The importance of metazoan parasites in the food web of the Ythan Estuary, Aberdeenshire
Food web theory purports to describe properties that are common to all natural communities. Unfortunately, the data upon which the theory is based is fragmentary, and one of the most obvious omissions is that of parasites. The present study attempts to address this problem. The Digenean parasites found in the Ythan in two species of estuarine snail, Littorina littorea and Hydrobia ulvae, were identified, and prevalences were recorded. Field and laboratory experiments were conducted to assess the impact of infection on the mortality, behaviour and fecundity of the snail hosts. The importance of the parasites as predators of the snails in this ecosystem was assessed relative to the impact of other species recorded in the published Ythan food web. The parasites found in 12 species of fish occurring in the Ythan were recorded. In addition, five species of birds taken from Loch Leven, Perthshire, were autopsied, and parasites present were identified. Using this data, along with information available from the literature, the published food webs of the Ythan and Loch Leven systems were augmented with host-parasitic links. The new webs thus created were analysed for five patterns (food chain length, proportion of top, intermediate and basal species, degree of omnivory, linkage density and degree of triangulation). The values for these patterns were compared with those obtained from published webs without parasites, and with the predictions of the cascade model. A revised version of the cascade model, incorporating scale variant linkage density, was found to accurately predict maximum food chain length.