Noah's Ark could never have contained them: there are probably about 8.7 million species of living things, the vast majority of them undiscovered, according to what is believed to be the most authoritative estimate ever carried out of the scale of life on earth.
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So far, about 1.2 million species, ranging from microscopic insects to the blue whale, the largest living creature, have been described, but it has always been recognised that the true total is very much higher. Previous estimates have ranged from three million, right up to 100 million, but the new figure, based on an innovative analytical technique, dramatically narrows the range of possibilities.
The assessment, by scientists from the Census of Marine Life, a 10-year international study of life in the oceans which reported last year, 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 per cent of the terrestrial species and 91 per cent of the marine species have yet to be discovered, described and catalogued, with many of them likely to be small creatures in inaccessible locations, such as beetles in the rainforest canopy or marine animals in the deep ocean (although large animals are still being found in countries such as Vietnam and Papua New Guinea).
"The question of how many species exist has intrigued scientists for centuries and the answer... is particularly important now because a host of human activities and influences are accelerating the rate of extinctions," said Camilo Mora of the University of Hawaii and Dalhousie University in Halifax, Canada, one of the lead authors of the study.
"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," he added.
The estimate is based on the way species are classed together, using the taxonomic classification system developed by the Swedish biologist Linnaeus in the 18th century, which is still in use today.
This 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 scientists spotted a numerical pattern linking the higher taxonomic levels to species numbers, which allows the latter to be predicted.
Applied to all five known kingdoms of eukaryotes – living organisms with structured cells that exclude bacteria and viruses – the technique 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.