||Range of conditions
||Full, although salinity in pools and crevices in the littoral
zone can vary considerably with evaporation and dilution by rain.
||Extremely exposed, Very exposed, Exposed
The structure of
ecological communities on rocky shores is affected by a horizontal gradient of exposure to
wave action, from sheltered bays to exposed headlands. The degree of wave action on a
particular shore is determined by the aspect of prevailing winds coupled with the
fetch: the distance over which winds blow. Shores with a long fetch can have
strong wave action because the wind has a greater distance to generate the height of
waves. Such shores may also receive swell on windless days, resulting from distant storms.
Exposure to wave action affects the distribution of species, according to their
tolerances. Increased exposure favours certain sessile, filter -feeding species. At the
same time, increasing exposure carries an increased risk of dislodgement and physical
damage, limiting the range of susceptible and physically fragile species.
Bedrock; large boulders.
Hard rocks provide a more secure anchorage for large plants and animals such as fucoids
||Upper shore, Mid shore, Lower shore
||At the interface between land and water, species spend part
of their time immersed in the sea, or at least splashed by its spray, and part of their
time in contact with the air, with a vertical gradient of emersion up the shore. Air
temperatures commonly fluctuate by 10 to 20oC in a 24 hour period whereas sea
temperatures usually fluctuate by less than 10oC in a year. Intertidal areas
will also be exposed to the rigors of sunlight at low water especially when low water
spring tides occur around midday.
||Tidal ranges from 0.5 m to 12 m in the British Isles.
||In temperate zones, the risk of desiccation due to heat and
low humidity is highly significant. The ability of species to tolerate desiccation will
effect community structure, as will wave exposure which can modify the extent of the
vertical gradient. As wave action increases, so does the amount of spray produced. Waves
with greater amplitude break at higher shore levels, showering still higher areas with
spray. At very exposed sites, shore levels well above the highest tide level may regularly
be wetted by the action of waves. It is not uncommon to find high shore species many tens
of meters above the theoretical tidal limit on very exposed cliffs (Lewis 1964).
||Rock type influences the slope and topographical complexity
of the shore, and slope determines the area available for littoral species. Barnacles and
limpets are successful on steep shores, while mussels and seaweeds are more common on
gently-sloping or horizontal shores.