8.7 million species live on Earth
Earth is home to 8.7 million species of animals and plants, only a tiny proportion of which are known to science, experts have said.
Scientists arrived at the figure after adopting a new approach to estimating species numbers which they believe is more accurate.
Previous "guesstimates" have put the number at anything from three to 100 million.
The research indicates there are 6.5 million species living on the land and 2.2 million, about a quarter of the total, in the oceans. Yet 86% of all terrestrial species and 91% of marine species have yet to be discovered, described and catalogued.
Lead scientist Dr Camilo Mora, from the University of Hawaii, US, and Dalhousie University in Halifax, Canada, said: "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.
"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 improve human well-being."
Experts from the Census of Marine Life, a major investigation of life in the oceans, devised the new way of estimating species numbers. They reported their findings in the online journal Public Library of Science Biology.
Swedish scientist Carl Linnaeus created the system still used to name and describe species in 1758. In the 253 years since, around 1.2 million species, roughly a million on land and 250,000 in the oceans, have been described and catalogued.
Linnaeus developed the taxonomic classification system which groups forms of life in a pyramid-like hierarchy, ranked upwards from species to genus, family, order, class, phylum, kingdom and domain.
Analysing data on ocean species, the Census of Marine Life scientists spotted a numerical pattern linking the higher taxonomic levels to species numbers. The scientists applied the technique to all five known kingdoms of eukaryotes - living organisms with structured cells that exclude bacteria and viruses. This predicted 7.77 million species of animals, 298,000 species of plants, 611,000 species of fungi, 36,400 species of single-celled organisms, or protozoa, and 27,500 species of chromists which include brown algae, diatoms and water moulds.