needed to understand how these differences in habitat and transmission mode drive the different gradients of parasite species diversity.

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).