There are 8.7 million species on Earth, according to the latest estimate, claimed to be far more precise than any before.
Census of Marine Life scientists say there are around 6.5 million species found on land and 2.2 million in the ocean. And, they say, a staggering 86 percent of all species on land and 91 percent of those in the seas have yet to be discovered, described and catalogued.
"The question of how many species exist has intrigued scientists for centuries and the answer, coupled with research by others into species' distribution and abundance, is particularly important now because a host of human activities and influences are accelerating the rate of extinctions," says
Says lead author Camilo Mora of the University of Hawaii and Dalhousie University in Canada.
"Many species may vanish before we even know of their existence, of their unique niche and function in ecosystems, and of their potential contribution to improved human well-being."
About 1.25 million species have been described and entered into central databases, with another 700,000-odd described but yet to reach the central databases.
Up to now, the best approximation of Earth's species total was based on educated guesses, with experts variously pegging the figure at something between three and 100 million.
The new research gives an estimated figure of 8.7 million by identifying a series of reliable numerical relationships between the more complete higher taxonomic levels and the species level.
"We discovered that, using numbers from the higher taxonomic groups, we can predict the number of species," says Sina Adl of Dalhousie University.
"The approach accurately predicted the number of species in several well-studied groups such as mammals, fishes and birds, providing confidence in the method."
The authors suggest that describing all remaining species using traditional approaches could require up to 1,200 years of work by more than 300,000 taxonomists - and cost $364 billion.
"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," says Mora.
"Renewed interest in further exploration and taxonomy could allow us to fully answer this most basic question: What lives on Earth?"