Are We Currently In A 6th Mass Extinction?
by Ann Gibbons
2 March 2011
Earth’s creatures are on the brink of a sixth mass extinction, comparable to the one that wiped out the dinosaurs. That’s the conclusion of a new study, which calculates that three-quarters of today’s animal species could vanish within 300 years. “This is really gloom-and-doom stuff,” says the study’s lead author, paleobiologist Anthony Barnosky of the University of California, Berkeley. “But the good news is we haven’t come so far down the road that it’s inevitable.”
Species naturally come and go over long periods of time. But what sets a mass extinction apart is that three-quarters of all species vanish quickly. Earth has already endured five mass extinctions, including the asteroid that wiped out dinosaurs and other creatures 65 million years ago. Conservationists have warned for years that we are in the midst of a sixth, 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. But how does this new mass extinction compare with the other five?
Barnosky and colleagues took on this challenge by looking to the past. First, they calculated the rate at which mammals, which are well represented in the fossil record, died off in the past 65 million years, finding an average extinction rate of less than two species per million years. But in the past 500 years, a minimum of 80 of 5570 species of mammals have gone extinct, according to biologists’ conservative estimates—an extinction rate that is actually above documented rates for past mass extinctions, says Barnosky. All of this means that we’re at the beginning of a mass extinction that will play out over hundreds or thousands of years, his team concludes online today in Nature.