Damage caused by introduced species results from the high population densities and large body sizes that they attain in their new location. Escape from the effects of natural enemies is a frequent explanation given for the success of introduced species. Because some parasites can reduce host density and decrease body size, an invader that leaves parasites behind and encounters few new parasites can experience a demographic release and become a pest. To test whether introduced species are less parasitized, we have compared the parasites of exotic species in their native and introduced ranges, using 26 host species of molluscs, crustaceans, fishes, birds, mammals, amphibians and reptiles. Here we report that the number of parasite species found in native populations is twice that found in exotic populations. In addition, introduced populations are less heavily parasitized (in terms of percentage infected) than are native populations. Reduced parasitization of introduced species has several causes, including reduced probability of the introduction of parasites with exotic species (or early extinction after host establishment), absence of other required hosts in the new location, and the host-specific limitations of native parasites adapting to new hosts.

Parasitism is the most common consumer strategy among organisms, yet only recently has there been a call for the inclusion of infectious disease agents in food webs. The value of this effort hinges on whether parasites affect food-web properties. Increasing evidence suggests that parasites have the potential to uniquely alter food-web topology in terms of chain length, connectance and robustness. In addition, parasites might affect food-web stability, interaction strength and energy flow. Food-web structure also affects infectious disease dynamics because parasites depend on the ecological networks in which they live. Empirically, incorporating parasites into food webs is straightforward. We may start with existing food webs and add parasites as nodes, or we may try to build food webs around systems for which we already have a good understanding of infectious processes. In the future, perhaps researchers will add parasites while they construct food webs. Less clear is how food-web theory can accommodate parasites. This is a deep and central problem in theoretical biology and applied mathematics. For instance, is representing parasites with complex life cycles as a single node equivalent to representing other species with ontogenetic niche shifts as a single node? Can parasitism fit into fundamental frameworks such as the niche model? Can we integrate infectious disease models into the emerging field of dynamic food-web modelling? Future progress will benefit from interdisciplinary collaborations between ecologists and infectious disease biologists.

Parasitism is the most common animal lifestyle, yet food webs rarely include parasites. The few earlier studies have indicated that including parasites leads to obvious increases in species richness, number of links, and food chain length. A less obvious result was that adding parasites slightly reduced connectance, a key metric considered to affect food web stability. However, reported reductions in connectance after the addition of parasites resulted from an inappropriate calculation. Two alternative corrective approaches applied to four published studies yield an opposite result: parasites increase connectance, sometimes dramatically. In addition, we find that parasites can greatly affect other food web statistics, such as nestedness (asymmetry of interactions), chain length, and linkage density. Furthermore, whereas most food webs find that top trophic levels are least vulnerable to natural enemies, the inclusion of parasites revealed that mid-trophic levels, not low trophic levels, suffered the highest vulnerability to natural enemies. These results show that food webs are very incomplete without parasites. Most notably, recognition of parasite links may have important consequences for ecosystem stability because they can increase connectance and nestedness.connectance ͉ parasitism ͉ trophic ͉ predation ͉ trematode F ood webs trace the flow of energy through an ecosystem. In revealing how consumer-resource interactions lead to trophic cascades, apparent competition, and diversity-stability relationships, food webs provide a unifying theme for ecology (1). Ironically, the most common consumer strategy, parasitism (2), is usually left out of food webs because parasites are often more difficult to quantify by standard ecological methods (3). To fit the resulting parasite-free food webs, prominent theoretical concepts, such as the cascade (4) and niche (5) models, generally assume that consumers eat species smaller than themselves, suggesting that new theoretical models may be needed to accommodate parasites, which eat species larger than themselves. Before taking this step, however, it is expedient to consider how, if at all, parasites affect food webs.Insect parasitoids are the only type of parasite commonly included in food webs. This is because parasitoids are large relative to host size, are easy to sample, and can affect insect population dynamics. However, insights gained from adding parasitoids to food webs may not extend to other infectious agents that have very different life histories (6). In comparison with highly host-specific parasitoids, individual life stages of typical parasites (an adult tapeworm in a dog's gut) can have a broad host range (7). Furthermore, many typical parasites have complex multiple-host life cycles that are embedded in food webs (3). Calls for considering typical parasites in food webs (3,(8)(9)(10) have gained few responses. The initial efforts have shown little effect of including parasites in food webs beyond obvious increases in species richness, number of links, trophi...

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