Researchers of biodiversity agree that we are in the midst of the seventh mass extinction. Even if the current rate of habitat destruction were to continue in forests and coral reefs alone, half the species of plants and animals would be gone by the end of the 21st century. Our descendants would inherit a biologically impoverished and homogenized world. Not only would there be many fewer life forms, but also faunas and floras would look much the same over large parts of the world, with disaster species such as fire ants and house mice widely spread. Humanity would then have to wait millions of years for natural evolution to replace what was lost in a single century.

In the long term, I am convinced, the quenching of life's exuberance will be more consequential to humanity than all of present-day global warming, ozone depletion and pollution combined. Why? For practical reasons, if nothing else. Humanity's food supply comes from a dangerously narrow sliver of biodiversity. Throughout history, people have cultivated or gathered 7,000 plant species for food. Today only 20 species provide 90% of the world's food and threeÑmaize, wheat and riceÑsupply more than half. Tens of thousands of species of the world's still surviving flora can be bred or provide genes to increase production in deserts, saline flats and other marginal habitats.

Natural pharmaceuticals offered by biodiversity are also underutilized. Only a few hundred wild species have served to stock our antibiotics, anticancer agents, pain killers and blood thinners. The biochemistry of the vast majorityÑmillionsÑof other species is an unfathomed reservoir of new and potentially more effective substances. The reason is to be found in the principles of evolutionary biology. Caught in an endless arms race, these species have devised myriad ways to combat microbes and cancer-causing runaway cells. We have scarcely begun to consult them for the experience stored in their genes.

If the future enhancement of agriculture and medicine is not thought enough to merit conservation, then consider survival. The biosphere gives us renewed soils, energy, clean water and the very air we breathe, all free of charge. The more species that compose wild communities, the more stable and resilient becomes the planet as a whole.

Then consider ethics. More and more leaders of science and religion now pose this question: Who are we to destroy or even diminish biodiversity and thus the creation? Look more closely at nature, they say; every species is a masterpiece, exquisitely adapted to the particular environment in which it has survived for thousands to millions of years. It is part of the worldÑpart of Eden if you preferÑin which our own species arose.

The profligacy of the 20th century has led humanity into a bottleneck of overpopulation and shrinking natural resources. Through this bottleneck humanity and the rest of life must now pass. By the end of the new century, if we are both lucky and wise, we will exit in better shape than we entered, with the population peaked around 8 billion or less and a gradual decline begun.

People everywhere will have acquired a decent quality of life, with the expectation of more improvement to come. One of the defining goals of the century must also be to settle humanity down before we wreck the planet. To that end it is important to accept the challenge and responsibility of global conservationÑand to do so right now, before it is too late. We will be judged by the amount of biodiversity we carry through the bottleneck with us. There are reasons to be warily optimistic that biodiversity may be salvageable. Whether it happens in time depends fundamentally on the shift to a new ethic, which sees humanity as part of the biosphere and its faithful steward, not just the resident master and economic maximizer. That change of heart has begun in most countries among a few farsighted leaders and a growing part of the general public, albeit very slowly.

Success also depends on attention to sustainable management of the environment, including protection of biodiversity. Conservation experts now give top priority to "hot spots," pockets of wild nature that contain high concentrations of endangered species, which give hope that a great deal can be accomplished in a short span of time. From the coastal sage of California to the rain forests of West Africa, the hottest of the terrestrial hot spots occupy only 1.4% of the world's land surface yet are the exclusive home of more than a third of the terrestrial plant and vertebrate species. Similarly, from the streams of Appalachia to the Philippine coral reefs, aquatic hot spots occupy a tiny fraction of the shallow water surface. This much of the world can be set aside quickly without crippling economic or social consequences. More difficult but equally important are the preservation and long-term nondestructive use of the remaining fragments of the old-growth forests, including the tropical wildernesses of Asia, Central Africa and Latin America. None of this will be easy, but no great goal ever was. Surely nothing can be more important than to secure the future of the rest of life and thereby to safeguard our own.

Wilson is University Research Professor at Harvard. His most recent book is "Consilience: The Unity of Knowledge"