Just how many different species are there? To date nobody has a precise answer – but it doesn’t stop scientists trying to work it out…
The astonishing diversity of life on our planet is one of its most distinctive features. Actually counting how many species of animals and plants – not to mention fungi and microscopic creatures – exist is extremely difficult, and to date nobody has a precise answer.
Species new to science are turning up all the time. Estimates from experts in taxonomy differ enormously, from as little as three million to as great as 100 million. A study in the journal PLoS Biology suggests that there are roughly 8.7 million species of eukaryote (that is, all complex life such as animals, plants and fungi, but excluding bacteria and viruses), of which about 2.2 million species live in the sea.
Though some 1.2 million species have been catalogued over the past 250 years, the same report estimated that a staggering 86 per cent of species on land and 91 per cent of marine creatures have yet to be discovered.
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The problem is that the results you get depend strongly on how you use existing knowledge to extrapolate the number of species yet to be found.
The PLoS study used a metric based on the idea that ‘higher’ taxonomic ranks (such as classes or orders) are discovered more rarely than lower ones (such as species). But this is just one of a whole 200 of methods to estimate species number.
For example, ecological theorist Robert May used the idea that larger creatures were rarer than smaller ones to estimate that there were 10-50 million species; fungal specialist David Hawksworth used a rough estimate that there were six species of fungus for every plant to estimate 1.6 million fungi species; and analysis of the rate at which new species of flowering plant are named led to a prediction that 13-18 per cent of flowering plant species remain unknown.
Given how rapidly we are wiping out species, many organisms will probably become extinct before they are known to science.