On top of Green Mountain, something strange has been happening. This remote mountain poking into the trade winds from the British imperial outpost of Ascension Island in the mid-Atlantic has created its own fully functioning cloud rainforest ecosystem virtually from scratch in just150 years. It did it from odds and ends of botanical scrap brought in by the Royal Navy. According to ecological theory, rainforests are supposed to evolve slowly over millions of years, as species co-evolve and ecological niches are created and filled. Discovering the Green Mountain cloud forest is like finding that a pile of used car parts in a scrapyard has spontaneously reassembled into a functioning car. Unless, that is, ecologists have gottheirtheories hopelessly wrong.
WHEN Charles Darwin stopped off at the mid-Atlantic island of Ascension in July 1836, homeward bound after his long journey aboard the Beagle, he described an island “entirely destitute of trees”. The near-naked island was no casualty of human activity, however. Eighty years before, when Ascension was still uninhabited except for a few passing sailors, Peter Osbeck, a Swedish priest, dropped by on his way home from China. He wrote of “a heap of ruinous rocks” with a bare, white mountain in the middle.
At about a million years old, the volcanic island was a geological upstart and scarcely able to get going biologically because of its remoteness – some 2000 kilometres from the nearest continent. All it boasted was a couple of dozen species of plant, most of them ferns and some of them found nowhere else.
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And so it might have remained. But in 1843, British plant collector Joseph Hooker made a brief call on his return from Antarctica. Surveying the bare earth, he concluded that the island had suffered some ecological calamity that had denuded it of vegetation and triggered a decline in rainfall that was turning the place into a desert.
The Royal Navy, which by then maintained a garrison on the island, was keen to improve the place and asked Hooker’s advice. He suggested an ambitious scheme for planting trees and shrubs that would revive rainfall and stimulate a wider ecological recovery. And, perhaps lacking anything else to do, the sailors set to with a will.
In 1845, a naval transport ship from Argentina delivered a batch of seedlings. More than 200 species of plant arrived from the Cape Botanic Gardens in South Africa in 1858. In 1874, Kew sent 700 packets of seeds, including those of two species that especially liked the place: bamboo and prickly pear.
With sailors planting several thousand trees a year, the bare white mountain was soon cloaked in green – and renamed Green Mountain. An Admiralty report in 1865 praised the new forest growing in the clouds on top of Ascension. The island “now possessed thickets of upwards of 40 kinds of trees besides numerous shrubs”, it said. And it noted that “through the spreading of vegetation, the water supply is now excellent”.
By the early 20th century the mountain’s slopes were covered in guava, banana and wild ginger, the white-flowered Clerodendrum and Madagascan periwinkle, as well as the Norfolk Island pine and mighty eucalyptus from Australia. Up on the summit a bamboo forest, buffeted by the fierce trade winds, was soon howling like a giant wind chime.
Modern ecologists throw up their hands in horror at what they see as Hooker’s environmental anarchy. The exotic species wrecked the indigenous ecosystem, squeezing out the island’s endemic plants. In fact, Hooker knew well enough what might happen. “The consequences to the native vegetation of the Peak will, I fear, be fatal, and especially to the rich carpet of ferns that clothed the top of the mountain when I visited it.” However, he and the navy saw greater benefit in improving rainfall and encouraging a more prolific vegetation on the island.
But there is a much deeper issue here than the relative benefits of sparse endemics versus lush aliens. And as botanist David Wilkinson of Liverpool John Moores University in the UK pointed out after a recent visit to the island, it goes to the heart of some of the most dearly held tenets of ecology. Conservationists’ understandable concern for the fate of Ascension’s handful of unique species has, he says, blinded them to something quite astonishing – the fact that the introduced species have been a roaring success.
Today’s Green Mountain, says Wilkinson, is “a fully functioning man-made tropical cloud forest” that has grown from scratch from a ragbag of species collected more or less at random from all over the planet.
The artificiality troubles Wilkinson a little. “Emotionally, I hate it, but intellectually I love it,” he says. But how could it have happened? Conventional ecological theory says that complex ecosystems such as cloud forests can emerge only through evolutionary processes in which each organism develops in concert with others to fill particular niches. Plants co-evolve with their pollinators and seed dispersers, while the microbes in the soil evolve to deal with the peculiarities of the biochemistry of the leaf litter.
But that’s not what happened on Green Mountain. And the experience suggests that perhaps natural rainforests are constructed far more by chance than by evolution. There is a term for this among dissident ecologists. They call it “ecological fitting”. Species, they say, don’t so much evolve to create ecosystems as make the best of what they have. What works works.
“The Green Mountain system is a spectacular example of ecological fitting,” says Wilkinson. “It is a man-made system that has produced a tropical rainforest without any co-evolution between its constituent species.”
Not everyone agrees. Alan Gray, an ecologist at the University of Edinburgh in the UK, argues that the surviving endemic species on Green Mountain, though small in number, will still be co-evolving and may form the framework of the new ecosystem. The incomers may just be an adornment with little structural importance for the ecosystem. Even the new species may not be quite such a random selection as at first appears. “Many of the imports may have come from the same place, importing their co-evolutionary relationships,” he suggests.
But to Wilkinson this sounds like clutching at straws. And the idea of the instant formation of rainforests sounds increasingly plausible as research reveals that supposedly pristine tropical rainforests from the Amazon to south-east Asia may in places be little more than the overgrown gardens of past rainforest civilisations.
The most surprising thing of all is that no ecologists have thought to conduct proper research into this human-made rainforest ecosystem. A survey of the island’s flora conducted six years ago by the University of Edinburgh and a subsequent conservation strategy by the British government were concerned only with endemic species. They characterised everything else – the majority of the island’s flora and fauna – as a threat. The Ascension authorities are currently turning Green Mountain into a national park where introduced species, at least the invasive ones, are earmarked for culling rather than conservation.
Conservationists, Wilkinson says, have understandable concerns. At least four endemic species have gone extinct on Ascension since the exotics started arriving. And five endangered plants are just clinging on. But in their urgency to protect endemics, ecologists are missing out on the study of a great enigma.
“As you walk through the forest, you see lots of leaves that have had chunks taken out of them by various insects. There are caterpillars and beetles around,” says Wilkinson. “But where did they come from?” Are they endemic or alien? If alien, did they come with the plant on which they feed or discover it on arrival? Such questions go to the heart of how rainforests happen.
The Green Mountain forest holds many secrets. And the irony is that the most artificial rainforest in the world could tell us more about rainforest ecology than any number of natural forests.