Quiz Time: How Many Different Species on Earth?

As countless creatures remain undocumented, a study offers an answer to a longstanding scientific mystery

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For a real conversation-stopper at a late-summer cocktail party, forget Tripoli, Dominique Strauss-Kahn, the East Coast earthquake, or the threatening hurricane. Instead, you might stump your fellow guests with a casual question: "Hey, have you ever wondered how many species of organisms there are on earth?"

While they're looking enviously toward the bar, shoot back this quick answer: "8,700,000, give or take a cool million. Oh, and about three-quarters live on land, the rest in the deep blue sea."

That's the latest estimate -- really a sophisticated scientific guesstimate -- from a new study that takes a crack at answering an age-old scientific mystery.

And, the strange thing is, we don't yet know, and may never know, what most of these creatures are. But, happily, scientists are busy trying to count the number of plants and animals -- and environmentalists are busy worrying about the ones we may be losing. And so should you, if you're not too busy being a smug Homo sapiens.

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"We are astonishingly ignorant about how many species are alive on earth today, and even more ignorant about how many we can yet lose," chides the Australian scientist Robert M. May. "It is a remarkable testament to humanity's narcissism that we know the number of books in the US Library of Congress on 1 February 2011 was 22,194,656, but cannot tell you -- to within an order-of-magnitude -- how many distinct species of plants and animals we share our world with."

In the 250 years since Linnaeus created his universal naming system, some 1.2 million species have been described and cataloged, about 1 million of them land-based and 250,000 in the oceans. And every year another 15,000 new creatures are uncovered. But estimates over the years about what the total might be have swung widely, from 3 million to 100 million.

In the latest attempt at narrowing that range down, a team of researchers from the University of Hawaii in Honolulu and from Dalhousie University in Halifax, Nova Scotia, developed a complex mathematical model that uses what we do know to calculate what we don't know: how many creatures inhabit our planet? Their study, published Tuesday in the online Public Library of Science journal, PLoS Biology, suggests that animals -- with some 7.8 million species -- dominate, followed by fungi (molds to mushrooms) at 610,000 and plants with nearly 300,000 species

The new research suggests that roughly nine out of 10 species have yet to be discovered. Not surprisingly, it's likely that we already know about most of the larger animals that swim the seas, roam the planet, or fly over it. So the great big unknown out there are likely to be millions of tiny animals and thousands of plants and fungi, many in narrow-range "hotspots" or less-explored areas of the deep sea and soil (although some speculate that many could be unseen in a shovel of soil in your own backyard).

Presented by

Cristine Russell is a senior fellow at Harvard Kennedy School of Government and the president of the Council for the Advancement of Science Writing.

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