Homage to Linnaeus How Many Parasites How Many Hosts

ANDY DOBSON,* KEVIN D. LAFFERTY,+ ARMAND M. KURIS,* RYAN F. HECHINGER,* and WALTER JETZ§

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 tropi-

*EEB, Guyot Hall, Princeton University, Washington Road, Princeton, NJ 08544; +Western Ecological Research Center, U.S. Geological Survey, Marine Science Institute, University of California, Santa Barbara, CA 93106; ^Department of Ecology, Evolution, and Marine Biology, and Marine Science Institute, University of California, Santa Barbara, CA 93106; and §Division of Biological Sciences, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093.

cal 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.

The year 2008 marks the tercentenary of the birth of Linnaeus, the scientist who first provided a formal classification for biological diversity. In the initial edition of Systema Naturae (Linnaeus, 1735), Linnaeus included a group of species—the Paradoxa—that confounded his classification or whose actual existence he questioned. Pelicans, for example, were placed in Paradoxa because Linnaeus thought they might reflect the over-fervent imaginations of New World explorers. Parasitic worms were also placed in Paradoxa because Linnaeus initially thought that they might be confused, or misplaced, earthworms. In later editions of Systema Naturae, Linnaeus revised his opinions about both pelicans and parasitic worms. We now know much about parasites but still rarely think of them as major components of biodiversity. One primary goal of this chapter is to revise this misconception and quantify the ubiquity of parasitism as a lifestyle. We then attempt to quantify how many parasite species are threatened with extinction.

To quantify the abundance and potential loss rates of parasite biodiversity, we initially need to quantify these measures for their host species. For this we have briefly synthesized the work of May (1988, 1990a), Stork (1993), Purvis and Hector (2000), and Erwin (2004). We then restrict our tally of parasite diversity to parasitic helminths of the vertebrates: trematodes, cestodes, acanthocephalans, and the parasitic nematodes. This tally will synthesize and update an excellent book-length treatment of this question by Poulin and Morand (2000, 2004). Although our approach uses the best available data for the most comprehensively studied groups of parasites and hosts, our attempts to quantify species numbers and extinction rates for parasites still provide underestimates of the true global values of these parameters for several taxonomic and pragmatic reasons: vertebrates are a small component of host diversity, vertebrates are parasitized by a subset of the helminths, and helminths are not the most fully described parasite taxa.

HOW MANY SPECIES ARE THERE ON EARTH?

Beginning in 1988, Robert May (1988, 1990a, 1992) cogently argued that our inability to estimate the diversity of species on Earth provided a sad and somewhat self-centered testimony to human inquisitiveness.

Homage to Linnaeus / 65

After collating data on the numbers of species in each major taxon, May (1988) concluded that our knowledge of vertebrates far exceeded that of invertebrates and protists. The principal reason for the deficient quantitative assessment of diversity in invertebrates and protists was the limited number of trained taxonomists (especially in the tropics, where most of the world's biodiversity resides). Although strides have been made to build capacity in these areas over the last 20 years (Janzen, 1994; Smith and Rogo, 2005), the number of taxonomists working in the museums of most tropical countries today is roughly comparable to the number that worked in Sweden's museums 250 years ago (in Linnaeus's time, at the dawn of taxonomy). Consequently, classifying and naming species continues to proceed at a slow and uneven rate.

Erwin's (1982) work on beetles in tropical forest canopies provided a dramatic illustration of our lack of comprehension of how many extant species exist. Erwin's initial estimates suggested there might be as many as 30 million species of beetles in the world's tropical forests [considerably more than the 20,000 species initially estimated by John Ray (1627-1705) and cataloged by Linnaeus in Systema Naturae (Linnaeus, 1735)]. Erwin's estimate of global insect diversity stimulated a series of articles that used a variety of different approaches to estimate total species numbers. Erwin (2004) recently reviewed this literature, and his summary table is illustrated in Fig. 4.1. Two key patterns emerge. First, estimates of global spe-

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FIGURE 4.1 Estimates since the time of Linnaeus of the number of metazoan species. Data are from Erwin (2004), and the dates for Linnaeus (1735) and John Ray (1691) were estimated from time of publication of their major books on this topic (Erwin, 2004). The most recent sets of estimates sometimes provide a range, or an upper bound, and less frequently a ''best estimate'' of total species numbers.

cies diversity have increased almost exponentially since Linnaeus's and Ray's original estimates. Second, various numerical estimates of global biodiversity made during the past 20 years concentrate between 3 and 10 million species, of which only 1.4 million have been formally described. It seems unlikely that we will ever achieve a secure estimate of extant species, particularly because many species seem destined for extinction before they are counted, classified, and formally named.

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