. Simpson, 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).
Analyzing the taxonomic clustering of the 1.2 million species today 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.
Says Dr. Adl: "We discovered that, using numbers from the higher taxonomic groups, we can predict the number of species. The approach accurately predicted the number of species in several well-studied groups such as mammals, fishes and birds, providing confidence in the method."
When applied to all five known eukaryote* kingdoms of life on Earth, the approach predicted:
- ~7.77 million species of animals (of which 953,434 have been described and cataloged)
- ~298,000 species of plants (of which 215,644 have been described and cataloged)
- ~611,000 species of fungi (moulds, mushrooms) (of which 43,271 have been described and cataloged)
- ~36,400 species of protozoa (single-cell organisms with animal-like behavior, eg. movement, of which 8,118 have been described and cataloged)
- ~27,500 species of chromista (including, eg. brown algae, diatoms, water moulds, of which 13,033 have been described and cataloged)
Total: 8.74 million eukaryote species on Earth.
(* Notes: 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: certain micro-organisms and virus "types", for example, which could be highly numerous.)
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