"We aren't quite sure who is cutting our forests and who is going to flood our land, but we know they live in towns, where rich people are getting richer, and we poor people are losing what little we have."

Biological diversity is being eroded as fast today as at any time since the dinosaurs died out some 65 million years ago. The crucible of extinction is believed to be in tropical forests. Around 10 million species live on earth, according to the best estimates, and tropical forests house between 50 and 90 percent of this total. About 17 million hectares of tropical forests -- an area four times the size of Switzerland -- are now being cleared annually, and scientists estimate that at these rates roughly 5 to 10 percent of tropical forest species may face extinction within the next 30 years.

This estimate may prove conservative, however. Rates of tropic forest loss are accelerating, and some particularly species-rich forests are likely to be largely destroyed in our lifetime. Some scientists believe that about 60,000 of the world's 240,000 plant species, and perhaps even higher proportions of vertegrated and insect species, could lose their lease on life over the next three decades unless deforestation is slowed immediately.

Tropical forests are by no means the only sites with endangered biodiversity. Worldwide, nearly as much temperate rainforest -- once covering an area nearly the size of Malaysia -- has also been lost. Although the total extent of forest in the northern temperate and boreal regions has not changed much in recent years, in many areas the species-rich, old-growth forests have been steadily replaced by second-growth forests and plantations. Evidence of accelerating clearnace of temperate forests is also appearing: between 1977 and 1987, 1.6 millon hectares of forest was lost in the United States alone.

In several spots in europe, fungal species diversity has dropped by 50 percent or more over the past 60 years. In such "Mediterranean" climes as California, South Africa, central Chile, and Southwest Australia, at least 10 percent of all plant and animal species are imperilled. The largest number of recent extinctions have been on oceanic islands: some 60 percent of plant species endemic to the Galápagos Islands are endangered, as are 42 percnet of the Azores' endemic species and 75 percent of the endemic plan species of the Canary Islands.

The biodiversity of marine and freshwater systems face serious loss and degradation. Perhaps hardest hit of all are freshwater ecosystems, battling long-term pollution and the introduction of many alien species. Marine ecosystems too are suffering from the loss of unique populations of many species and are undergoing major ecological changes.

The number of documented species extinctions over the past century is small compared to those predicted for the coming decades. This difference is due, in part, to the acceleration of rates of habitat loss over recent decades but also the difficulty of documenting extinctions. The vast majority of species has not yet even been described, and many may disappear before they are even known to science. Moreover, species are generally not declared to be extinct until years after they have last been seen -- so figures for documented extinctions are highly conservative. Finally, some species whose polutions are reduced by habitat loss below the level necessary for long-term survival may hang on for several decades without hope of recovery as their population dwindles -- these are the "living dead."

Still, evidence of extinction, especially of distinct populations of species, is only too plentiful. In 1990, the otter died out in the Netherlands, and in 1991 Britain declared the mouse-eared bat extinct. In the eastern Pacific, elevated sea temperatures in the 1980s caused the extinction of a hydrocoral. In the past decade, at least 34 species or unique populations of plants and vertebrates have become extinct in the United States while awaiting federal protection. Worldwide, over 700 extinctions of vertebrates, invertebrates, and vascular plants have been recorded since 1600. How many species went extinct elsewhere, unnoticed?

Habitat loss not only precipitates species extinctions, it also represents a loss of biodiversity in its own right. In many countries, relatively little natural vegtation remains untouched by human hands. In Bangladesh, only 6 percent of the original vegetation remains. Forests around the Mediterranean Sea probably once covered 10 times their current area, and in the Netherlands and Britain, less than 4 percent of lowland raised bogs remain undamaged.

The dramatic losses of species and ecosystems obscure equally large and important threats to genetic diversity. Worldwide, some 492 genetically distinct populations of tree species (including some full species) are endangered. In the northwestern United States, 159 genetically distinct populations of ocean-migrating fish are at high or moderate risk of extinction, if they have not already slipped into oblivion.

Loss of genetic diversity could imperil agriuculture. How much the genetic base has already eroded is hard to say, but since the 1950s, the spread of modern "Green Revolution" varieties of corn, wheat, rice, and other crops has rapidly squeezed out native landraces. Modern varieties were adopted on 40 percent of Asia's rice farms within 15 years of their release, and in the Philippines, Indonesia, and some other countries, more than 80 percent of all farmers now plant the new varieties. In Indonesia, 1500 local rice varieties have become extinct in the last 15 years. A recent survey of sites in Kenya with wild coffee relatives found that the coffee plants in two of the sites had disappeared, three sites were highly threatened, and six were possibly threatened. Only two were secure.

The impact of such losses of genetic divesrity often registers swiftly. In 1991, the genetic similarity of Brazil's orange trees opened the way for the worst outbreak of citrus canker recorded in the country. In 1970, U.S. farmers lost $1 billion to a disease that swept through uniformly susceptible corn varieties. Simlarly, the Irish potato famine in 1846, the loss of a large portion of the Soviet wheat crop in 1972, and the citrus canker outbreak in Florida in 1984 all stemmed from reductions in genetic diversity. In such countries as Bangladesh, where some 62 percent of rice varieties come from a single maternal plant, Indonesia (74 percent), and Sri Lanka (75 percent), such outbreaks could occur at anytime.

Gene banks have slowed the loss of genetic diversity, but the high costs of periodically regenerating the seeks and the risk of mechanical failures make seekbanks less than fail-safe. In 1980, experts estimated that even in developed countries between one-half and two-thirds of the seeds collected in past decades had been lost. In 1991, representatives of 13 national germplasm banks in Latin America reported that between 5 and 100 percent of the maize seed collected between 1940 and 1980 is no longer viable.

The loss of genetic, species, and ecosystem diversity both stems from and invites the loss of cultural diversity. Diverse cultures have bred and sustained numerous varieties of crops, livestock, and habitats. By the same token, the loss of certain crops, the replacement of traditional crops with export crops, the extinction of species embedded in religion, mythology, or folklore, and the degradation or conversion of homelands are cultural as well as biological losses. Since 1900, experts say, about one Indian tribe has disappeared from Brazil each year. Almost one half of the world's 6000 languages may die out in the next 100 years. Of the 3000 languages expected to survive for a century, nearly half will probably not last much longer.