The wave exposure gradient has a considerable effect on community structure, as a result of the stresses and benefits experienced at different levels of wave energy. The general patterns of zonation on rocky shores can be explained in terms of physical factors affecting the outcome of biological interactions. Steeper moderately exposed shores generally have stable patterns of zonation over time. Flatter moderately exposed shores are often characterised by highly dynamic communities with patches of one species giving way to another over time.

As mentioned above, moderately exposed rocky shores are often made up of a mosaic of communities, each cycling through a number of successional stages and structured by a number of positive and negative interactions between the main species but with fluctuations generated by recruitment variation. The communities are each dominated by a particular group of species, which may give way to others and sometimes to bare rock over time. These have been particularly well studied on the Isle of Man (Burrows & Lodge 1950; Hawkins & Hartnoll 1983, 1985; Hawkins et al. 1992) where the following effects on the mid shore have been shown: The limpet Patella vulgata is an important grazer, feeding on the young Fucus vesiculosus plants. Mature F. vesiculosus plants dislodge settling barnacles Semibalanus balanoides as their fronds sweep over the rock. Juvenile limpets, which dislodge newly-settled barnacles as they move, and dogwhelks Nucella lapillus, which are predators of barnacles, aggregate under mature clumps of F. vesiculosus. Thus, barnacles are scarce in patches dominated by mature F. vesiculosus; however, these patches last for only about 3 to 4 years. The sweeping action of F. vesiculosus fronds and the presence of limpets minimise the successful settlement of young fucoids. However, limpet grazing is inefficient amongst mature barnacles; as a result, some fucoids are able to settle and survive. Fucus vesiculosus clumps appear amongst the barnacles, reducing barnacle recruitment and encouraging the aggregation of limpets.

Many rocky shore species have a planktonic dispersal phase. These species produce propagules or larvae that spend their early life in the open sea and may eventually settle on shore some distance from where they originated. This strategy allows species to rapidly colonise new areas that become available. The level of larval supply and its fluctuation plays a considerable role in structuring rocky shore communities and has been appreciated for a long time (Southward & Crisp 1956; Lewis 1964; Kendall et al. 1985).

Macroalgae exude considerable amounts of dissolved organic carbon which are taken up readily by bacteria and may even be taken up directly by some larger invertebrates. Only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins 1996). Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may then enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shore organisms also make a contribution to the food of many marine species through the production of planktonic larvae and propagules which supply essential nutrients to pelagic and benthic food chains.

Limpet Patella vulgata, barnacle Semibalanus balanoides & the fucoids Fucus vesiculosus, F.spiralis and F.serratus

Fish and crustaceans, migrating into the intertidal to feed as the tide rises, are important predators of rocky shore species. Corkwing wrasse Crenilabrus melops rely heavily on the intertidal. Juvenile wrasse are commonly found in rockpools. Shore birds also feed on the rocky shore (Feare & Summers 1985) e.g. the invertebrates attracted to seaweed on the strandline are a particularly important food source. Rich pickings can also be had under macroalgae canopies.

Communities on moderately exposed shores show dynamics caused by physical disturbance events, which create space for recolonization. Stochastic (chance) events contribute greatly to variability in the community and the major cause of this is the supply of settling planktonic propagules of key species in the community (Hawkins & Hartnoll 1982,1985; Gaines & Roughgarden 1985; Gaines & Bertness 1992). Disturbance due to major climatic events, such as storms and cold winters (e.g. Crisp 1964) or small-scale physical damage (Paine & Levin 1981; Shanks & Wright 1986) can also have important effects.

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