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This estimate is open to several questions. For one thing, the total includes relatively few species of bacterial, protozoan, and helminth parasites, largely because such parasites are usually studied in connection with economically important animal hosts. But it could be that essentially every animal species is host to at least one specialized such parasitic species (47), which would immediately double the estimated total. For another thing, the Acarina (mites), both tropical and temperate, are even less well studied than tropical insects it was largely tropical insects that carried the estimate from the known 1.5 million to 3 million to 5 million, and mites could carry it significantly higher. See Wartt et al. 2010 PMID 20847295: "There are estimated to be
48–58,000 vertebrate species and about two million invertebrates". For 10^7 different species of organisms on
Earth see Encyclopedia of Molecular Cell Biology and Molecular Medicine, Werner Arber, Genetic Variation and
Molecular Darwinism p.21 right column 3rd paragraph |