How Many Species? A Study Says 8.7 Million, but It’s Tricky

In the foothills of the Andes Mountains lives a bat the size of a raspberry. In Singapore, there’s a nematode worm that dwells only in the lungs of the changeable lizard.

These are certainly not the last two species that scientists will ever discover. Each year, researchers report more than 15,000 new species, and their workload shows no sign of letting up. “Ask any taxonomist in a museum, and they’ll tell you they have hundreds of species waiting to be described,” says Camilo Mora, a marine ecologist at the University of Hawaii.

Scientists have named and cataloged 1.3 million species. How many more species there are left to discover is a question that has hovered like a cloud over the heads of taxonomists for two centuries.

“It’s astounding that we don’t know the most basic thing about life,” said Boris Worm, a marine biologist at Dalhousie University in Nova Scotia.

On Tuesday, Dr. Worm, Dr. Mora and their colleagues presented the latest estimate of how many species there are, based on a new method they have developed. They estimate there are 8.7 million species on the planet, plus or minus 1.3 million.

The new paper, published in the journal PLoS Biology, is drawing strong reactions from other experts. “In my opinion this is a very important paper,” said Angela Brandt, a marine biologist at the University of Hamburg in Germany. But critics say that the method in the new paper can’t work, and that Earth’s true diversity is far greater.

In 1833, a British entomologist named John Obadiah Westwood made the earliest known estimate of global biodiversity by guessing how many insect species there are. He estimated how many species of insects lived on each plant species in England, and then extrapolated that figure across the whole planet. “If we say 400,000, we shall, perhaps, not be very wide of the truth,” he wrote.

Today, scientists know the Westwood figure is far too low. They’ve already found more than a million insect species, and their discovery rate shows no signs of slowing down.

In recent decades, scientists have looked for better ways to determine how many species are left to find. In 1988, Robert May, an evolutionary biologist at the University of Oxford, observed that the diversity of land animals increases as they get smaller. He reasoned that we probably have found most of the species of big animals, like mammals and birds, so he used their diversity to calculate the diversity of smaller animals. He ended up with an estimate 10 to 50 million species of land animals.

Other estimates have ranged from as few as 3 million to as many as 100 million. Dr. Mora and his colleagues believed that all of these estimates were flawed in one way or another. Most seriously, there was no way to validate the methods used, to be sure they were reliable.

For the new estimate, the scientists came up with a method of their own, based on how taxonomists classify species. Each species belongs to a larger group called a genus, which belongs to a larger group called a family, and so on. We humans, for example, belong to the class of mammals, along with about 5,500 other species.

In 2002, researchers at the University of Rome published a paper in which they used these higher groups to estimate the diversity of plants around Italy. At three different sites, they noted the number of genera, families and so on. There were fewer higher-level groups than lower ones at each site, like the layers of a pyramid. The scientists could estimate how many species there were at each site, much as it’s possible to estimate how big the bottom layer of a pyramid based on the rest of it.

The paper drew little notice at the time, but Dr. Mora and his colleagues seized on it, hoping to use the method to estimate all the species on Earth. They charted the discovery of new classes of animals since 1750. The total number climbed steeply for the first 150 years and then began to crest — a sign that we’re getting close to finding all the classes of animal. They found that the discovery rate of other high-level groups has also been slowing down. The scientists built a taxonomic pyramid to estimate the total number of species in well-studied groups, like mammals and birds. They consistently made good predictions.

Confident in their method, the scientists then used it on all major groups of species, coming up with estimates of 7.7 million species of animals, for example, and 298,000 species of plants. Although the land makes up 29 percent of the Earth’s surface, the scientists concluded that it is home to 86 percent of the world’s species.

“I think it is an interesting and imaginative new approach to the important question of how many species actually are alive on earth today,” said Lord May.

But Terry Erwin, an entomologist at the Smithsonian Institution, think there’s a big flaw in the study. There’s no reason to assume that the diversity in little-studied groups will follow the rules of well-studied ones. “They’re measuring human activity, not biodiversity,” he said.

David Pollock, an evolutionary biologist at the University of Colorado who studies fungi — a particularly understudied group — agrees. “This appears to be an incredibly ill-founded approach,” he said. There are 43,271 cataloged species of fungi, based on which Dr. Mora and his colleagues estimate there are 660,000 species of fungi on Earth. But other studies on fungus diversity suggest the number may be as high as 5.1 million species.

The authors of the new study acknowledge that their method doesn’t work well with bacteria. Scientists have only started to really dig into the biodiversity of microbes, and so they are finding high-level groups of bacteria at a brisk pace. Dr. Mora and his colleagues write that their estimate — about 10,000 species — should be considered a “lower bound.”

Microbiologists, on the other hand, are fairly sure the diversity of microbes will turn out to dwarf the diversity of animals. A single spoonful of soil may contain 10,000 different species of bacteria, many of which are new to science.

Jonathan Eisen, an expert on microbial diversity at the University of California, Davis, said he found the new paper disappointing.

“This is akin to saying, ‘Dinosaurs roamed the Earth more than 500 years ago,’ ” he said. “While true, what is the point of saying it?”