Earth’s creatures are on the brink of a sixth mass extinction, comparable to the one that wiped out the dinosaurs, concludes a new study that calculates that 3/4 of today’s animal species could vanish within 300 years.

http://news.sciencemag.org/sciencenow/2011/03/are-we-in-the-middle-of-a-sixth-.html?ref=hp

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.