Sisters' secret is out

Geologist Hamish Campbell surveys one of the Chatham Islands' characteristic flat-topped outcrops at the Sisters. Picture / Richard Robinson

On one of the tiny rocks called the Sisters, north of the main Chatham Islands, scientists found two kowhai seeds. The find, last week, was surprising because kowhai trees are rare in the Chathams. But seabirds are abundant, and the kowhai seeds were found between nests of the northern royal albatross.

Massey University biologist Steve Trewick believes the seeds were probably spotted by an albatross at sea, swallowed, and regurgitated on the Sisters, along with fish caught on the bird's foraging expedition to feed its chick.

On this remote rock stack in the ocean Trewick had stumbled on a crucial piece of evidence in a 150-year-old scientific debate which still rages. It was the kowhai's plant type, Sophora (formerly Edwardsia), whose widespread distribution around the Southern Hemisphere inspired Charles Darwin to think about how plant and animal species might have spread around the globe.

Contemporaries, including his friend Joseph Hooker, believed the only way to explain such patterns was that there must once have been "land bridges" between what are now separate land masses, during ice ages when the sea level was lower than it is today. But Darwin believed in wind, ocean currents and seabirds such as the albatross. In 1857 he wrote to Hooker: "I believe you are afraid to send me a ripe Edwardsia pod for fear I should float it from N. Zealand to Chile!"

A century and a half later, Trewick and 21 other scientists have gone to the Chatham Islands, 850km east of the New Zealand mainland towards Chile, to look again at this historic argument.

Hooker's view has been refined by the revolution in geological understanding that came with the discovery, about 40 years ago, that the Earth's crust is broken into enormous plates, like jigsaw pieces, which rise out of mid-ocean fissures and spread, then dive under neighbouring plates.

Mainland New Zealand has been lifted out of the sea in the past five million years or so by the huge pressures created at a plate boundary. Tracing the movements of these plates, geologists have worked out that during the dinosaur era, about 200 to 100 million years ago, the separate lands of New Zealand, Australia, Antarctica, India, Africa and South America were bunched together in a super-continent, Gondwana.

Biologists in the Hooker tradition believe that between 100 and 50 million years ago, spreading plates pulled these lands apart. As they did so, remnants of Gondwanan plants and animals were trapped on each drifting land mass. Later, these ancient species evolved independently on the separate continents and islands into the diversity of plants and animals of today.

On the Chatham Islands, unique giant stag-beetles, a giant flightless click-beetle, a knobbly speargrass weevil and other insects led entomologists John Dugdale and Rowan Emberson to speculate in 1996 that there must once have been "a land connection with New Zealand, as all their relatives are flightless or poorly flighted".

But they were puzzled by what they called the "enigma" of the Chathams: "The fauna has southern and northern mainland elements and, although the presence of some flightless groups implies a land connection with the mainland, other groups one might expect to travel overland are absent."

Meanwhile, geologists were gathering evidence that may support Darwin's argument. Far from having drifted slowly eastwards, with some land above water, for 50 million years, the Chatham Islands may have re-emerged from the sea as recently as four million years ago.

The islands contain some of New Zealand's oldest rocks - schist that is about 160 million years old and formed part of the ancient Gondwana land mass - but on top of this ancient surface volcanoes sprouted from about 70 million to only 1.6 million years ago, including small ones that formed rock outcrops such as the Sisters.

On the Sisters, geologists Hamish Campbell and Chris Adams found xenoliths (meaning "foreign rocks") made of limestone, indicating the volcano there burst through a layer of soft limestone rock created by soil and dead plants and animals were washed off the ancient land into the sea to accumulate on the seabed.

The Sisters and other volcanic outcrops, such as Star Keys, appear from a distance to have flat tops - a classic sign that they were once levelled by the sea before being raised again.

The suggestion is that most, perhaps all, of the Chathams were under water as recently as two to four million years ago. In that case, the plants and animals found there today could not possibly be remnants of ancient Gondwanan species and must have arrived by air, sea or birds.

The 22 scientists who have been crawling all over the islands include geologists led by Campbell, and biologists led by Trewick. They were funded by an $870,000, three-year Marsden grant from the Royal Society of New Zealand. Their Chathams Emergent Ark research survey breaks new ground internationally.

Since Darwin's time, scientists have recognised plants and animals can cross narrow seas to colonise new land but no one has been able to show that plants and flightless animals could cross huge distances of ocean - such as that between New Zealand and the Chathams - without land bridges or human intervention.

Even in the nine years since Dugdale and Emberson's work, another scientific revolution, genetics, has given biologists a new set of tools to analyse the differences between species of living things.

Trewick says analysis shows that the plants and animals of the Chathams are genetically similar and in some cases identical to those on the New Zealand mainland, despite sometimes different appearances.

"There are communities of plants that are specific to the islands. Chathams forget-me-nots really do look different. The pigeons are a bit bigger," he says. "But a lot of the plant species are called a different species because they are on an island."

Christchurch Botanic Gardens' curator David Given says the Chathams' gigantic species of forget-me-nots, sowthistles and button daisies are genetically close to those in New Zealand and Chile, just much bigger, perhaps an effect of more sun and fewer browsing animals.

Even Emberson, who joined the expedition to the Sisters, says his earlier work did not support theories that plants and animals evolved independently on the Chathams for anything like the 70 million years or so since Gondwana broke up.

"Based on the material I accumulated, I thought it was not as old as 70 million years, which is about the time New Zealand has been separated from Australia," he says. "I was not sure that it was as young as three to four million years. But it seemed to be of the same order of difference that you find on the Three Kings [north of Northland], which is perhaps 10 to 15 million years."

He says his speculation in 1996 about land bridges was simply quoting what was then accepted geological theory.

Trewick believes that flightless beetles, other insects and plants could have arrived in the Chathams within the past three to four million years.

He believes beetles, wetas and other creepy-crawlies probably stowed away inside logs which were then washed out to sea and blown eastwards by storms.

Plant seeds may have drifted for days on the ocean and germinated when they reached land. Others may have been swallowed by seabirds and carried to their nesting places, like the kowhai on the Sisters.

What are now flightless birds may have once had the power of flight and reached the Chathams and the New Zealand mainland, then evolved to lose their wings because there were fewer ground predators.

Trewick's analysis shows a takahe species once found in the North Island was genetically closer to a flying ancestor species than it was to the modern South Island takahe, which must have evolved independently from its flying ancestor.

Trewick concludes new characteristics - such as those caused by changing from flight to ground-dwelling - can happen much faster than previously thought.

Given draws the same conclusion for plants. New Zealand has more than 100 species of the woody shrub koromiko. They all look quite different yet are almost the same genetically. "That suggests an incredibly rapid burst of evolution," he says.

Piecing together all the evidence suggests plants and flightless animals could have crossed vast distances of ocean, then evolved to adapt to new lands much more quickly than previously thought.

A major storm doesn't have to be a common event to explain a species jumping explaining a species' sudden arrival in a new land at some moment during several million years. "It doesn't have to happen in our lifetime," Given says. "It may be only once in 100,000 years. But we can make some feasible assumptions.

"So we are looking at the ocean currents. I really want to look at what you come up with on the beaches, what happens when there is a big flood in New Zealand and how far that floodwater can carry out to sea. I am not aware of any other studies that combine the geology and biology. This is pretty unique, as we are trying to bring the two together and looking for plants and insects.

"So it's a bit of a scientific gamble. But what the hang is the use of putting a lot of money into a project if you know what the answer is?"