Rocky shore communities are structured by interactions, such as competition for space, among the key species. The effects of competition are often influenced by other species. Grazing by limpets, for example, prevents fucoid species from establishing on more exposed shores (Jones, 1948; Southward and Southward, 1978; Hawkins and Hartnoll, 1983). The influence of the grazers therefore has a dramatic effect on the structure of the whole community on more exposed shores. Similarly canopy interactions shape sheltered shores and the sublittoral fringe.

Rocky shores are occupied by many species, some of which have a relatively minor effect on the abundance of others. Others directly influence the abundance of other species, either through competition, predation or grazing. As the Fucus - barnacle mosaic example shows, interactions between two species can have knock on effects to other neighbouring species. In Chapter VI we advocate a monitoring strategy based on abundance estimates for a limited number of important species. This should include indicator species for specific impacts but should be based mainly on those species which characterise and structure rocky shore communities. Foremost among these are those which occupy most of the available space including barnacles, mussels, fucoids and kelps. Abundant mobile animals such as littorinids should also be included. Of particular importance are those which exert an influence on the rest of the community through affecting the outcome of competitive interactions between other important species. Limpets are the best example on UK shores. There is little evidence that dogwhelks play a substantial role in structuring communities, though they are abundant predators on many rocky shores. The implications for the community of predation by species which migrate onto the shore to feed when the tide is in are not fully understood. Many rocky shore communities are also affected by the actions of animals which are not seen at low tide; for example, seasonal invasions of the lower shore by predators such as Asterias rubens and Echinus esculantus. The outcome of these biological interactions result in the observed dynamics and distribution patterns. Major space occupying species provide substantial secondary habitat for smaller species. This is particularly the case for canopy forming algae (Seed and Williams, 1992) and mussels (Seed, 1995).

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