GLOBAL: Nine million species on Earth?

Some 8.7 million species, give or take 1.3 million, is the new estimated total number of species on Earth. Said to be the most precise calculation ever offered, 6.5 million species are on land and 2.2 million, or about 25% of the total, dwell in the ocean depths. Census of Marine Life scientists say the estimates are 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 three million and 100 million. The study, published last Tuesday by PLoS Biology, says a staggering 86% of all species that live on land and 91% of those in the seas have yet to be discovered, described and catalogued.

Lead author Dr Camilo Mora of the University of Hawaii and Dalhousie University in Halifax, Canada, says the question of how many species exist has intrigued scientists for centuries. 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 well-being," Mora says.

Co-author Dr Boris Worm of Dalhousie University says the study deduces the most basic number needed to describe our living biosphere:

"If we did not know - even by an order of magnitude of one million, 10 million or 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."

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 report is published with 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 that 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," 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."

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 1.2 million species, roughly one million on land and 250,000 in the oceans, have been described and entered into central databases while some 700,000 more are thought to have been described but have yet to reach the central databases.

Up 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 three to 100 million. Such wildly differing numbers were questioned because there is no way to validate them.

Mora and Worm, together with Dalhousie colleagues, 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 listed 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.

They discovered that, using numbers from the higher taxonomic groups, they could predict the number of species in other groups; the method accurately predicted the number of species in several well-studied groups such as mammals, fishes and birds, thus providing confidence in the technique used.

When applied to all five known eukaryote kingdoms of life on Earth, the approach predicted:

* 7.77 million species of animals, of which only 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 (moulds, mushrooms), of which 43,271 have been described and catalogued.
* 36,400 species of protozoa (single-cell organisms with animal-like behaviour such as movement), 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.

Total: 8.74 million eukaryote species on Earth. 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 are certain micro-organisms and virus 'types', for example, which could be highly numerous.

Within the 8.74 million total is an estimated 2.2 million (plus or minus 180,000) marine species of all kinds, about 250,000 (11%) of which have been described and catalogued.

When it formally concluded in October 2010, the Census of Marine Life offered a conservative estimate of one million+ species in the seas.

Based on current costs and requirements, the study suggests that describing all remaining species using traditional approaches could require up to 1,200 years of work by more than 300,000 taxonomists at a cost of $US364 billion. Fortunately, new techniques such as DNA bar coding are radically reducing the cost and time involved in new species identification.

"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?"