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Science: Expedition to mystery island: Michael Brooke and fellow scientists went to the Pacific to study the animals and plants of Henderson Island, a unique treasure-trove

SEEN FROM some distance away, across the deep blue of the Pacific, Henderson Island appears to be flat, so flat as to make the Fens seem altogether hilly. But closer up, the hard, grey, limestone cliffs girding the island are evident. These cliffs, pocked with caves and wheezing blowholes, are cleft by horizontal notches at various levels, caused by erosion by the sea. The notches record temporary hiccups, maybe several-thousand- year hiccups, in the continuing rise of Henderson from a sea-level atoll to the raised coral island it is today.

Around much of the island, the Pacific swells crash tirelessly against the cliffs. Elsewhere the cliffs are protected from the waves' onslaught by a reef, a narrow lagoon and a beach fit for the beautiful people who tipple in Bacardi adverts.

The first members of the Sir Peter Scott Commemorative Expedition, none of them potential Bacardi models, arrived on Henderson in January 1991; their first task was to ferry ashore, by inflatable, the several tons of equipment needed for a detailed scientific study of one of the world's most remarkable islands.

Up until a million years ago Henderson was a coral atoll atop a volcanic mount rising from the ocean bed, some 3,500m below. Then Pitcairn Island, nowadays home to the descendants of the Bounty mutineers, erupted 110 miles to the south-west. The seafloor was depressed and, by a seesaw process, a geological spasm known as lithospheric flexure, Henderson Island started to rise. It has reached about 30 metres.

The raising has several consequences. The first manifests itself in damage to footwear: the flat top of Henderson Island is the remains of the old lagoon floor. Underfoot is a jumble of coral debris, giant clam shells and razor-sharp limestone pinnacles which slice shoe leather.

The next is that the island is waterless. Although rainfall is ample, around 1.5m per year, it all drains immediately into the porous limestone. There are neither streams nor permanent standing water. (The expedition relied on water caught by the camp tarpaulins.) Despite the absence of standing water, the bulk of today's plateau is swathed in a forest some six metres tall. It is so dense that, away from paths, progress is only possible with a machete.

Henderson Island's uniqueness, and the need to protect it from human interference, led to its being designated a World Heritage Site by Unesco in 1988. It joined an elite 300-strong group including such cultural jewels as the Taj Mahal and such natural wonders as Yosemite National Park. Meanwhile, the need for a scientific expedition to document the site in more detail than had been achieved during occasional brief visits was brought into focus.

So it was that, after a lengthy gestation, the Sir Peter Scott expedition was born. During the 15 months of fieldwork, 34 people from seven countries with varied scientific skills were to participate in the expedition. Only I stayed throughout, and thus, for better or worse, spent longer on Henderson Island than anyone since the Polynesians. Various shipwrecked sailors had all died or made an escape in less than a year.

What makes the island unique? The Henderson plateau, now well above sea level, is never washed over by the sea during the occasional cyclones - the soggy, salty fate of other low atolls in the area. As a result of such devastation, atolls generally have few endemic plant and animal species. Rather, they are repeatedly recolonised by species good at hopping across the ocean. By contrast Henderson's elevation has allowed the evolution of a suite of endemic plants and animals. These, combined with the island's geographic isolation, are what make it so special.

There are 10 unique flowering plants including an endemic sandalwood and a bushy daisy, four landbird species including a tiny all-black flightless rail and a host of insects and snails. Nearly all these specialities seem to have originated by island-hopping from the west against the prevailing south-east trade winds. The landless expanse of ocean to the east, between Henderson and South America, has proved much less bridgeable.

In the early Eighties a wealthy American mine-owner, Smiley Ratliff, proposed to build an airstrip and set up a holiday hideaway on uninhabited Henderson. His massive wealth enabled the contemplation of such a project despite the island's isolation, about 1,400 miles from the nearest international airport in Tahiti and more than 3,000 miles to continental land, be it New Zealand to the west or South America to the east. But the project drew howls of protest from the conservation community and was eventually quashed by Her Majesty's Government (Henderson, part of the Pitcairn Islands, is still a UK dependent territory). Thereafter the feeling grew that Henderson deserved special recognition, since, with the exception of Aldabra in the Indian Ocean, all our planet's other large raised atolls have been damaged by man's clumsy hand.

Ours was not the first human expedition to Henderson. Polynesians, seafarers extraordinaires, voyaged by canoe across the vast width of the South Pacific during the first millennium after Christ. In general they settled islands in the west first, and then progressively colonised islands further eastward. The largest islands they settled remain populated to this day. Many smaller islands simply lacked the resources needed for permanent habitation. A dozen islands of intermediate size, known as the 'mystery islands', were occupied for some centuries but abandoned before the period of European exploration in the 17th and 18th centuries. Henderson, five and a half miles by three, is one such. There was no sign of Polynesian habitation when it was 'discovered' by the Portuguese navigator Quiros in 1606, nor when named by Captain Henderson aboard the Hercules in 1819.

Earlier scientists had noted Polynesian artefacts in the Henderson caves. But the scale of the Polynesian archaeological legacy came as a surprise to our team's archaeologist, Marshall Weisler from the University of California at Berkeley. After colonisation around 800AD, or possibly earlier, the population expanded, at times perhaps exceeding 100. The people harvested birds and turtles and caught fish on hooks fashioned from oyster shell. In the early phase, contact was probably maintained with Pitcairn, as artefacts from the lower archaeological layers include stone adzes shaped from hard volcanic rock which is obtainable on Pitcairn but not on Henderson. Later, communications seem to have deteriorated, as the Polynesians were driven to make adzes from local materials, primarily from the hinges of giant clams. Did Henderson's Polynesians eventually die out, or did they abandon the struggle against their harsh island, up sticks, and sail away? We do not know.

The Polynesians also exploited certain plants they deliberately brought to the island. Thus coconuts provided solid food for adults, liquid nourishment for babies, and leaves for shelter. The lily Cordyline has an edible sugar-rich tuber, and leaves which are useful for wrapping food during cooking in stone ovens. Little stands of Cordyline still dot the fringes of Henderson. Not so the candlenut - the nuts will burn - which seems to have vanished. The other introduced trees were two hardwoods useful for housing, miro (a rosewood) and toa. These are cut by modern Pitcairners on their annual visit to get wood for curio carving. The expedition oven which, on good days, yielded fish au gratin, treacle tart and deliciously yeasty bread, was fired by Pitcairners' miro offcuts .

Interestingly, these introduced plants are today found near the coast in a few flat acres between beach and cliff base. Meanwhile the thick forest of the interior is dominated by native plants and probably little altered. I sound a note of caution because, given occasional pockets of charcoal found by the expedition, it is just possible that the Polynesians extensively burnt the island. It would after all be easier to move on a burnt-over island than to cut new paths with stone-age tools. (Speaking from blistered experience I can report that progress is not much quicker with the benefit of metal machetes.)

If the Polynesian impact on the Henderson vegetation was probably marginal, the same is not true of the impact on bird life. Earlier excavations, in 1971 and 1987, uncovered the remains of at least three species, two pigeons and a storm petrel, no longer living on Henderson. One possibility is that these birds were cropped so heavily by hungry Polynesians that they were extirpated.

Another possibility is that their disappearance was due to predatory Polynesian rats. These were deliberately introduced to colonised islands where they thrived and provided the Polynesians with handy protein. The small storm petrel would have been especially vulnerable.

Today three species of ocean-

going gadfly petrel nest on Henderson. The parents, laying a single egg under bushes, are well able to hatch the egg. But a day or two after hatching trouble starts. The powder puff of a chick moves out from under its parent. Rats are then able to snatch and eat the chick while the parent petrel, about four times heavier than a 100g rat, displays an insouciance that seems wholly stupid. After all, the egg has taken 50 days to hatch, and the next opportunity to lay will be a year hence. With less than 20 per cent of eggs yielding fledglings, Henderson petrel populations may partly be sustained by immigration from other, healthier colonies.

At least today's four extant land birds seem able to cope with rats. Called, with a stunning lack of originality, the Henderson rail, the Henderson fruit dove, the Henderson lorikeet (a parrot) and the Henderson warbler, all maintain substantial populations - several hundreds for the lorikeets, several thousand for the other three. Having co-existed with Polynesian rats for at least 700 years, they are not imminently threatened. The little black flightless rail is astonishingly pugnacious, leaving its nest to buffet any rat passing too close. The rats, goliaths compared to the 70g rails, universally retreat, chastened by flurrying wings that are too tiny to lift the rail off the ground.

Intriguingly, three of the landbirds - warbler, rail and lorikeet - have been found in cave deposits pre-dating the arrival of Polynesians. The fruit dove is absent from these early layers. Could it have been brought to Henderson by the Polynesians, thrived in the absence of the two extinct pigeon species, and evolved into a distinct form in the past 900 years? Today it eats most of the types of fruit available on Henderson. On such a modest island it is hard to envisage this dove living alongside two other pigeon species with similar dietary habits.

I am hopeful that Henderson's treasures will be preserved; its isolation provides formidable protection. But the lesson of the Polynesian legacy is that ecological disturbances, however mild, can leave a long-lasting imprint.

Dr Michael Brooke is a zoologist at Cambridge University.

(Photograph omitted)