The wave exposure gradient has a considerable effect on community structure, as a
result of the stresses and benefits experienced at different levels of wave energy.
Certain species including the barnacles Semibalanus balanoides and Chthamalus
montagui are well adapted to survive on exposed shores. Dogwhelks Nucella lapillus,
which feed on barnacles, and mussels Mytilus edulis are also more abundant on
exposed shores. Algae found on these shores tend to be ephemeral or short turf forms.
The diversity of species on rocky shores increases towards the lower shore where the
habitat is wet for longer periods of the day. Only a limited number of species are able to
survive on extremely exposed shores, particularly those shores consisting mainly of steep,
smooth rock. Protected microhabitats on exposed shores, such as algal turfs or deep
crevices, can however support a surprising variety of species (Raffaelli & Hawkins
1996). A major biological influence on community structure is the presence of algal
canopies and shorter algal communities at mid and low shore levels. Macroalgae provide a
variety of resources which are not available on bare rock. Most importantly, they increase
the amount of space available for attachment, they provide shelter from wave action,
desiccation and heat and they are an important food source.
Many rocky shore species have a planktonic dispersal phase. These species produce
propagules or larvae that spend their early life in the open sea and may eventually settle
on shore some distance from where they originated. This strategy allows species to rapidly
colonize new areas that become available such as after storms. The level of larval supply
and its fluctuations play a considerable role in structuring rocky shore communities and
has been appreciated for a long time (Southward & Crisp 1956; Lewis 1964; Kendall et
Macroalgae exude considerable amounts of dissolved organic carbon which are taken up
readily by bacteria and may even be taken up directly by some larger invertebrates. Only
about 10% of the primary production is directly cropped by herbivores (Raffaelli &
Hawkins 1996). On exposed shores, grazers feed mainly on the microbial film. Dissolved
organic carbon, algal fragments and microbial film organisms are continually removed by
the sea. This may then enter the food chain of local, subtidal ecosystems, or be exported
further offshore. Rocky shores also make a contribution to the food of many marine species
through the production of planktonic larvae and propagules which supply essential
nutrients to pelagic and benthic species.
Keystone (structuring) species
Semibalanus balanoides, Chthamalus spp., Mytilus edulis.
Importance of habitat for other species
Fish and crusteaceans migrating into the intertidal zone to feed as the tide rises, are
important predators of rocky shore species. Corkwing wrasse Crenilabrus melops rely
heavily on the intertidal. Juvenile wrasse are commonly found in rockpools. Shore birds
also feed on the rocky shore (Feare & Summers 1985).
Communities on exposed shores show dynamics caused by physical disturbance events,
which create space for recolonization. Stochastic (chance) events contribute greatly to
variability in the community. The major cause of variability is the supply of settling
planktonic propagules of species in the community (Hawkins & Hartnoll 1982, Hartnoll
& Hawkins 1985; Gaines & Roughgarden 1985; Gaines & Bertness 1992).
Disturbance due to major climatic events such as storms and cold winters (e.g. Crisp 1964)
or small-scale physical damage (Paine & Levin 1981; Shanks & Wright 1986) can have
Time for community to reach maturity
No information available.