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Planet Earth

#63: How Many Species Inhabit the Earth?


It's a tough question, but we now have the best answer yet.

By Veronique GreenwoodDecember 22, 2011 6:00 AM

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Last year researchers at Dalhousie University in Nova Scotia released the most rigorous estimate yet of how many species live on our planet: 8.7 million, not counting bacteria. Nearly 6.5 million of these species live on land versus 2.2 million in the ocean, according to the analysis. “Humanity has committed itself to saving species from extinction, but until now we have not had a good idea of even how many there are,” says coauthor Boris Worm, a marine ecologist at Dalhousie.

Previous estimates have ranged from as few as 3 million species to as many as 100 million. To arrive at a more certain answer, marine ecologist Camilo Mora examined life’s diversity at higher levels of taxonomy (genus, family, order, et cetera). When he and his team analyzed the 1.2 million known species listed in two enormous databases, they found a consistent pattern in the number of species per genus, genus per family, and so on. Much in the way seeing the top of a pyramid lets you predict the width of the bottom, Mora extrapolated the total number of species from the higher, more complete taxonomic groups to arrive at his estimate.

Mora’s approach is not without its critics, but it is the first one that can be validated. When applied to mammals, for instance, it gave the correct number of species: 5,500.

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