Washington DC / Cambridge (UK), 23 August 2011 - Eight million seven hundred thousand (give or take 1.3 million) is the latest estimated total number of species on Earth and the most precise calculation ever offered, according to a new study co-authored by a researcher with the United Nations Environment Programme (UNEP).
Around 6.5 million species are found on land and 2.2 million (about 25 percent of the total) dwell in the ocean depths. The report, which was co-authored by Derek Tittensor at UNEP's World Conservation Monitoring Centre (UNEP-WCMC), in Cambridge, UK, also shows that 86% of all species on land and 91% of those in the seas have yet to be discovered, described or catalogued.
Announced today by Census of Marine Life scientists and published by PLoS Biology, the figure of 8.7 million species is based on an innovative, validated analytical technique that dramatically narrows the range of previous estimates. Until now, the number of species on Earth was said to fall somewhere between 3 million and 100 million.
Says lead author Camilo Mora of the University of Hawaii and Dalhousie University in Halifax, Canada: "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 improved human wellbeing."
"This work deduces the most basic number needed to describe our living biosphere," says coauthor Boris Worm of Dalhousie University. "If we did not know - even by an order of magnitude (1 million? 10 million? 100 million?) - the number of people in a nation, how would we plan for the future?"
"It is the same with biodiversity. Humanity has committed itself to saving species from extinction, but until now we have had little real idea of even how many there are."
Dr. Worm notes that the recently-updated Red List issued by the International Union for the Conservation of Nature assessed 59,508 species, of which 19,625 are classified as threatened. This means the IUCN Red List, the most sophisticated ongoing study of its kind, monitors less than 1% of world species.
The research is published alongside a commentary by Lord Robert May of Oxford, past president of the UK's Royal Society, who praises the researchers' "imaginative new approach."
"It is a remarkable testament to humanity's narcissism that we know the number of books in the US Library of Congress on 1 February 2011 was 22,194,656, but cannot tell you - to within an order-of-magnitude - how many distinct species of plants and animals we share our world with," Lord May writes.
"(W)e increasingly recognise that such knowledge is important for full understanding of the ecological and evolutionary processes which created, and which are struggling to maintain, the diverse biological riches we are heir to. Such biodiversity is much more than beauty and wonder, important though that is. It also underpins ecosystem services that - although not counted in conventional GDP - humanity is dependent upon."
Drawing conclusions from 253 years of taxonomy since Linnaeus
Swedish scientist Carl Linnaeus created and published in 1758 the system still used to formally name and describe species. In the 253 years since, about 1.25 million species - roughly 1 million on land and 250,000 in the oceans - have been described and entered into central databases (roughly 700,000 more are thought to have been described but have yet to reach the central databases).
To now, the best approximation of Earth's species total was based on the educated guesses and opinions of experts, who variously pegged the figure in a range from 3 to 100 million ? wildly differing numbers questioned because there is no way to validate them.
Drs. Mora and Worm, together with colleagues Derek P. Tittensor, Sina Adl and Alastair G.B. Simpson, refined the estimated species total to 8.7 million by identifying numerical patterns within 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 the taxonomic clustering of the 1.2 million species today in the Catalogue of Life and the World Register of Marine Species, the researchers discovered reliable numerical relationships between the more complete higher taxonomic levels and the species level.
Says Dr. Adl: "We discovered that, using numbers from the higher taxonomic groups, we can predict the number of species. The approach accurately predicted the number of species in several well-studied groups such as mammals, fishes and birds, providing confidence in the method."
When applied to all five known eukaryote* kingdoms of life on Earth, the approach predicted:
1) ~7.77 million species of animals (of which 953,434 have been described and cataloged)
2) ~298,000 species of plants (of which 215,644 have been described and cataloged)
3) ~611,000 species of fungi (moulds, mushrooms) (of which 43,271 have been described and
4) ~36,400 species of protozoa (single-cell organisms with animal-like behavior, eg. movement,
of which 8,118 have been described and cataloged)
5) ~27,500 species of chromists (including, eg. brown algae, diatoms, water moulds, of which 13,033 have been described and cataloged)
Total: 8.74 million eukaryote species on Earth.
(* Notes: Organisms in the eukaryote domain have cells containing complex structures enclosed within membranes. The study looked only at forms of life accorded, or potentially accorded, the status of "species" by scientists. Not included: certain micro-organisms and virus "types", for example, which could be highly numerous.)
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