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.
Estimates for the loss of biodiversity use a variety of methods to compare current rates of species extinction against background rates (May et al., 1995; Regan et al., 2001). All of these methods suggest that we are entering a period of mass extinction that is directly comparable to the mass extinctions recorded in the fossil record. Poulin and Morand (2004) used the proportion of threatened hosts in each major vertebrate taxon to estimate the potential threatened number of parasitic species. We have modified their projection to consider different levels of host specificity (Table 4.2). Poulin and Morand’s original calculation assumed a direct correspondence between the proportion of parasites threatened and the proportion of hosts threatened. This figure was then adjusted by the degree of host specificity of the parasites. Koh et al. (2004) performed a similar analysis, using more sophisticated models on select groups of hosts and parasites for which they acquired good data on host-use patterns. All