Colloquium paper: homage to Linnaeus: how many parasites? How many hosts?

Abstract

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 approximately 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 approximately 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.

Fig. 1.

Estimates since the time of Linnaeus of the number of metazoan species. Data are from Erwin (6), and the dates for Linnaeus (1735) and John Ray (1691) were estimated from time of publication of their major books on this topic (6). The most recent sets of estimates sometimes provide a range, or an upper bound, and less frequently a “best estimate” of total species numbers.

Fig. 2.

Relative abundance of different taxa, and the proportion of parasitic species in those taxa (data from Rohde, 1982, Ref. 23). Taxa are numbered along the x axis as follows: 1, Mastigophora; 2, Opalinata; 3, Sarcodina; 4, Apicomplexa/Microspora; 5, Ciliophora; 6, Mesozoa; 7, Porifera; 8, Cnidaria; 9, Ctenophora; 10, Platyhelminthes; 11, Priapulida; 12, Entoprocta; 13, Nemertina; 14, Nemathelminthes; 15, Annelida; 16, Pentastomida; 17, Arthropoda; 18, Tentaculata; 19, Mollusca; 20, Echiurida; 21, Sipunculida; 22, Hemichordata; 23, Echinodermata; 24, Pogonophora; 25, Chaetognatha; 26, Chordata. The area of a circle corresponds to the natural log of the total number of species in a taxon, and the center of the circle corresponds to the proportion of parasitic species in that taxon.

Fig. 3.

Three-dimensional visualization of the complexity of a real food web with parasites from the Carpinteria Salt Marsh web using WoW software. Balls are nodes that represent species. Parasites are the light-shaded balls, and free-living species are the dark-shaded balls. Sticks are the links that connect balls through consumption. Basal trophic levels are on the bottom, and upper trophic levels are on the top. Figure from Lafferty et al. (65).

Fig. 4.

Latitudinal relationship between taxonomic richness (left) and geographical range size (right) for all 9,754 bird species at three different taxonomic levels: species, family, and order. Only breeding distributions were included, and range sizes were measured over dry land and averaged across all species, families, or orders occurring at a given latitudinal band.

Fig. 5.

The relationship between geographic range size and percentage range transformations for all of the world's 8,750 land birds under two MEA Scenarios of future land-use change. (Left) “Adaptive mosaic” (which assumes a world with open political dialogue that deals proactively with environmental problems). (Right) “Order from strength” scenario (which assumes a more insular political world that only deals retroactively with environmental problems). Jetz et al. (39) provide complete detail for how the analyses were developed. The dots illustrate number of avian species, blue shading denotes range change due to climate change, and red illustrates land use change due to agricultural expansion.