If half of all species go extinct, will humans be next?

How many animal species do you think go extinct every year? Last week I conducted a highly unscientific polling of around 20 of my Facebook and Google Chat contacts, asking that same question. I'm not trying to brag, but I have some really smart friends, many of them with degrees in biology. Typical answers ranged from about 17 to a seemingly ludicrous 400. They were all wrong though - off by orders of magnitude*. In July, a summary article of nearly 80 papers, published in Science, stated that, "Of a conservatively estimated 5 million to 9 million animal species on the planet, we are likely losing ~11,000 to 58,000 species annually." If that finding is true, then every year, between .12% and 1.16% of all the animals on Earth vanish. Rodolfo Dirzo, the lead researcher on the Science study from Stanford University, points out that we've already lost 40% of the Earth's invertebrate species in the last 40 to 50 years. Almost half the animals without skeletons have gone extinct within half a human lifetime. The wide range of these estimates reflects our own uncertainty on this subject, but even our low-end assessments are alarming. Bugs and worms are gross, though; who cares if there are fewer spiders in my house now than in the arachnid-infested '60s? Unfortunately the future looks just as bleak for mammals. Dirzo says that if current trends hold, "in 200 years, 50% of the [mammal] species are going to be driven to the very edge of extinction."