Most biologists accept the estimate of American evolutionary biologist Edward O. Wilson that the Earth is losing approximately 27,000 species per year. This estimate is based primarily on the rate of disappearance of ecosystems, especially tropical forests and grasslands, and our knowledge of the species that live in such systems. We can measure the rate of loss of tropical rain forests, for example, by analyzing satellite photographs of continents from different periods that show rates and amounts of habitat destruction—and from these measurements calculate the approximate number of species being lost each year.
This extraordinary rate of extinction has occurred only five times before in the history of complex life on Earth. Mass extinctions of the geological past were caused by catastrophic physical disasters, such as climate changes or meteorite impacts, which destroyed
and disrupted ecosystems around the globe. In the fifth mass extinction, which occurred more than 65 million years ago, the Earth was shrouded in a cloud of atmospheric dust—the result of meteorite impact or widespread volcanic activity. The resulting environmental disruption caused the demise of 76 percent of all species alive at the time, including the dinosaurs. Today’s sixth extinction is likewise primarily caused by ecosystem disturbance—but this time the destroying force is not the physical environment, but rather humankind. The human transformation of the Earth's surface threatens to be every bit as destructive as any of the past cataclysmic physical disasters.
and disrupted ecosystems around the globe. In the fifth mass extinction, which occurred more than 65 million years ago, the Earth was shrouded in a cloud of atmospheric dust—the result of meteorite impact or widespread volcanic activity. The resulting environmental disruption caused the demise of 76 percent of all species alive at the time, including the dinosaurs. Today’s sixth extinction is likewise primarily caused by ecosystem disturbance—but this time the destroying force is not the physical environment, but rather humankind. The human transformation of the Earth's surface threatens to be every bit as destructive as any of the past cataclysmic physical disasters.
additional info :
| WORLD OF SCIENCE |
| The Biodiversity Issue: Is Nature at Risk? |
| Plant and animal species are disappearing from the earth at an alarming rate, and many scientists believe that human activity is largely responsible. Biodiversity—the biological variety that thrives in a healthy ecosystem—became the focus of intense international concern during the 1990s. What species are disappearing particularly fast and why? Is biodiversity essential to the health of the environment? If sustaining biodiversity requires modifying human behavior and limiting population growth, are there any benefits to reap? In a 1995 Collier’s Year Book article, environmental writer Stevenson Swanson explores this crucial environmental issue. |
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