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).