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ever, this perspective ignores several important “ecosystem services” that parasites perform. For example, parasites often act as regulators of host abundance, which in the case of generalist pathogens may lead to strong frequency-dependent control over relative host abundance throughout the host community (Dobson, 2004). Another example involves parasitic helminths that may play a major role in buffering levels of pollution in natural communities (Sures, 2003).

REGULATION OF HOST POPULATIONS AND RELATIVE ABUNDANCE IN COMMUNITIES

Parasites create a diversity of links in food webs that at first sight may appear atypical, but they are not unusual in nature—more than 75% of links in natural food webs probably involve parasites (Lafferty et al., 2006b). Because many parasites use multiple competing hosts on the same trophic level, their population dynamics may be modeled by sets of coupled differential equations that take the general form

where we assume that each host species i has species-specific birth and death rates (b and d) and experiences transmission of the pathogen at a rate βij from infected individuals of species j. Infection converts each susceptible host, S, into an infectious individual, I, that experiences an increased pathogen-induced mortality rate, α. When compared with single-species infectious disease models, the presence of interspecific transmission is usually strongly stabilizing for a wide range of interspecies transmission rates that are less than the rates of within-species transmission (Dobson, 2004). However, when rates of interspecific transmission approach rates of within-species transmission, the pathogen acts as a powerful mechanism of indirect competition [as a shared natural enemy (Holt and Lawton, 1994)] that can drive some host species extinct.

We can examine the potential consequences of this for more complex systems by recasting the differential equation models within the matrix framework that describes the initial trajectory of a perturbation to the whole food web. Thus, each element of the matrix represents a pairwise interaction between each pair of species in the food web (Pimm, 1982; Pascual and Dunne, 2005). If we retain our classification of each host as susceptible and infected, then the parasite in effect enters the food web as two species. Both have the phenotype of the host (although the feeding



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