The sixth great extinction

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, 16% are critically endangered--the highest threat level. This means that 184 of their species have suffered extreme and rapid reduction in population or habitat and may not survive the decade. Their remaining numbers range from under a few hundred to, at most, a few thousand. For birds, 182 of the 1,194 threatened species are critically endangered.

Although the status of most of the world's mammals and birds is fairly well-documented, we know relatively little about the rest of the world's fauna. A mere five percent of fish, six percent of reptiles, and seven percent of amphibians have been evaluated. Of those studied, at least 750 fish species, 290 reptiles, and 150 amphibians are at risk. Worrisome signs--like the mysterious disappearance of entire amphibian populations and fishermen's nets that come up empty more frequently--reveal that there may be more species in trouble. Of invertebrates, including insects, mollusks, and crustaceans, we know the least--but what is known is far from reassuring.

At the advent of agriculture some 11,000 years ago, the world was home to 6,000,000 people. Since then, our ranks have grown a thousandfold. Yet, the increase in our numbers has come at the expense of many other species.

The greatest threat to the world's living creatures is the degradation and destruction of habitat, affecting nine out of 10 threatened species. Humans have transformed nearly half of the planet's ice-free land areas, with serious effects on the rest of nature. We have made agricultural fields out of prairies and forests. We have dammed rivers and drained wetlands. We have paved over soil to build cities and roads.

Each year, the Earth's forest cover shrinks by 40,000,000 acres, with most of the loss occurring in tropical forests. Ecologically rich wetlands have been cut in half over the past century. Other freshwater and terrestrial ecosystems have been degraded by pollution. Deserts have expanded to overtake previously vegetated areas, accelerated in some cases by overgrazing of domesticated animals.

A study of 173 species of mammals from around the world showed that their collective geographical ranges have been halved over the past several decades, signifying a loss of breeding and foraging area. Overall, between two and 10% of mammal populations (groups of a single species in a specific geographical location) are thought to have disappeared along with their habitat.

Direct human exploitation of organisms, such as through hunting and harvesting, threatens more than one-third of the listed birds and mammals. Other threats to biodiversity include exotic species, often transported by humans, which can outcompete and displace native ones.

A survey of some 1,100 animal and plant species found that climate change could wipe out between 15-37% of them by 2050. Yet. the actual losses may be greater because of the complexity of natural systems. The extinction of key species could have cascading effects throughout the food chain.

Healthy ecosystems support us with many services--most fundamentally by supplying the air we breathe and filtering the water we drink. They provide us with food, medicine, and shelter. When ecosystems lose biological richness, they also lose resilience, becoming more susceptible to the effects of climate change, invasions of alien species, and other disturbances.

Consciously avoiding habitat destruction and mitigating the effects of land use alteration, reducing the direct exploitation of plants and wildlife, and slowing climate change can help us stop weakening the very life-support systems we depend on. While this may be the first time in history that a single species can precipitate a mass extinction event, it also is the only time that a single species can act to prevent it.

Janet Larsen is a research associate at the Earth Policy Institute, Washington, D.C.

COPYRIGHT 2004 Society for the Advancement of Education
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