Read more: “Rewriting the textbooks: When science gets it wrong“
Humans have been systematically naming species for just over 200 years. Discounting bacteria and viruses, which are not easily pigeonholed, we have logged 1.7 million so far. Ask a taxonomist to estimate Earth’s total inventory, and the most commonly touted figure is some 30 million species.
That is almost certainly a huge overestimate. It dates from 1982, when of the Smithsonian Institute in Washington DC took a headcount of beetles – Earth’s most richly speciated group of animals – living in the canopy of a single type of tropical tree in Panama. He extrapolated his tally of 1143 species to arrive at a figure of 30 million tropical arthropods worldwide (, vol 35, p 74). This phylum of invertebrates, which includes insects and spiders, is thought to account for around one-third of Earth’s total species. So even if Erwin’s number had been right, it should have translated into there being some 100 million species worldwide.
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Erwin’s work relied on a series of estimates and assumptions, such as the proportion of arthropods that are beetles, the total number of tropical tree species there are, and how picky beetles are when it comes to choosing the type of tree to live on. Last year, at the University of Melbourne, Australia, and his colleagues took all these factors into consideration in a new statistical analysis based on beetle counts in 56 species of tree in Papua New Guinea. They came up with a far lower figure for the arthropods – just 2.5 million species or thereabouts ().
Multiply this by three, and you come to a total of fewer than 8 million species. Hamilton goes even lower, arguing that vertebrates and plants have been more thoroughly catalogued than tropical arthropods. “The magic number is 5.5 million,” he says (New Scientist, 12 June 2010, p 4).
Having a firmer idea of the diversity of life makes it easier to conserve. But even with just 4 million more species to discover, it seems unlikely that the rate at which we find new ones will exceed that at which we are extinguishing others.
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