But Bebber thinks this method is the wrong one for the data, and that the team should have used a technique known as ordinal regression.Īs a result, the true number of species could be much higher or lower than 8.7 million, he said. The study team used a method called linear regression to calculate the number of Earth's species. "If I asked you to count out 8.7 million pennies, that would take you a while, even if you had a whole lot of people doing it."īut Dan Bebber, an ecologist at the environmental group Earthwatch Institute, said the study relies on improper statistical methods.
The new study "takes a hugely clever approach, and I think it's going to turn out to be a pretty important study," said Lucas Joppa, a conservation ecologist at Microsoft Research, the research branch of the software giant. Some experts called the research, published August 23 in the journal PLoS Biology, reasonable. Using complex statistics, Worm and colleagues used the number of genera, families, and so on to predict Earth's number of unknown species, and their calculations gave them a number: 8.7 million. That's a relatively easy task, since the number of new examples in these categories has leveled off in recent decades.īy contrast, the number of newly discovered species continues to rise sharply. Worm's team estimated the total number of genera, families, orders, classes, and phyla-a designation above class-in each kingdom. There are five kingdoms: animals, plants, fungi, chromists-including one-celled plants such as diatoms-and protozoa, or one-celled organisms. (See photos of species classification in National Geographic magazine.) Scientists lump similar species together into a broader grouping called a genus, similar genera into a still broader category called a family, and so on, all the way up to a supercategory called a kingdom. To gain a more precise answer, the authors examined the categories into which all species are grouped. (See "'Encyclopedia of Life' to Catalog All Species on Earth.") Previous guesses ranged from three million all the way to a hundred million. To calculate the percentage of unknown species, Worm and colleagues first had to answer one of the great questions of ecology: How many species live on the Earth? So far, some 1.2 million species are known to science. "There is an age of discovery ahead of us when we could find out so much more of what lives with us on this planet." What's been discovered so far are "those things that are easy to find, that are conspicuous, that are relatively large," Worm said.
But the inventories for other classes are woefully sparse.įor instance, only 7 percent of the predicted number of fungi-which includes mushrooms and yeasts-has been described, and less than 10 percent of the life-forms in the world's oceans has been identified. Two hundred and fifty years after Swedish botanist Carl Linnaeus devised a formal system for classifying the diversity of nature, the catalog for some classes of living things-such as mammals and birds-is nearly complete, the study says. The study was driven by a simple question: "Are we within reach of finding all species, or are we way off?" said study co-author Boris Worm of Canada's Dalhousie University. That means scientists have cataloged less than 15 percent of species now alive-and current extinction rates mean many unknown organisms will wink out of existence before they can be recorded. Even after centuries of effort, some 86 percent of Earth's species have yet to be fully described, according to new study that predicts our planet is home to 8.7 million species.