The sixth great extinction.

• Author: Larsen, Janet
• Position: Eye On Ecology - Scientist predict a mass extinction is on the way

ALMOST 440,000,000 YEARS AGO, some 85% of marine animal species were wiped out in the Earth's first known mass extinction. Roughly 73,000,000 years later, lane quantities of fish and. 70% of marine invertebrates perished in a second major extinction event. Then, about 245,000,000 years ago, up to 95% of all animals were lost in what is thought to be the worst extinction in history. Approximately 37,000,000 years hence, yet another mass extinction took a toll primarily on sea creatures, but also some land animals. Finally, 65,000,000 years ago, three-quarters of all species--including the dinosaurs--were eliminated.

Among the possible causes of these mass extinctions were volcanic eruptions, failing meteorites, and changing climate. After each extinction, it took upwards of 10,000,000 years for biological richness to recover. Yet, once a species is gone, it is gone for good.

The consensus among biologists is that we now are moving toward another mass extinction that could rival the past big five. This one is unique, however, in that it is largely caused by the activities of a single species. It is the sole mass extinction that humans will witness firsthand--and not just as innocent bystanders.

While scientists are not sure how many species inhabit the planet today, their estimates top 10,000,000. Each year, though, thousands of species, ranging from the smallest microorganisms to larger mammals, are lost forever. Some disappear even before we know of their existence.

The average extinction rate today is up to 10,000 times faster than the rate that has prevailed over the past 60,000,000 years. Throughout most of geological history, new species evolved faster than existing species disappeared, thus continuously increasing the planet's biological diversity. Now, evolution is falling behind.

Only a small fraction of the world's plant species has been studied in detail, but as many as half are threatened with extinction. South and Central America. Central and West Africa, and Southeast Asia--all home to diverse tropical forests--are losing plants most rapidly. Moreover, nearly 5,500 animal species are known to be threatened with extinction. The International Union for Conservation of Nature and Natural Resources-World Conservation Union's 2003 Red List survey of the world's flora and fauna shows that almost one in every tour mammals and one in eight birds are threatened with extinction within the next several decades.

Of 1,130 threatened mammals...