The destructive species Professor May had in mind when he addressed the 50th anniversary meeting of The World Conservation Union last November in Fontainebleu was, of course, Homo sapiens. Man's activities, notably his ruthless destruction of natural habitats and ever-growing consumption of the Earth's limited resources, will put thousands of species at risk of extinction over the next century. The problem scientists face is trying to assess the likely scale of the problem.
Few scientists who have studied the destruction of the planet's biodiversity - a measure of the wealth of animals and plants alive today - are in any doubt that we are facing another mass extinction. The difficulty, however, is proving it. There is no dispute over the scale of habitat destruction over the past few centuries, but how do we know that this has resulted in the extinction of species?
The recorded extinctions since 1600 of all types of animals, from molluscs to mammals, amount to no more than about 1,000 species, which is a tiny fraction of the many millions of species of animals and plants alive today. Sceptics also point out that the forest cover of the eastern United States now amounts to just 1 or 2 per cent of its original extent, yet only three forest birds went extinct during that period.
Georgina Mace, an expert on species extinction at the Institute of Zoology in London, says that such an interpretation underplays the true nature of the problem. "Many species go extinct unnoticed; some have never been described and some have been described but are so poorly known that we would not notice their passing," she says. It is a well-known phenomenon in biology - the more a group of animals or plants is studied, the greater the threat of extinction is realised.
Another reason why the problem can be easily underestimated is that the rules governing what is extinct are quite strict. The animal or plant in question must not have been observed for more than 50 years. Many species may be extinct already but not yet fulfilled the criteria of a recognised extinction. As Dr Mace points out, it is always harder to prove something does not exist than to show it does exist.
A central difficulty in assessing the scale of current extinction rates is that biologists have only formally described and named a fraction of the planet's lifeforms. Professor May and John Lawton, director of the Centre for Population Biology at Imperial College London, say that about 1.5 million species of animals and plants have names, but the total number of species could range from five million to 15 million. Furry and feathery animals are well described, but the same cannot be said for insects, worms and other "lower" lifeforms.
Ed Mathew, a campaigner with the World Wide Fund (WWF) for Nature says ignorance is the biggest problem. "To monitor the situation we need good information about the world's species. This information does not exist. Species are being destroyed faster than they are being described," he says.
Scientists estimate that the present biodiversity on Earth is so rich that it means there are more species alive now than at any other single period in the history of life. And yet, the millions of species living today constitute less than about 4 per cent of the total number of species that have existed during the past 600 million years following the "explosion" of the diversity of life during the Cambrian period. Extinction is an inevitable result of natural selection, which actually brings about new species in the continuing process of evolution.
Scientists have attempted to estimate the speed of the "background" extinction rate which must be taking place all the time, without the influence of human activity. They looked at what has happened in the past by studying extinction rates estimated from the fossil record, extending back over many hundreds of millions of years. This suggests that a typical lifetime of a species - from when it originated as a distinct interbreeding entity, to its final demise and removal - is a few million years. Some species, such as insects which last on average about five to 10 million years, have a longer lifetime than others, notably mammals which typically survive as a single species for about two million years.
When May and Lawton looked at extinction rates today, based on known extinctions of a species within a single group, say birds or mammals, they found that typical species lifetime was significantly shorter - about 10,000 years. "This may sound a long time, but the estimate for birds and mammals is 100 to 1,000 times shorter than the lifetimes of species in the fossil record," says Dr Mace.
Another method of estimating extinction looks at the destruction of natural habitats. The WWF says in its Living Planet Report published last year that the world's forest cover has decreased by 13 per cent between 1960 and 1990, which is equivalent to losing an area half the size of Norway each year. Scientists such as Brian Groombridge of the World Conservation Monitoring Centre in Cambridge are attempting to work out how this habitat loss is impinging on biodiversity.
A rule of thumb - and it is rough because different habitats carry different densities of animals and plants - is that a 90 per cent decrease in the area of natural vegetation will result in the loss of between 30 and 55 per cent of species. This means, for example, that if tropical forests are being lost at a rate of between 0.8 per cent and 2 per cent a year, the corresponding loss of species would amount to an annual loss of between 0.2 and 0.5 per cent of the total. "If there are five million species, then 10,000 to 25,000 species are committed to extinction each year as a result of habitat loss," says Dr Mace.
These figures tie in quite well with those based on the known loss of individual species within well studied groups. They also match the rapid loss seen during the five mass extinctions of evolutionary history. Dr Mace reaffirms the view of colleagues such as May, Lawton and Groombridge: "We do seem to be on the brink of a largescale extinction spasm, but a major difference is that now almost all extinctions are due directly or indirectly to the impact of human activities. People now so dominate the Earth that there are very few species completely unaffected by our existence."
John Lawton puts is more starkly: "Whatever view one takes, the impending sixth mass extinction will be unique in the history of the planet."