Several mass extinctions have occurred throughout Earth's history. The most famous is the extinction that wiped out the dinosaurs at the end of the Cretaceous period. But the demise of dinosaurs paled in comparison to an extinction event that occurred at the end of the Permian period, about 251 million years ago. When the dying was over, 95 percent of all marine species and 70 percent of all land vertebrates had vanished. But here's the thing: These extinctions were caused by major geologic or meteorological events, like an asteroid impact.
Scientists believe that the Ordovician mass extinction was caused by a change in climate. As the Earth cooled off, glaciers formed, causing sea levels to drop. In turn, this caused half of all the animal families to become extinct. Scientists aren't sure what caused the late Devonian mass extinction but they do know that it resulted in the extinction of 25 percent of all marine families and 50 percent of all marine genera. Finally, no one is sure what caused the Cretaceous-Tertiary event (called the K-T event), but the result was the dinosaurs' mass extinction.
A species becomes extinct when none of its members are left alive, and a species can arrive at this point for a number of reasons, including:
Loss of habitat: Members of a species often find themselves with no place to live as the nearby human population grows. This has happened for many reasons over history, but man has accelerated this process. As people move farther into unsettled areas and cities expand, the native species in those areas lose their homes. If the species are able to move and find new homes, they have a chance of surviving. Otherwise, habitat loss often leads to extinction.
Hunting: People often hunt specific animals to the point of extinction.
Climate change: This can lead to extinction, and in fact likely caused the extinctions at the end of the last ice age.
Many modern extinctions result from human activity, not natural phenomena at the planetary level. Ancient extinctions played out over millions of years, but we've seen plants and animals disappear in decades. Some scientists estimate that species are disappearing at 50 to 100 times the natural rate -- a number predicted to rise dramatically over the next several years.
And if you're wondering how many species have disappeared, well that's difficult to say since scientists aren't sure how many species have existed. They can estimate, however, that more than 3 billion species are now extinct. Remarkably, scientists think that less than a third of these species were victims of mass extinctions Newman. Species usually move toward extinction very gradually, and until recently, the average rate of extinction, or background rate, was one to five species per year Newman. The number has increased significantly, however, and is now estimated at between 100 and 5,000 species per year Holsinger. Nevertheless, these are all educated guesses; no one's really sure how many species are alive today.
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