Are humans causing a sixth mass extinction of Earth’s creatures?

Science : Although species naturally come and go over long periods of time, a new study says that Earth’s creatures are on the brink of a mass extinction, the sixth in its history, writes Ann Gibbons for Science. A mass extinction is defined as when three-quarters of all species vanish quickly, such as the dinosaur disappearance 65 million years ago after an asteroid struck Earth. Conservationists have warned for years that we are in the midst of a human-caused extinction, with species from frogs to birds to tigers threatened by climate change, disease, loss of habitat, and competition for resources with nonnative species. The study’s lead author, Anthony Barnosky of the University of California, Berkeley, says, “This is really gloom-and-doom stuff. But the good news is we haven’t come so far down the road that it’s inevitable.” If humans work quickly to protect endangered and threatened species and their habitats now, the mass extinction can be prevented or at least delayed by thousands of years.