BOOK REVIEW / The inestimable value of life on Earth: 'The Diversity of Life' - Edward O Wilson: Allen Lane, 22.50 pounds

TOLD he had shot one of the world's last two Imperial woodpeckers - the largest woodpecker of all - a Mexican truck driver said it was 'un gran pedazo de carne (a great piece of meat)'. There you have the simplest explanation of why life on Earth is passing through its most violent mass extinction spasm since the death of the dinosaurs 65 million years ago. Short-term, short-sighted human desires.

There is, mind you, rather more worth saying on the subject, as Professor Wilson demonstrates in this fine, fat book on biodiversity, the buzz word at last year's Earth Summit in Rio de Janeiro. The first two- thirds are an explanation and celebration of the abundance, variety and weirdness of living things. The remainder sets out why a quarter or more of all species may become extinct in the next 30 years and how this slaughter might be slowed.

We do not know exactly how many animal and plant species are known to humanity, in the sense that they have been described and named, because nobody is keeping proper count. This Harvard professor, whose own speciality is ants, puts the number at about 1.4 million. As for how many as yet undiscovered types are out there in places like tropical rainforests, the deep ocean floor and coral reefs, his estimate is between 10 and 100 million.

Yet nature appears to be utterly callous and careless. Half of its creations spend their time trying to eat or become parasitic on other living things. Extinction is commonplace; 99 out of every 100 species that have ever existed have vanished, according to Wilson. Every 50 million years or so something comes along - a giant meteorite, a change in sea levels and climate, volcanic eruptions - that wipes millions out at a stroke. .

Clearly, nature is profligate in creating and eliminating species - so why all the fuss about us destroying them? Because their uniqueness is not trivial, like the inanimate, boring uniqueness of every grain of sand and pebble on the beach. As a product of evolution, each species has its own complex, ordered way of being different, of grabbing a toehold in its niche. On our foreheads alone live two different kinds of mite, one in the hair follicles and one in the sebaceous glands.

So although it could be seen as merely sad that a quarter of all contemporary species will be rubbed out in a couple of generations as a by-product of the human population explosion, Wilson argues that it is much worse than that. He tells of the many useful chemicals and genes that have been found in wild plants and animals. A quarter of all prescriptions dispensed in the United States are based on substances extracted from plants. There are myriad undiscovered species out there that we might one day be able to use to grow new foods, make new drugs and give our crop plants resistance to pests and disease, if only we would let them live.

Alas, that argument is unlikely to wash with the people who decide the fate of ecosystems. How do you value possible future use, compared with the immediate gain from exploiting a forest for timber and farmland right now? And even if the spread of humanity wipes out a quarter of all species, a huge number remain to be investigated and perhaps exploited one day.

He has other arguments that serve biodiversity better. Damage to some 'keystone' species and ecosystems may have large unforeseen consequences. Rainforests, for example, are known to have a powerful influence on local climate and rainfall.

Finally, there is his simple, emotional belief that killing entire species is ugly and wrong, even if we have not yet even discovered what we are destroying. In this, Wilson, whose genius for popularising biology has already won him two Pulitzer prizes, should win many converts. You put down this book appreciating the centrality of species' diversity in the confusion and grandeur of life on Earth.