On a remote island, lessons in how ecosystems function

I was standing on the summit of an extinct volcano in the center of one of the most remote islands on the planet: Ascension Island in the tropical South Atlantic. Midway between Brazil and Africa, Ascension is a thousand miles from the nearest speck of land. Below me was a harsh treeless moonscape of volcanic clinker, baking in the sun. But in the cool mountain air, 800 meters up, I was surrounded by lush greenery and a light mist from a cloud settled over the mountaintop.

They call it Green Mountain. But the greenery is new. My guide, the island's conservation development officer, Stedson Stroud, peered around us and smiled. "Nothing you see here is native," he said. "Except for a few ferns, everything has been introduced in the past 200 years."

On our way up the mountain, we had walked through New Zealand flax, Bermuda cedar, Chinese ginger, South African yews, guava from Brazil, European blackberries, and screw pines that grow higher here than they do at home on the islands of the Pacific. The summit, improbably covered in a dense stand of Asian bamboo, rattled like a huge wind chime in the brisk trade winds. All this had been introduced by the British Navy during the early- and mid-19th century, along with rabbits, cats, donkeys, hedgehogs, mynah birds, bees, and much else.

The forest that covers Green Mountain is said to be the only tropical forest in the world - apart from monoculture plantations - that is entirely man-made. It is an ecosystem, but like no other. Stroud admitted that, as a conservationist and member of the International Union for Conservation of Nature's Species Survival Commission group for the South Atlantic, he should perhaps be rooting out all those alien species. But if he did, there would be almost nothing left. And in any case, he mused, he is presiding over something profoundly interesting - a functioning ecosystem in which a ragbag of species shipped in from all over the world thrive as if they had been together for millennia.

Though badly under-researched because of the island's remoteness, Green Mountain is being hailed as the icon for a fundamental reassessment of many nostrums of both environmentalist and ecological science. It is also creating controversy among ecologists. What are we to make of this confected cloud forest? Is it nature or a garden? Is it a beacon for re-greening the planet or a biological abomination?

Ascension Island, which is just under twice the size of Manhattan, erupted from the Atlantic floor a million years ago. The last new lava flows were 700 years ago. The island has been in British hands for almost 200 years, as a base for controlling the Atlantic. These days, the island is peppered with aerials that track orbiting spacecraft, communicate with nuclear submarines, and listen in secretly on satellite-relayed communications. The electronic spies of Britain's GCHQ, and possibly also the NSA, are here.

It also has one of the longest airstrips in the world. Built by the U.S. Army during World War II on land leased from Britain, it provides a secure stopping point for U.S. military flights into Africa. When I arrived on a British flight in early July, no fewer that nine large U.S. military aircraft were sitting on the tarmac, all busy protecting President Obama during his visit to Africa.

Inevitably, such human traffic brings alien species. Some lava flows and cinder cones are now covered in fast-spreading tobacco plants and Mexican thorns, whose seeds are eaten and distributed by feral donkeys and sheep. They are accidental introductions. But what sits on Green Mountain is largely deliberate.

When Charles Darwin visited Ascension Island aboard the Beagle in 1836, he complained about Ascension's "naked hideousness." But the Royal Navy garrison, established in 1815, set about changing that. First, it put together a farm on the few patches of natural soil on the mountain. Then British colonial botanist Sir Joseph Hooker - a future head of the famous botanical gardens at Kew in London who visited the island in 1843 - came up with the idea of growing trees to green the arid island and increase its rainfall. The idea was that the new vegetation on the mountaintop would scavenge moisture from the passing clouds. Further down the slopes, planting would encourage soil growth. Hooker's ambition was nothing less than "terra-forming" the volcanic island, says Stroud in a recent paper with David Catling of the University of Washington, Seattle.

Today, the island has about 300 introduced species of plants to add to its 25 native species, says David Wilkinson of Liverpool John Moores University in the UK. Many are spreading. Above about 660 meters, Green Mountain is completely vegetated, with coffee bushes, vines, monkey puzzle trees, jacaranda, juniper, bananas, buddleia, Japanese cherry trees, palm trees, clerodendrum, green aloe and the pretty pink flowers of the Madagascan periwinkles. And Stroud says the vegetation captures more cloud moisture, just as Hooker had hoped, even though rainfall has declined in the lowlands around.

The invasion has been double-edged. The invaders have damaged some of the handful of endemic species that had found a foothold on the island during its million-year. Three of the endemic ferns are believed now extinct. But as we stood on the mountain, gazing south over an abandoned NASA tracking station, Stroud pointed out below us the spot where, in 2009, he rediscovered on a cliff face a single specimen of a species believed lost, Anogramma ascensionis. Now it is being propagated in the UK ready for a reintroduction.

In fact, many of the endemics seem to get on remarkably well with the motley collection of invaders, says Stroud. The ferns that once clung to the bare mountainside now prosper on the branches of introduced trees like bamboo. Stroud showed me ferns that he believes now thrive only on the mosses that grow on such branches.