Biodiversity & Conservation

While physical factors, such as wave exposure, as indicated in the title of the biotope, clearly influence the occurrence of this biotope, the interactions between physical and biological factors are responsible for much of the structure and dynamics within the biotope. The diversity of species within the ELR.BPat biotope, and on rocky shores in general, increases towards the lower shore where the habitat is wet for longer. Damp crevices at the lower parts of the biotope may support macroalgae such as Mastocarpus stellatus, Osmundia pinnatifida and encrusting coralline algae as well as some faunal organisms.

Exposed conditions favour the growth of barnacles, limpets and mussels. Fucoid algae are largely excluded because of the effect of wave action, but also from grazing pressure on exposed shores. For example, in New England, where Patella is absent, fucoid plants extend into more exposed conditions. A dynamic balance probably exists between fucoids and limpets plus barnacles, and is mediated by wave action. In sheltered conditions the balance shifts in favour of fucoids and in exposure the balance shifts towards limpets, barnacles and mussels. Communities on exposed shores are more stable than those of moderately exposed shores (see £MLR.BF£) where a mosaic of patches of fucoid cover, dense barnacles and limpets are subject to small scale temporal variations.

Although there are relatively few species or abundance of predators on rocky shores predation can play a role in structuring the biotope. The most obvious predator, particularly in those exposed to wave action such as the ELR.Bpat biotope, is the dogwhelk Nucella lapillus, which feeds on mussels and barnacles. When present in high abundance the dog whelk can affect the density of mussels and barnacles on the shore. Birds, which invade the shore at high and low tide respectively, can also be important predators on the shore.

A dense covering of barnacle species is effective in limiting the efficiency of limpet grazing which adversely affects limpet growth. Bulldozing by grazing limpets may cause high post-settlement mortality of barnacles (Jenkins et al., 2000).

Rocky shore communities are often highly variable in time, due to the combined influences of physical disturbance, competition, grazing, predation and variation in recruitment. However, exposed shores tend to be less variable than moderately exposed shores and are therefore more stable. Exposed conditions favour the development of a relatively stable covering of barnacles and limpets. The barnacle population can be depleted by the foraging activity of the dogwhelk Nucella lapillus from spring to early winter and replenished by settlement of Semibalanus balanoides in the spring and Chthamalus spp. in the summer and autumn. There will also be seasonal changes in the growth rates of the algae that may be present in the biotope.

Apart from cracks and crevices in the bedrock and overhangs which provide refugia for a variety of species there is very little habitat complexity in the ELR.BPat biotope. Most of the surface of the bedrock or boulders in the biotope will be covered in barnacles and limpets to which few other species can attach. The barnacles may be covered by Porphyra sp. on the upper shore. Empty barnacle shells provide shelter for small littorinids such as Littorina neglecta and Littorina saxatilis.

In the absence, or low abundance, of macroalgae primary production in this biotope will be limited to microalgae growing on rock surfaces so productivity in the ELR.BPat biotope is probably not as high as some other rocky shore biotopes. Detrital input will be important for the suspension feeding barnacles and mussels. Rocky shores can make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.

Most species present in the biotope possess a planktonic stage (gamete, spore or larvae) which float in the plankton before settling and metamorphosing into the adult form. This strategy allows species to rapidly colonize new areas that become available such as in the gaps often created by storms. Thus, for organisms such as those present in this biotope, it has long been evident that recruitment from the pelagic phase is important in governing the density of populations on the shore (Little & Kitching, 1996). Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.

• Barnacle settlement and recruitment can be highly variable because it is dependent on a suite of environmental and biological factors, such as wind direction and success depends on settlement being followed by a period of favourable weather. Long term surveys have produced clear evidence of barnacle populations responding to climatic changes. During warm periods Chthamalus spp. predominate, whilst Semibalanus balanoides does better during colder spells (Hawkins et al., 1994). Release of Semibalanus balanoides larvae takes place between February and April with peak settlement between April and June. Release of larvae of Chthamalus montagui takes place later in the year, between May and August.
• Recruitment of Patella vulgata fluctuates from year to year and from place to place. Fertilization is external and the larvae is pelagic for up to two weeks before settling on rock at a shell length of about 0.2mm. Winter breeding occurs only in southern England, in the north of Scotland it breeds in August and in north-east England in September.

Some of the species living in the biotope do not have pelagic larvae, but instead have direct development of larvae producing their offspring as 'miniature adults'. For example, many whelks such as Nucella lapillus and some winkles do this, as do all amphipods. Adult populations of these species are governed by conditions on the shore and will generally have a much smaller dispersal range. Nucella lapillus breeds throughout the year but there is a maximum in reproductive output in the spring and autumn. The species lays eggs in protective egg capsules on hard substrata in damp crevices and under stones.

Bennell (1981) observed that barnacles that were removed when the surface rock was scraped off in a barge accident at Amlwch, North Wales returned to pre-accident levels within 3 years. However, barnacle recruitment can be very variable because it is dependent on a suite of environmental and biological factors, such as wind direction, so populations may take longer to recruit to suitable areas. Recolonization of Patella vulgata on rocky shores is rapid as seen by the appearance of limpet spat 6 months after the Torrey Canyon oil spill reaching peak numbers 4-5 years after the spill (Southward & Southward, 1978). However, it does seem likely that a barnacle-limpet community would reach maturity within five years.