How Many Parasites And What Is Their Role In An Ecological Food

An alternative approach to ascertain global estimates of parasite diversity is simply to examine how many parasites are in a specific habitat or ecosystem. We have been undertaking this for salt marshes along the coasts of California and Baja, Mexico (Lafferty et al., 2006a,b; Kuris et al., 2008). The initial results confirm that «40% of the species in any location are parasitic on the 60% of species that are free-living. However, consideration of the trophic links of the parasitic species significantly changes our perception of how ecological food webs are structured.

The standard ecological food web is normally considered to be a trophic pyramid, with primary producers on the bottom, fewer species of herbivores on the next level, and even fewer predatory species higher up (Lindeman, 1942). When parasites are included, this pattern is almost literally ''turned on its head'' (Fig. 4.3); essentially, a second web appears around the free-living web, and this completely changes the level of con-

FIGURE 4.3 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. (2008).

FIGURE 4.3 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. (2008).

nectivity. The addition of =40% more species to the community leads to four times the number of trophic connections between species, thus creating a web that is much more tightly coupled. In many ways, parasite species appear as hidden ''dark matter'' that holds the structure of the web together, and in ways that are very different from those of free-living species (Fig. 4.3). Furthermore, the web's structure changes from a pyramid to an inverted rhomboid. Predatory species at high trophic levels are now seen to be consumed from within by a diversity of parasites. Animals at lower trophic levels have fewer parasites, but they are often essential hosts for specific stages of parasites that need hosts from two or three different trophic levels to complete the life cycle. When transmitting between trophic levels, only a minority of parasites successfully infect a host; most parasite individuals are consumed as planktonic prey items by many of the species they are trying to parasitize.

Even if a parasite successfully establishes in a host, it is often consumed when the host becomes a prey item in the diet of a predator. Natural selection has made considerable use of this resource-consumer link and allowed parasites to continue their life cycle in the viscera of predatory species. In many cases, the parasites have evolved to modify the behavior of the prey to make it more accessible to the predator, thus significantly increasing transmission efficiency through this stage of the life cycle (Dobson, 1988; Lafferty, 1992). We suspect that the food-web structure observed in salt-marsh communities is common to most natural ecological communities, with parasite species comprising =40% of the local species diversity but exerting significant stabilizing forces that hold together the structure of much of the free-living web.

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