Biodiversity is critical for human life, which makes the latest study on it especially troubling. In research published in the Proceedings of the National Academy of Sciences on the sixth mass extinction we're currently undergoing, scientists looked at threatened-species data from the International Union for the Conservation of Nature, analyzing nearly 30,000 land vertebrate species. They found 388 species that have between 1,000 and 5,000 individuals, as well as 515 species that are "on the brink," meaning fewer than 1,000 animals are left in their population. Of those, more than half are believed to have just 250 or fewer remaining. Those on-the-brink species are in danger of vanishing completely over the next two decades, joining the 543 species that have disappeared over the last century—a number the New York Times notes would naturally have taken 10,000 years if humans weren't involved.
"In other words, every year over the last century we lost the same number of species typically lost in 100 years," study co-author Gerardo Ceballos says. Meanwhile, there's a percentage that's especially worrying—84% of the "under 5,000" species live in the same areas (mainly in the tropics) as the "under 1,000s," which can have an interspecies domino effect, per the scientists: "Extinction breeds extinctions." And, unlike the other five mass extinctions that have taken place, caused by such natural events as asteroid collision or volcanic eruptions, this time the finger is pointed squarely at humans. "It is entirely our fault," Ceballos tells CNN. "When humanity exterminates other creatures, it is sawing off the limb on which it is sitting," says Paul Ehrlich, another study co-author, per the Guardian. Adds a Center for Biological Diversity scientist: "The very survival of humanity is at stake." (Read more discoveries stories.)