Skip to main content
U.S. flag

An official website of the United States government

Homage to Linnaeus: How many parasites? How many hosts?

August 1, 2008

Estimates of the total number of species that inhabit the Earth have increased significantly since Linnaeus's initial catalog of 20,000 species. The best recent estimates suggest that there are ≈6 million species. More emphasis has been placed on counts of free-living species than on parasitic species. We rectify this by quantifying the numbers and proportion of parasitic species. We estimate that there are between 75,000 and 300,000 helminth species parasitizing the vertebrates. We have no credible way of estimating how many parasitic protozoa, fungi, bacteria, and viruses exist. We estimate that between 3% and 5% of parasitic helminths are threatened with extinction in the next 50 to 100 years. Because patterns of parasite diversity do not clearly map onto patterns of host diversity, we can make very little prediction about geographical patterns of threat to parasites. If the threats reflect those experienced by avian hosts, then we expect climate change to be a major threat to the relatively small proportion of parasite diversity that lives in the polar and temperate regions, whereas habitat destruction will be the major threat to tropical parasite diversity. Recent studies of food webs suggest that ≈75% of the links in food webs involve a parasitic species; these links are vital for regulation of host abundance and potentially for reducing the impact of toxic pollutants. This implies that parasite extinctions may have unforeseen costs that impact the health and abundance of a large number of free-living species.

Citation Information

Publication Year 2008
Title Homage to Linnaeus: How many parasites? How many hosts?
DOI 10.1073/pnas.0803232105
Authors Andy Dobson, Kevin D. Lafferty, Armand M. Kuris, Ryan F. Hechinger, Walter Jetz
Publication Type Article
Publication Subtype Journal Article
Series Title Proceedings of the National Academy of Sciences of the United States of America
Series Number
Index ID 70179521
Record Source USGS Publications Warehouse
USGS Organization Western Ecological Research Center