Known as the biosphere to scientists and as the creation to theologians, all of life together consists of a membrane around earth so thin that it cannot be seen edgewise from a satellite yet so prodigiously diverse that only a tiny fraction of species have been discovered and named. The products of billions of years of evolution, organisms occupy virtually every square centimeter of the planet's surfaces and fill nearly every imaginable niche.

Biologists estimate that more than half the species occur in the tropical rain forests. From these natural greenhouses, many world records of biodiversity have been reportedÑ425 kinds of trees in 2.5 acres (1 hectare) of Brazil's Atlantic forest and 1,300 butterfly species from a corner of Peru's Manu National Park, both more than 10 times the number from comparable sites in Europe and North America. At the other extreme, the McMurdo Dry Valleys of Antarctica, with the poorest and coldest soils in the world, still harbor sparse communities of bacteria, fungi and microscopic invertebrate animals.

A few remarkable species, the "extremophiles," have achieved astonishing feats of physiological adaptation at the ends of habitable Earth. In the most frigid polar waters, fish and other animals flourish, their blood kept fluid by biochemical antifreezes. Populations of bacteria live in the spumes of volcanic thermal vents on the ocean floor, multiplying in water above the boiling point.

And far beneath Earth's surface, to a depth of 2 miles (3.2 km) or more, dwell the slimes (subsurface lithoautotrophic microbial ecosystems), unique assemblages of bacteria and fungi that occupy pores in the interlocking mineral grains of igneous rock and derive their energy from inorganic chemicals. The slimes are independent of the world above, so even if all of it were burned to a cinder, they would carry on and, given enough time, probably evolve new life-forms able to re-enter the world of air and sunlight.

Earth's biodiversity (short for biological diversity) is organized into three levels. At the top are the ecosystems, such as rain forests, coral reefs and lakes. Next down are the species that compose the ecosystems: swallowtail butterflies, moray eels, people. At the bottom are the variety of genes making up the heredity of each species. How much biodiversity is there? Biologists have described a total of between 1.5 million and 1.8 million species. Yet this impressive achievement is only a small beginning. Estimates of the true number of living species range, according to the method employed, from 3.6 million to more than 100 million.

Least known are the smallest organisms. By repeated sampling, biologists estimate that as few as 10% of the different kinds of insects, nematode worms and fungi have been discovered. For bacteria and other microorganisms, the number could be well below 1%. Even the largest and most intensively studied organisms are incompletely cataloged. Four species of mammals, for example, have recently been discovered in the remote Annamite Mountains along the Vietnam-Laos border. One of them, the saola or spindlehorn, is a large cowlike animal distinct enough to be classified in a genus of its own. Earth, as far as life is concerned, is still a little-known planet.

Biologists who explore biodiversity see it vanishing before their eyes. To use two of their favorite phrases, they live in a world of wounds and practice a scientific discipline with a deadline. They generally agree that the rate of species extinction is now 100 to 1,000 times as great as it was before the coming of humanity. Throughout most of geological time, individual species and their immediate descendants lived an average of about 1 million years. They disappeared naturally at the rate of about one species per million per year, and newly evolved species replaced them at the same rate, maintaining a rough equilibrium. No longer. Not only has the extinction rate soared, but also the birthrate of new species has declined as the natural environment is destroyed. The principal cause of both extinction and the slowing of evolution is the degrading and destruction of habitats by human action. While covering only 6% of Earth's land surface, about the same as the 48 contiguous United States, the rain forests are losing an area about half the size of Florida each year.

Damage to intact forests, which occurs when they are broken up into isolated patches or partly logged, or when fires are set, threatens biodiversity still more. With other rich environments under similar assault, including coral reefs (two-thirds degraded) and salt marshes and mangrove swamps (half eliminated or radically altered), the extinction rate of species and races is everywhere rising.

Not all doomed species disappear immediately. Most first suffer loss of their ranges and gene pool to dangerously low levels, eventually descending to join what biologists call the "living dead." Throughout the world, 976 tree species, for example, are classified as critically endangered. Two are down to three or four surviving individuals and three others to only one. I have been grimly compiling what I call the Hundred Heartbeat Club of animal speciesÑthose consisting of a hundred or fewer individuals, hence that number of heartbeats away from total extinction. The club's more familiar members include the Javan rhinoceros, Philippine eagle, Hawaiian crow, Spix's macaw and Chinese river dolphin. Other endangered species lined up for early admission are the giant panda, Sumatran rhinoceros and mountain gorilla.