Rocky shores exist where the effect of waves on the coastline is mainly erosive, wearing down the softer materials and carrying them away, leaving the hardest rocks exposed. Most of the substrate is therefore stable and permanent, forming a secure surface upon which can grow a variety of organisms requiring attachment; for example, large algae, barnacles, mussels and limpets. The appearance of the shore depends largely upon the type of rock exposed. Horizontal strata often erode to a stepped series of fairly uniform level platforms which provide little shelter from the waves. Tilted strata running across the shore usually produce a very varied shore with numerous protruding rock ledges and overhangs, and deep pools in the gullies between them. Certain types of rock erode to a smooth surface, while some laminated rocks readily gape to form deep narrow fissures.
Many rocky shores are heavily populated. The agitation of the water keeps it well oxygenated, and favours plant growth by continually replenishing the supply of nutrients. The plants provide a primary food supply for animals, and copious additional food is available from the plankton.
Rocks present a variety of habitable environments - exposed rock faces, sheltered overhangs, crevices, deep or shallow pools, silt within fissures or under boulders, in the shelter of algae or in their ramifying holdfasts - each offers a domain which some species can occupy.
The size and composition of rocky shore communities are profoundly influenced by the intensity of wave action because this is one of the major factors determining the amount and type of algal growth on the rocks (Moyse and Nelson-Smith, 1963; Stephenson and Stephenson, 1972). Where wave intensity is moderate, large algae cover the shore and give shelter to many small animals which cannot tolerate complete exposure to air and sun, for example coelenterates, sponges, bryozoans and small crustaceans. Stronger waves prevent the growth of plants, and the rock surface then becomes covered mainly with barnacles and limpets, or sometimes at the lower levels by mussels. In extreme conditions of wave exposure, rock faces are swept virtually bare and the population is restricted to fissures and crevices. Because of the wetting effects of splash, heavy wave action tends to raise the levels to which sublittoral and littoral populations extend up the shore, and greatly increases the width and height of the littoral fringe (Figure 8.11). Where sand is deposited between rocks, their lower parts may be kept bare by the scouring effects of wave-tossed sand.
The intensity of wave action is a difficult parameter to evaluate. Attempts have been made to measure it with dynamometers (Jones and Demetropoulos, 1968), but an alternative approach has been to define degrees of wave exposure in terms of their biological effects. The most obvious contrast to be made is between shores where the rock surface is mainly encrusted with barnacles or mussels, i.e. barnacle-dominated and mussel-dominated shores, and those where the rocks are covered by a copious growth of seaweeds, i.e. algal-dominated shores. The former occur where the force of wave action is too fierce to allow the survival of large plants; the latter where the intensity of wave action is much gentler. By studying the differences of population between exposed headlands and sheltered inlets, numerical values for wave intensity can be assigned to particular patterns of population on the assumption that the differences are due mainly to wave effects, i.e. a biologically defined scale of exposure to wave action (Ballantine, 1961; Lewis, 1964).
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