The rainforest harbours most of the as yet undescribed species. Photo taken by Thomas Hesselberg

How many species are found on Earth? You might think that this is a relatively simple question that would elicit a straightforward answer, but you would be wrong. It is very difficult to estimate the total number of species on our planet. Around 1.2 million species have been described by taxonomists, but all scientists agree that many more species remain to be discovered. Previous estimates range from 3 and up to 100 million species. However, a new study in PLoS Biology uses a newly develop method to estimate that a total number of 8.7 million eukaryotic species can be found on land and in the oceans.

A group of Canadian, American and English scientists have developed a new method to assess the total number of eukaryotic species on Earth. Eukaryotes are higher organisms with complicated cells containing organelles and comprising all animals, algae, plants, fungi and protist micro-organisms, but excluding bacteria.

The new method uses the fact that although the rate of discoveries of new species continues to rise, the discoveries of higher taxonomic classes (i.e. phylum, order, family, genus) is levelling off. This is then combined with a demonstrated relationship between the number of species and the higher taxonomy to estimate the number of species in each group.

Using this method, the scientists estimate a total of 8.7 million species of eukaryotes, and 10,000 of prokaryotes (i.e. bacteria) although they acknowledge that the latter estimate is very uncertain. Of these species, 6.5 million are terrestrial and 2.2 million are found in the oceans. The vast majority of species are animals with an expected number of 7.7 million followed by 600,000 species of fungi and 300,000 species of plants.

If this estimate is true a lot of species remain to be discovered as only about 14% of terrestrial and 9% of oceanic species have been formally described by taxonomists. Extrapolating from the current rate of discoveries there will be jobs for 300,000 taxonomists for the next 1,200 years before all species are described for an estimated total cost of $360 Billion.

Source
Mora C, Tittensor DP, Adl S, Simpson AGB, Worm B (2011) How Many Species Are There on Earth and in the Ocean? PLoS Biol 9(8): e1001127.doi:10.1371/journal.pbio.1001127

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