A gradient of environmental conditions extends across the shore, due mainly to the different durations of submergence at each level. The lowest parts of the shore are uncovered only during the lowest spring tides, and then only for brief periods. The highest levels are seldom fully submerged and are mainly wetted by wave splash. Intermediate levels experience intermediate durations of alternating exposure and submersion. Even on non-tidal shores there are gradients between permanently submerged and fully terrestrial conditions, with intermediate levels wetted by splash or irregularly covered and uncovered as wind action alters the water level.

The requirements for life in air and water are so different that no organism is equally well suited to every level of the shore. Different levels are therefore occupied by different assemblages of plants and animals, each species having its main abundance within a particular zone where conditions are most favourable for it. Above and below this zone it occurs in reduced numbers, or is absent, because environmental conditions are less suitable: physical conditions may be too difficult to allow its survival; it may be ousted by competition with other species better suited to these levels; it may be eliminated by predation; or some biotic factor essential for its life, such as suitable food, may be lacking. Some examples of these types of interactions are given in papers by Barnett, 1979; Connell, 1961; Coombs, 1973; and Schonbeck and Norton, 1978, 1979.

Alternations of uncovering and submergence present somewhat different problems to different organisms, depending on the type and level of shore they inhabit, and their mode of life. For surface-living forms, drying, wave action, illumination and temperature are major factors influencing zonation but their relative importance must vary with level. The occupants of the upper shore are exposed to air for long periods between spring tides, and must therefore be able to withstand conditions of prolonged drying, extremes of temperature and strong illumination. Direct wave impact is a relatively brief and infrequent hazard, and strong wave action favours the upward spread of this population by increasing the height to which spray regularly wets the shore.

Lower on the shore, surface populations are never uncovered for more than a few hours at a time, and here the problems of desiccation, temperature fluctuations and excessive illumination are less severe. These organisms experience longer and more frequent periods of wave action with the attendant risks of damage or dislodgement, and during submergence the illumination may be inadequate for certain species of plants. Other factors which vary with shore level, and influence distribution, include the duration of periods during which feeding is possible, the maximum depth of water, and often the type of substrate. These factors may largely account for the zonation of many of the forms which are not ordinarily left exposed, but hide in crevices or under the algae, or burrow into the rock or sediment. Because the vegetation varies at each level of the shore, the distribution of herbivorous animals and their predators may be influenced by the dependence of grazers on certain preferred algae.

Although the tides exert a major influence on zonation, the distribution of littoral (shore-dwelling) species relative to particular tidal levels is by no means constant. Wide variations occur from place to place due to differences of geography, geology and climate. Factors which modify zonation and vary with locality include the intensity of wave action, the range of temperatures and humidities, the aspect of the shore with respect to the sun and prevailing wind, the type of rock or sediment, the amount of rainfall and fresh water run-off, and the period of day or night when extreme low tides occur. Lewis (1964) has emphasized that the littoral zone cannot be satisfactorily defined solely in relation to sea level, but is better described in biological terms as the strip between sea and land inhabited by characteristic communities of organisms which thrive where the shore surface undergoes alternations of air and seawater.

These communities comprise three main groups occupying different levels, and the littoral zone can be correspondingly subdivided (see Section 8.8.1). The three-zone system was first proposed by two biologists in the 1940s (Stephenson and Stephenson, 1972) and was extended by Lewis (1964). Today the system still provides a useful descriptive framework.

The highest zone, with the uppermost communities which require mainly aerial conditions, is called the littoral fringe. This zone is submerged only at spring tides or wetted only by wave splash. On very sheltered shores this zone is a narrow belt below EHWS level, but it becomes higher and wider with increasing exposure to wave action, until, on the most wavebeaten rocky coasts, it lies entirely above EHWS level and may extend upwards for 20 m or more. Below the littoral fringe is the broad eulittoral zone, occupied by communities tolerant of short periods of exposure to air between tides but requiring regular submersion, or at least thorough wetting, at each tidal cycle. The low-shore zone is called the sublittoral fringe and really forms part of the permanently submerged sublittoral zone. It is only exposed at spring low tides. At their lowest levels the populations of the eulittoral zone overlap those of the sublittoral fringe (see Figure 8.11). Examples of intertidal zonation around the British Isles are given later (see page 284 ff).

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