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).
Stephenson and Stephensons universal zonation scheme
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.
Stephenson and Stephenson's universal
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).
Zonation and direct responses to emersion
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 20th 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 Interactions controlling upper and lower limits: competition
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: Habitat selection
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,
General synthesis of factors setting limits
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
Patterns in community structure along and
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).