could only have got here by somehow coming across the ocean."

In Aotearoa's first 15 million years of isolation, the New Zealand environment was very much like both Australia and Antarctica, he says.

Then, 66 million years ago, the mass extinction of dinosaurs changed everything forever.

"[When dinosaurs disappeared] the aforementioned placental mammals and marsupials and so on, they became the new carnivores and herbivores ... but New Zealand never got those animals, so instead what managed to get to New Zealand were the birds.

"Our ecology, especially our vertebrate ecology, was built around these bird arrivals, so they were the ones that became the main herbivores and carnivores."

About two million years ago, New Zealand suffered a tremendous loss of floral diversity and fauna during the Ice Ages.

"A lot of plant life would've basically slowly migrated north to more equable climates, but they could only go so far ... so basically anything that couldn't withstand life in the north of New Zealand, because it was still cold there, just became extinct.

"We lost dozens of genera of plants in that time, we used to have eucalyptus ... we probably had hundreds of species of proteas, we've only got two now."

If you were to set foot in New Zealand back then you'd have first noticed the dominance of birds and forestland, Thomsen says.

"There nine species [of moa], and each of the species was doing its own thing. The big ones, Dinornis, were specialising in eating from the foliage from the lower trees and shrubs and so on, then there were birds like rails, terrestrial ones which were just foraging around in vegetation, eating the low plants.

"Not just in daylight but in the night-time as well, a lot of these animals were nocturnal.

"And the little frogs which are so rare now, these previous little relics from the Pangea times, the Leiopelma frogs, they must've been so abundant ... far more lizards than we would see now, and the short-tailed bat."

On the forest floor, native flightless wrens would have been rustling around, Thomsen says.

The last sighting of a flightless wren - before their extinction - was by a lighthouse keeper on Stephens Island in Cook Strait.

"The only description of the behaviour of that last flightless wren was ... like a mouse going around. So there would've been four types of flightless wren scurrying around."