Consultant in Environment and Development, Oxford, United Kingdom
There is strong evidence that we are into the opening stages of an extinction spasm. That is, we are witnessing a mass extinction episode, in the sense of a sudden and pronounced decline worldwide in the abundance and diversity of ecologically disparate groups of organisms.
Of course extinction has been a fact of life since the emergence of species almost 4 billion years ago. Of all species that have ever existed, possibly half a billion or more, there now remain only a few million. But the natural background rate of extinction during the past 600 million years, the period of major life, has been on the order of only one species every year or so (Raup and Sepkoski, 1984). Today the rate is surely hundreds of times higher, possibly thousands of times higher (Ehrlich and Ehrlich, 1981; Myers, 1986; Raven, 1987; Soulé, 1986; Western and Pearl, in press; Wilson, 1987). Moreover, whereas past extinctions have occurred by virtue of natural processes, today the virtually exclusive cause is Homo sapiens, who eliminates entire habitats and complete communities of species in super-short order. It is all happening in the twinkling of an evolutionary eye.
To help us get a handle on the situation, let us take a lengthy look at tropical forests. These forests cover only 7% of Earth’s land surface, yet they are estimated to contain at least 50% of all species (conceivably a much higher proportion [see Erwin, Chapter 13 of this volume]). Equally important, they are being depleted faster than any other ecological zone.
There is general agreement that remaining primary forests cover rather less than 9 million square kilometers, out of the 15 million or so that may once have existed