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What a plethora of books on biodiversity. At least a dozen distinguished ones have appeared during 1994 alone, and twice as many are in the publishing pipeline. And rightly so. The decline of biodiversity can be regarded as the most critical of Earth's environmental crises insofar as it is irreversible. All our other problems are intrinsically reversible. The biodiversity crisis, aka biodepletion, is different. When a single species is gone, that is it - and we are already into the opening phase of a mass extinction consuming millions of species. The Earth's experience of mass extinctions in the geologic past suggests that the present phenomenon, if allowed to persist unabated, will impoverish the biosphere for five million years, possibly several times longer.

Yet biodiversity receives less attention than it deserves. That is to say, biodiversity in its proper full sense, not rhinos or whales or a few other of the "charismatic mega-vertebrates", but rather the biodiversity that involves invertebrates and especially insects for the most part, creatures that with their many millions comprise 90 per cent of all species. By contrast, vertebrates of whatever size and shape make up a maximum of 0.4 per cent of all species.

When people hear that we may well be losing 50 to 100 species every day, they may respond: "What has biodiversity ever done for me?" The answer is that there is a one in four chance that the medicine or drug they purchase in a chemist's shop derives from a wild plant. One of the biggest breakthroughs against cancer in the past 30 years came from two biocom? obtained from the rosy periwinkle, which are potent against Hodgkin's disease, childhood leukemia and other blood cancers. Scientists believe that tropical forests contain at least another 12 plants potent against other forms of cancer - provided plant experts can study them before their habitats are destroyed.

The commercial value of plant-based medicinals is around $50 billion a year in just the rich nations, while the "green medicine" that forms the medicinal mainstay for developing nations is worth another $50 billion. Yet a mere 1 per cent of species has been examined to date. And according to the current rate of extinction, a full quarter of Earth's plant species will have gone by the time today's infants are old enough to read The THES. Much the same can be said of species' contributions to new foods and industrial raw materials and even sources of energy.

The world is now stirring itself to stem this biotic holocaust. At the Rio Earth Summit in 1992 virtually all nations signed a Biodiversity Convention. Alas, government signatures alone do not save much, and thus far we have scarcely slowed the annual acceleration in the extinction rate (already 100,000 times the "natural" rate before the arrival of humans). What will help to push governments into action is public opinion; and this will be aided by the four books under review.

The first, The Biophilia Hypothesis, deals with what one of its editors, E. O. Wilson, terms "our innate affinity for the natural world" - or "biophilia". In an earlier book on the topic, Wilson has postulated that there might well be a biological need underpinning our intimate bonds with nature, and that this need could play a vital role in our development as individuals and as a species.

The thesis plainly appeals to a number of front-rank analysts, as witness the contributors to this book who include Gordon Orians, Gary Nabhan, Lynn Margulis, Madhav Gadgil and David Orr. In 15 position papers we read about biophilia in relation to the conservation ethic, human evaluation of nature, the "spiritual ecology" of hunter-gatherers, the Gaia concept, and many more variations on the biophilia theme. Along the way we encounter much evidence in support of biophilia, together with its opposite biophobia. Consider, for instance, our apparently inborn delight in flowers, trees, water and diversified landscapes, and the equally inborn dislike of spiders and snakes. Many traditions and cultural icons depend upon symbols drawn from nature, especially animals, such as the lions that appear in legends from far and wide. As biodiversity disappears and we become further estranged from the natural world, we may well find that we are not only impoverished in biological and material senses, but that we have been made bereft in cultural, symbolic, psychological and even spiritual senses. To the extent that the biophilia thesis stands up, it provides an additional potent argument for conserving biodiversity.

Does it stand up? Certainly the book makes the case well, though it might have been improved with the addition of the odd paper by a sceptic. It may indeed be that the savannah environments of the formative periods of human evolution are the reason why we now enjoy open and lightly-wooded landscapes, and esteem their more powerful denizens such as lions; and that, conversely, we eschew thick forests and disparage their beasts such as wolves and bears, and hence few monuments feature wolves as appendages to human heroes, while the wood-cutter often features as the guardian of human interests. But this is not self-evidently true.

The second book, Essentials of Conservation Biology, is much more of a textbook though perfectly readable. Covering the new sub-discipline of conservation biology, its authoritative text presents analyses of such basic concepts as keystone species, genetic variability, trophic levels, island biogeography, rareness, endangerment, exotics, endemics, extinction vortices, population viability, minimum gene pools, and the latest conservation imperative, restoration ecology. The book does not limit itself to biology, ecology, genetics and evolution so as to embrace such a wide-ranging agenda; it draws too on economics, law, anthropology, history, philosophy, ethics and politics. A broad approach indeed, which none the less works in R. B. Primack's skilful hands.

There are concise discussions of such issues as how species come to occur, the reasons for the global decline of amphibians, the wisdom or otherwise of spending $15 million on the Californian condor, hot-spot concentrations of species, the decline of the songbird in North America, the marketing of elephant feet for wastepaper baskets, debt-for-nature swaps, and the non-utilitarian value of species. The book raises questions such as why most species live in the tropics and especially in tropical forests: with 6 per cent of earth's land surface, these forests contain at least 60 per cent of all species, yet they are the most threatened of all the large-scale ecological zones. And there are surprising findings on numerous topics. For instance, advancing technology is still the cause of far more job losses among lumberjacks in Oregon's forests than are Oregon's much-publicised efforts to preserve the spotted owl. Finally, the book's abundant illustrations and its 1,000 references in almost 600 pages make it exceptional value. Altogether, it is a first-rate overview of the subject.

The third book, Paradise Lost?, approaches the problem of biodepletion from an economic standpoint. Written under the auspices of the Beijer International Institute of Ecological Economics in Stockholm, it seeks to promote a dialogue about biodiversity between ecologists and economists. In my opinion - and I have a foot in both camps - there has been no greater organisational roadblock in the whole environmental arena than the disinclination of these two parties to talk to each other during the past few decades. I must salute a book that at least tries to foster an exchange and even an integration of views.

Ecologists tend to say that we need all the biodiversity we can preserve: every last bit is precious. Economists respond that human ingenuity and technological know-how will compensate for biodepletion - just as they have always overcome problems to do with our finite resources. This book takes the debate far beyond this basic stand-off. Ecologists are compelled to reflect on the fact that we cannot afford to save everything and that no species is "beyond value". At the same time, economists would do well to consider that there is next to no market in the goods and services supplied by, say, the Californian condor, and hence hardly any way for "conservationist consumers" to register their preferences with their dollar votes. It is no solution to ask people what they would pay to preserve the bird and take this as a proxy evaluation. Most people have little understanding of the condor's ecological worth, let alone its many other values, moreover surveys of "willingness to pay" take no account of people as yet unborn.

There are two central problems, the first of which is well explored in the book, the second less so. We face huge uncertainty as to the ultimate overall value of biodiversity, especially in terms of its many environmental functions and ecosystem services. We have hardly a preliminary idea of "the minimum threshold level (of biodiversity) to sustain human welfare and even existence". Hence we should deploy extreme caution in deciding how much biodiversity we should allow to disappear. Because of this uncertainty, we should maintain the maximum amount of biodiversity. The authors of Paradise Lost?, by contrast, assert that we should aim to safeguard whatever "minimum level of biodiversity is required to maintain human welfare", and hence that "biodiversity conservation does not require complete preservation of all species in the world". If only we had the wisdom of Solomon to make such momentous judgements.

Second, what is the most suitable criterion to guide our decisions? Does it truly lie with "human welfare"? The economist would reply with Alexander Pope that man is the measure of all things, and that we cannot even talk of value except within a human context (implying, presumably, that values do not exist on the Moon except when earthling astronauts are there to hold them). So the traditional approach of economics has been that we should mobilise the earth's resources to support humankind's cause. Yet at a time when whole segments of the planetary ecosystem face terminal threat, should we not seek to mobilise humanity's resources in support of the planetary cause - and thereby give ourselves our best and perhaps only chance of worthwhile existence? Fortunately the authors broach this challenge, albeit in ultra-tentative terms, in their final chapter where they express the hope that we need to "change our current understanding (of the biodiversity problem) based on a more integrated ecological-economic approach". If only they had taken a more powerful stance on this need in what is otherwise a fine book, strongly recommended.

The final book, The Economic Value of Biodiversity, postulates that if we could only calculate the full value of, say, an elephant, we would be in a much better position to persuade politicians and policy-makers that the species is worth preserving in competition with other ways of expending public money. But it is next to impossible to assess the true overall value of a species. An elephant is worth far more than its ivory and other physical products. Each of the few hundred elephants in Kenya's Amboseli Park brings in thousands of tourist dollars year after year. Elephants perform prodigious environmental services through their impact on vegetation - in opening up woodlands, for example, to the benefit of associated species. And what price shall we put on what John Donne called "the only gentle great thing"? Unless we can do a better job of "capturing" the myriad values represented by elephants, including those bestowed through their mere existence, we shall likely be doomed to watch them disappear. David Pearce and Dominic Moran readily recognise the limitations of economics. More books like this will help to push back the boundaries of our understanding, all too constrained as it is.

To return to the irreversibility of lost biodiversity. Biodepletion will not only visit deprivation on people today, it will impoverish our descendants for 20 times longer than the period since humans first emerged as a species. As E. O. Wilson points out, the world could recover within a decade from a seismic economic slump, and it could even pick itself up from a nuclear exchange within a century at most. The biodiversity crisis is different, the most far-reaching decision humankind has faced.

Norman Myers is a visiting fellow, Green College, Oxford.