News in Science

8.7 million species on Earth, say experts

Species counted Scientists have come up with the first precise, accurate and verifiable estimate of the number of species on Earth.

Ecologist Professor Camilo Mora of the University of Hawaii and colleagues report their findings today in the journal PLoS Biology.

One of the most important characteristics of life is diversity, says Mora, yet for 250 years scientists have been unable to answer the critical question of how many species there are on the planet.

"It's the most basic question, however practically it has been very challenging to answer," he says.

"This is the first time we've delivered a method that gives a number. The number we have come out with 8.7 million species. It is not only precise, but it is also accurate - we have validated it."

Estimates to date of the number of species on Earth range from 3 million to 100 million species, says Mora.

These have been based on the unvalidated opinions of experts, says Mora.

New method

Mora and colleagues based their estimate on a pattern in the relationship between the number of groups at different taxonomic levels.

The Linnean system categorises each species as being part of a genus, which in turn is part of a family, order, class, phylum and finally, kingdom.

Most is known about the higher taxonomic groups (genera, families, orders, classes and phyla) and least is known about the lower taxonomic groups such as species.

Scientists now believe they have a handle on the final number of higher taxonomic groups in the animal and plant kingdoms, since there has been a levelling off in discovery of new groups.

Mora and colleagues plotted the relationship between the number of each of these better documented taxonomic groups.

Using statistics they then found the best mathematical model to fit the data and to extrapolate from it to estimate the number of species.

Validation

To validate their model, Mora and colleagues used it to predict the number of species in 18 taxonomic groups from different kingdoms for which species numbers are well known.

They found that their predicted number of species agreed with the actual number in all of the taxonomic groups tested.

Using their validate model the researchers predicted the approximate number of species in each kingdom to be:



7.77 million species of animals (of which 953,434 have been described and catalogued)



298,000 species of plants (of which 215,644 have been described and catalogued)



611,000 species of fungi (of which 43,271 have been described and catalogued)



36,400 species of protozoa - single-cell organisms with animal-like behaviour (of which 8,118 have been described and catalogued)



27,500 species of chromists - including brown algae, diatoms and water moulds (of which 13,033 have been described and catalogued)



Mora says the new findings should spur future research to focus on discovery of new species of fungi, of which only a small proportion have been described.

He says the number of prokaryote species, which include bacteria, is only approximately 11,000.

Missing knowledge

Overall, the researchers estimate that 86 per cent of all species on land and 91 per cent of those in the seas have yet to be discovered, described and catalogued.

"The air we breathe, the water we drink and the food we eat - all of these things are delivered to us by species," says Mora.

"The possibility of finding species that will improve human welfare are just massive."

At the same time species are being lost due to climate change, ocean acidification, habitat loss, invasive species and pollution.

"We cannot really appreciate the magnitude of that loss until we know how many species are there."

The researchers estimate that it would cost $US 364 billion and take 300,000 taxonomists 1200 years to describe all remaining species using traditional approaches.

Fortunately, they add, new techniques such as DNA barcoding are making new species identification faster and cheaper.