Quiz Time: How Many Different Species on Earth?

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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).

Recent discoveries of new critters, such as the psychedelic frogfish, a tiny lizard the size of a dime, and the blind hairy mini-lobster, have piqued interest in the "small and weird" animals that might be out there, writes veteran AP science reporter Seth Borenstein.

The new study is already starting to spark scientific debate, of course, about the way the estimate was calculated and whether the numbers are even in the ballpark. In The New York Times, science writer Carl Zimmer says that while some describe the work as "very important," other critics charge that 8.7 million species could be a gross underestimate and that "Earth's true diversity is far greater."

Some cynics may ask, why should we care? That answer may be existential for some, practical for others. On the practical side, there are potential benefits in medicine and agriculture. In his crisp commentary in the journal, Lord May, a distinguished Oxford zoologist and former president of Britain's Royal Society, noted that a Victorian physicist once dismissed the quest as little more than stamp collecting. Nonsense, he says: 

Such knowledge is important for full understanding of the ecological and evolutionary processes which created, and which are struggling to maintain, the diverse biological riches we are heir to. Such biodiversity is much more than beauty and wonder, important though that is. It also underpins ecosystem services that -- although not counted in conventional GDP -- humanity is dependent upon.

May cites the development of a new strain of rice in the 1970s that crossed a conventional type with a new variety discovered in the wild. The resulting hybrid yielded 30 percent more grain.

Unfortunately, however, human activities, including the loss of habitat and the changes accelerated by climate change, threaten the very survival of many forms of life. As one of the study authors, Dr. Camilo Mora, of the University of Hawaii, writes: "With the clock of extinction now ticking faster for many species, I believe speeding the inventory of earth's species merits high scientific and societal priority. Renewed interest in further exploration and taxonomy would allow us to fully answer this most basic question: What lives on earth?"

At the conventional pace and current funding, that task could take from 500 to 1,200 years to complete the job. But new molecular biology methods, such as "barcode taxonomy," could help shorten the job along with more people joining the quest. Places like Costa Rica are adopting novel approaches that train local people (so-called "parataxonomists") to help experts recognize new species. May's "optimistic guess" would be about a century to finish the assessment of life on earth. 

That may be too heavy for idle cocktail party chatter. But before your companion escapes for another drink, you could always try.

Image: Keri Wilk

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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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