Through his 1993 book The Diversity of Life, Wilson has probably done more to state the importance of biological diversity than any other scientist. Biodiversity, he has written, "is the key to the maintenance of the world as we know it".
Tonight at the Natural History Museum, Wilson will describe how that biodiversity has been tipped into sharp decline by the ignorance and foolishness of just one very familiar species. However, he is not one of those conservationists who wishes to fence nature off from human contact. Biodiversity should be cherished for our sake, not its own.
Which is where the turtle comes in. If farmed in the flood plains where it occurs naturally, it would yield 400 times the amount of meat produced by cattle raised in the same area of cleared forest. Wilson sees Earth's biodiversity as a vast potential resource - for food, medicines, education, entertainment, even mental health.
More than 40 years of field work has taken Wilson everywhere from Cuba to Fiji. His pioneering work on the biogeography of islands showed how diversity is related to the area of an ecosystem. Today this knowledge tells us what we can expect when habitats are eroded. It is not good.
The picture is made bleaker by recent reassessments of data on endangered species populations. A fortnight ago the International Union for the Conservation of Nature (IUCN) published new "red lists" that count 25 per cent of all mammals as "vulnerable" or "endangered" - up from 18 per cent two years ago. The US Nature Conservancy has raised the number of species at risk in the dozen groups that it focuses on (including mammals, birds, flowering plants and butterflies) from 33 to 44 per cent. In the past 100 years, 1.5 per cent have become extinct.
Yet life on Earth is more diverse than at any time in its 600-million- year history. Naturalists have identified only a fraction of all species. Estimates of the number once ranged from 5 to 100 million, but are gradually converging to a figure of about 10 million species. Estimates of rates of extinction vary too. Methods based on island biogeography produce high figures. Recent analysis of the drift of species across IUCN categories, from "vulnerable" to "endangered" to "critically endangered" is more optimistic.
But the extinction of an individual species is almost bound to pass unnoticed. Only rarely do we know how a species met its end. Two cases: in 1844, two Icelandic fishermen clubbed the last pair of great auks to death. And 20 years ago, a lorry driver shot the last imperial woodpecker, in Mexico. "It was a great piece of meat," he said afterwards.
Biodiversity is under attack even where species should be safest. Wilson cites a recent study by William Newmark of the Utah University of Natural History of the situation in American and Canadian national parks. "You can witness the decline of mammal species park by park. They are disappearing exactly as predicted, namely the smaller the park the faster they are disappearing. No park is big enough."
Global extinction rates are perhaps thousands of times higher than before the coming of man. But we have been here before - or rather, our planet has. There have been five waves of extinction, each of which wiped out between 10 and 40 per cent of animal and plant families. "Our" extinction, the sixth, is projected to eliminate up to 20 per cent of species. But then, most species that have ever lived are now extinct. And aren't we a species too? Anything we do is still done within our ecosystem.
"Why should we care?" Wilson asks. "What difference does it make if some species are extinguished, if even half of all the species on Earth disappear?" There are three main arguments. First and most venal, there is the potential benefit to humanity from chemical and genetic "prospecting" of little- known species which might yield new crops or medicines. "I live in the real world. I have discovered, talking to national leaders from Newt Gingrich to business groups across the country, that you have to start there. People do not immediately understand the other arguments. I think the utilitarian argument is valid; in fact, it's exciting. There are so many beneficial effects that are possible."
Wilson describes a collaboration between the pharmaceuticals company Merck and Costa Rica's National Institute of Biodiversity to collect and assay samples of flora and fauna. A share of royalties from the sales of any commercial products derived from these organisms goes back to fund local conservation programmes. Others are following. Brazil is currently drafting legislation which would regulate land use along these lines. "It's a movement that's beginning to spread around the world, but not fast enough to suit me."
Second is the aesthetic argument - that biodiversity should be preserved for our pleasure. If we grow to love our ecosystems, that very familiarity will help to save them. Some say ecotourism merely brings pollution and disruption, but Wilson disagrees.
"This is a startling misconception. The way I see it, it's vastly better to have some trails and a couple of camps in a rainforest than no rainforest. That's what it comes down to in a lot of cases."
Third, Wilson believes there is a deeper reason for preserving biodiversity and for guaranteeing human access to it. "This is part of my conception of 'biophilia', admittedly a subject not yet studied in any depth by psychologists. But there's some evidence that humanity responds in a positive way, and in fact enjoys better mental health, with access to natural environments. Our spirit needs the feeling that there are untamed regions" n
'The Diversity of Life' lecture is a 7.30pm tonight at the Natural History Museum, London. For tickets contact Amanda de la Rosa at the museum's Development Trust on 0171 938 8975.Reuse content