PARASITES AND FOOD-WEB PATTERNS

1. We expanded two previously studied food webs, those of the Ythan es tuary and Loch Leven, by the documentation of metazoan endoparasites p resent in each system. The resulting webs are the first in the literat ure to contain substantial numbers of parasites. 2. Four versions of e ach web were produced, containing increasing numbers of parasite speci es and parasite-host links. Each version was analysed for four food-we b patterns. food-chain length, linkage density, proportions of top, in termediate and basal species and omnivory. 3. The patterns were not ge nerally consistent with those previously reported in the literature. I n particular, increasing the numbers of parasitic links increased the proportions of top species, and the percentage of omnivory, in both th e well (Ythan) and poorly (Loch Leven) documented webs. 4. These resul ts were dependent on the choice of biological or 'tropho' species. Spl itting the parasites documented in the Ythan web into separate life-hi story stages produced a greater conformity of omnivory and linkage den sity with previous webs, but a larger deviation of the proportion of t op species from previously recorded values. 5. The cascade model accur ately predicts the length of food chains in the Ythan web. This sugges ts that the feeding hierarchy assumed in the model may exist in nature , but that body size alone is not sufficient to explain it.