Vertical Zonation

Patterns in community structure along and between shores

Rocky shores are often characterised by striking horizontal bands of species or species assemblages. A particularly good example is the neatly delineated stands of fucoid species on most sheltered UK shores (Lewis, 1964).

An early and quite useful attempt to characterise the main zones seen on rocky shores was made by Stephenson and Stephenson (1949) and a much more detailed account of zonation on rocky shores in Britain and Ireland is given by Lewis (1964. The Stephensons identified three main zones common to many shores around the world.

• The upper zone, called the supralittoral fringe (also described as the littoral fringe by Lewis, 1964), is mainly characterised by lichens, cyanobacteria and small grazing snails, the periwinkles.
• The much broader midlittoral (eulittoral sensu Lewis, 1964) zone exists in the midshore and is dominated by suspension feeding barnacles and mussels.
• Finally, the narrow, low shore infralittoral (sublittoral sensu Lewis, 1964) fringe is dominated by red algae and kelps, species that usually extend into the permanently immersed sublittoral zone.

Figure - Stephenson and Stephenson's universal zonation scheme

The width and upper limit of each zone generally increase as wave action becomes more intense (Lewis, 1964). The wave action gradient also affects the species which might be found in any zone. The Stephensons’ three zone system can be applied to UK shores with some degree of wave exposure. On more sheltered shores, however, mid shore levels are often dominated by fucoids and the zones are less clear cut. The terminology used by Lewis (1964) is used more often in the U.K., for example it was adopted in the textbooks by Hawkins and Jones (1992) and Raffaelli and Hawkins (1996).

An obvious factor contributing to these zonation patterns is the emersion gradient. It is well known that zonation of communities occurs along stress gradients. The best known examples are at the larger scales of altitude and latitude. An intuitive explanation for observed zonation patterns might be that the vertical distribution of each species is set by its tolerance to the stresses of prolonged exposure to the air during emersion and prolonged submergence during immersion (see linked figure). The causes of zonation have received a good deal of attention in laboratory and field studies dating back to the beginning of the 20^th century (Baker, 1909. These studies have shown that while physical factors directly influence the upper limits of the distribution of many species, biological interactions play a significant role in shaping zonation patterns. Biological factors act primarily on lower limits but can sometimes set upper limits.

The majority of species found in the littoral zone are of marine origin. For these species, stress increases with shore height. It is usually true that higher shore species are more tolerant of the emersion stress than species found at lower shore levels (Norton, 1985). Physiological, behavioural and morphological adaptations allow high shore species to survive periods of emersion. However, low shore species are usually able to withstand periods of emersion greater than they experience in the field, even at the upper limits of their distribution. Many species do not occupy higher shore levels despite being able to tolerate the physical conditions found there.

Field observations have demonstrated that some species are killed by physical factors associated with emersion. For example, cyprids of the barnacle Semibalanus balanoides will settle above the upper limit of adult conspecifics during years of heavy settlement. If that settlement occurs during a hot spring, the cyprids die (Connell, 1961, Foster, 1971). If it occurs during a cool spring, the cyprids may survive until metamorphosis. Adults are more tolerant to emersion than cyprids. However, ‘out-of-zone’ adults eventually die as a result of desiccation stress during hot summers. Similarly, the high shore algae Pelvetia spp. and Fucus spiralis and even occasionally the midshore Ascophyllum nodosum can be killed during periods of hot weather (Schonbeck and Norton, 1978; Hawkins and Hartnoll, 1985; Norton, 1985).

The chlorophyte Prasiola stipatata is often abundant the high shore in the winter but dies back in the summer. During the same periods, several mid and low shore species, especially fucoids were not observed to die at their upper distributional limits. Thus, while it might directly set the upper limits for some sessile high and mid shore species (e.g. F. vesiculosus and F. serratus), physical stress on its own is not sufficient to determine the distribution patterns of every species. The experimental removal of higher shore algae can allow F. vesiculosus to extend its range upwards (Hawkins and Hartnoll, 1985). The reduced grazing pressure caused by the death of limpets following the Torrey Canyon oil spill led to an upward extension of low shore red algae and kelps (Southward and Southward, 1978). Thus the effects of grazing and competition are also important in defining the upper limits of species. Unpublished work in the U.K. (Boaventura and Hawkins) has shown that red algal turfs can be induced to extend higher up the shore if limpets are removed (see also Hawkins and Jones, 1992). Similar work was first done in Australia by Underwood and co-workers (Underwood, 1980; Underwood and Jernakoff, 1981). Further unpublished work (Hill, personal communication) has also shown that, in the short term, the kelp Laminaria digitata can grow higher up the shore if Fucus serratus is removed.

Biological factors are of great importance in determining the lower limits of rocky shore plants and sessile animals. Competition for space is perhaps the single most important biological interaction shaping rocky shore communities although grazing and predation also have significant effects. The dominant species on rocky shores are usually sessile and attached to the rock itself. Generally more of the available space is occupied at lower shore levels and new settlement can occur only when predation, grazing or physical disturbance removes previous occupants.

On Great Cumbrae the barnacle Chthamalus montagui is restricted to the high shore while the mid-shore is dominated by Semibalanus balanoides. Connell (1961a,b found that C. montagui transplanted to lower shore levels were displaced by S. balanoides which grew faster and undercut or crushed the smaller species. However, when S. balanoides were removed, Chthamalus was able to survive and grow at mid-shore levels. Furthermore, S. balanoides was able to extend its range into lower zones when protected from predation by the dogwhelk, Nucella lapillus. It has subsequently been shown that competition from large fucoids and red algal turfs can prevent Semibalanus from extending into lower shore levels (Hawkins, 1983). Pelvetia canaliculata is usually prevented from spreading down the shore by competition from Fucus spiralis. However, both species can grow better at even lower levels than they are usually found (Schonbeck and Norton, 1980).

Behaviour is an important factor determining the distribution of animals on the shore. Even sessile animals have mobile larvae which often preferentially settle close to adult conspecifics, thereby reinforcing existing zonation patterns. Limpets, dogwhelks and other mobile species can actively select areas of shore. Limpets rarely stray beyond their tolerance limits (Wolcott, 1973). Dogwhelks retreat to crevices and low shore levels to avoid the risk of dislodgement in storms (Burrows and Hughes, 1989).

While the vertical zonation seen on rocky shores is clearly a response to the emersion gradient, physical factors do not set the upper limits of the distribution of all species. In general, the upper limits of most high shore species are set by physical factors. However, biological interactions, especially competition for space, grazing and predation can set the boundaries between many species at mid and lower shore levels, although their ultimate extension up the shore would be set by physical factors. In other words, physical factors set the ultimate upper limits to organisms of marine ancestry but proximate factors often prevent this physiological barrier from being reached.

The wave exposure gradient also has a considerable effect on community structure, through the stresses and benefits experienced at different levels of wave energy (see linked figure). Certain species are well adapted to survive on exposed shores including the barnacles Semibalanus balanoides and Chthamalus montagui. Dogwhelks, which feed on barnacles and mussels are also more abundant on exposed shores. On exposed shores, eulittoral seaweeds are usually ephemeral or short turf forms. In contrast, the more foliose fucoids perform better in sheltered conditions where their presence reduces the abundance of barnacles in the mid-shore.

It is well known that most fucoids are prevented from establishing on exposed shores by limpet grazing (Jones, 1948, Southward and Southward, 1978, Hawkins and Hartnoll, 1983, Hawkins et al., 1992). The direct effects of wave action have been shown to be important for limiting recruitment of Ascophyllum spp. on more exposed shores (Vadas et al., 1990). Conversely, the factors responsible for excluding limpets and barnacles from sheltered shores are less well known. Larval supply, direct and indirect effects of canopies including competition from sub-dominant turf forming algal species, siltation, post-settlement predation have all been proposed (Hawkins and Hartnoll, 1983, Hawkins and Jones, 1992, Hawkins et al., 1992) although the only attempt to test such ideas was made by Jenkins (1995).

Lower on the shore, Alaria spp. replaces Laminaria digitata in more exposed conditions, whilst L. saccharia predominates in shelter (see linked figure), particularly on unstable boulders being an annual. If L. digitata is removed on moderately sheltered shores, L. saccharia can be induced to colonise from shelter and Alaria spp. from exposure. This suggests that moderate shelter to moderate exposure are ideal conditions for sublittoral fringe kelp but that the more opportunistic species (Alaria spp., L. saccaria) are confined to sub-optimal conditions at either end of the wave exposure gradient (Hawkins and Harkin, 1985).

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