Parasites and Food Web Patterns

1. We expanded two previously studied food webs, those of the Ythan estuary and Loch Leven, by the documentation of metazoan endoparasites present in each system. The resulting webs are the first in the literature to contain substantial numbers of parasites. 2. Four versions of each web were produced, containing increasing numbers of parasite species and parasite-host links. Each version was analysed for four food-web patterns: food-chain length, linkage density, proportions of top, intermediate and basal species and omnivory. 3. The patterns were not generally consistent with those previously reported in the literature. In particular, increasing the numbers of parasitic links increased the pro- portions of top species, and the percentage of omnivory, in both the well (Ythan) and poorly (Loch Leven) documented webs. 4. These results were dependent on the choice of biological or 'tropho' species. Splitting the parasites documented in the Ythan web into separate life-history stages produced a greater conformity of omnivory and linkage density with previous webs, but a larger deviation of the proportion of top species from previously recorded values. 5. The cascade model accurately predicts the length of food chains in the Ythan web. This suggests that the feeding hierarchy assumed in the model may exist in nature, but that body size alone is not sufficient to explain it