The general patterns of zonation on rocky shores can be explained in terms of physical factors affecting the outcome of biological interactions. However, even clearly delimited zones of conspicuous space occupiers or canopy forming seaweeds also consist of numerous subsidiary species, the populations of which undergo fluctuations in space and time. Sheltered rocky shores generally have vary stable patterns of zonation over time. This can be true of steeper moderately exposed shores. Flatter moderately exposed shores are often characterised by highly dynamic communities with patches of one species giving way to another over time (see below). Communities on very exposed shores show dynamics caused by physical disturbance events which create space for recolonization.

Many fluctuations seen in rocky shore communities are due to biological interactions. However, all ecological communities are influenced to some degree by physical factors. The interaction of the emersion gradient, the wave exposure gradient and disturbance regime, the local hydrodynamic regime and the geographical position of the shore determine which species are likely to be found there. Thus any effect of species interactions (grazing, predation or competition) on community structure may ultimately depends on the influence of the physical environment on the species involved.

Stochastic (chance) events contribute greatly to variability in the community. The major cause of this type of variability is stochastic variation in the supply of settling planktonic propagules of key species in the community (Hawkins and Hartnoll, 1982; Gaines and Roughgarden, 1985; Hartnoll and Hawkins, 1985; Gaines and Bertness, 1992; see section E below), but the disturbance due to major climatic events (e.g. Crisp, 1964) or small-scale physical damage (Paine and Levin, 1981; Shanks and Wright, 1986) can have important effects.

Moderately exposed rocky shores are often made up of a mosaic of patches, each cycling through a number of successional stages (Figure 5) and structured by a number of positive and negative interactions between the main species but with fluctuations generated by recruitment variation. The community is composed of such patches, each dominated by a particular species or 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 and Lodge, 1950; Hawkins and Hartnoll, 1983; Hartnoll and Hawkins; 1985, reviewed by Hawkins et al., 1992). 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, 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 minimises the successful settlement of young fucoids. When the old plants die back, the sheltering limpets disperse and the bare rock is then colonised by barnacles. Limpet grazing is inefficient amongst mature barnacles; as a result, some fucoids are able to settle and survive. F. vesiculosus clumps appear amongst the barnacles, reducing barnacle recruitment and encouraging the aggregation of limpets.

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