Barnacles and fucoids (moderately exposed shores)
Ecological and functional relationships
Ecological relationships within this biotope are very complex resulting in dynamic communities with a mosaic of patches of fucoid cover, dense barnacles and limpets subject to small scale temporal variations due to seasonal and non-seasonal factors. While physical factors clearly influence the distribution and abundance of species on rocky shores it is the interaction between physical and biological factors that is responsible for much of the structure and dynamics of rocky shore communities.
The diversity of species within the MLR.BF biotope, and on rocky shores in general, increases towards the lower shore where the habitat is wet for longer. Macroalgal cover increases the structural complexity of the habitat providing refugia for a wide range of mobile and sessile animals. The MLR.BF biotope occurs in the eulittoral zone, extending from the upper shore where barnacles and limpets are present in quantity with fucoids although often this belt has only sparse algal cover compared with the lower eulittoral.
Grazing on rocky shores can exert significant controlling influences on the algal vegetation, particularly by patellid limpets and littorinid snails which are usually the most prominent grazers. There are probably also significant effects caused by 'mesograzers' - amphipods such as Hyale prevostii and isopods, which are much smaller but may occur in high densities.
Predation can be an important force in the structuring of rocky shore communities. However, there are relatively few species or abundance of predators on rocky shores, a reflection of the species position at the top of the food web. The most obvious predator on rocky shores, particularly those exposed to wave action, is the whelk Nucella lapillus. At lower levels on the shore, starfish may become abundant and are predators especially of mussels. Crabs are more hidden from view on many rocky shores, often because they migrate up and down with the tides, or lurk in crevices at low tide. At low tide level the diversity of predators increases and nudibranch gastropods, polychaetes and nemertines may be abundant. Fish and birds, which invade the shore at high and low tide respectively, are also important predators on the shore.
In addition to barnacles, other sessile suspension feeding animals may be abundant on the lower shore in barnacle-fucoid biotopes. Organisms such as tunicates, sponges, bryozoans, hydroids and spirorbid worms are typically found on various parts of macroalgal plants or attached to the bedrock.
The presence of a fucoid canopy inhibits the settlement of barnacles by blocking larval recruitment mainly by 'sweeping' the rock of colonizers. However, the canopy offers protection against desiccation which promotes the clumping of adults and the recruitment of young in several species of mobile animals. The number of limpets increases with maturing fucoid clumps.
Limpets are the dominant grazers in the system and their home scars tend to be aggregated with a preference for mature algal patches. A spatially uneven pattern of grazing pressure is thought to lead to new algal patches in areas of low local limpet density (Hartnoll & Hawkins, 1985).
A dense covering of barnacle species is effective in limiting the efficiency of limpet grazing which adversely affects limpet growth. The development of an increasing barnacle cover would contribute, together with decreased limpet grazing to the re-establishment of the fucoid canopy.
The dense beds of fucoid plants provide substratum and shelter for a very wide variety of species, including the tube worm Spirorbis spirorbis, herbivorous isopods, such as Idotea, and amphipods like Hyale prevostii, and surface grazing snails, such as Littorina obtusata, and also provide considerable substratum for epiphytic species. They may also act as nursery grounds for various species including Nucella lapillus.
Seasonal and longer term change
Fucoid-barnacle mosaics on rocky shores are highly variable in space and time and considerable natural change is seen, especially in seaweed cover and number of limpets (Hartnoll & Hawkins, 1985). Natural changes can easily cause a given area to progress through a number of biotopes over time. Seasonal changes are also apparent on rocky shores with seasonal variation in growth and recruitment. Fucus serratus
plants, for example, lose fronds in the winter, followed by regrowth from existing plants in late spring and summer, so that summer cover can be about 250% of the winter level (Hawkins & Hartnol, 1980). The barnacle population can be depleted by the foraging activity of the dog whelk 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.
Habitat structure and complexity
Barnacle-fucoid shores provide a variety of habitats and refugia for other species. Macroalgae increases the structural complexity of the habitat providing a variety of resources that are not available on bare rock. Fronds provide space for attachment of encrusting or sessile epifauna and epiphytic algae and provide shelter from wave action, desiccation and heat for invertebrates. Empty barnacle shells provide shelter for small littorinids such as Littorina neglecta
and Littorina saxatilis
The littoral community of fucoids, barnacles and limpets on moderately exposed shores is relatively unstable, existing in a state of dynamic equilibrium in which biological or physical changes can create quite drastic effects on the pattern of the community (Southward & Southward, 1978) and so the biotope itself is subject to change and may cycle between different biotopes or sub-biotopes.
Rocky shore communities are highly productive and are an important source of food and nutrients for members of neighbouring terrestrial and marine ecosystems (Hill et al., 1998). 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, 1999). Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.
Many rocky shore species, plant and animal, possess a planktonic stage: gamete, spore or larvae which float in the plankton before settling and metamorphosing into adult form. This strategy allows species to rapidly colonize new areas that become available such as in the gaps often created by storms. For these organisms 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.
- Community structure and dynamics on barnacle-fucoid shores are strongly influenced by larval supply. Annual variation in recruitment success, of algae and barnacles particularly, can have a significant impact on the patchiness of the shore. For example, a low recruitment of limpets, or high recruitment of barnacles might lead to reduced limpet grazing and, therefore, more Fucus spp. escapes resulting in a fucoid dominated community.
- Recruitment of Fucus serratus from minute pelagic sporelings takes place from late spring until October. There is a reproductive peak in the period August - October and plants can be dispersed long distances (up to 10km). Germlings have a high mortality during winter due to storms and heavy wave action with up to 83% being recorded lost in 77 days on the Isle of Man.
- Ascophyllum nodosum is also recruited from pelagic sporelings, but recruitment is generally poor with few germlings found on the shore.
- Barnacle recruitment can be very 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.
- Among sessile organisms, patterns fixed at settlement, though potentially altered by post settlement mortality, obviously cannot be influenced by dispersal of juveniles or adults.
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 than species with a pelagic larvae.
Time for community to reach maturity
Although the recruitment of many species in the barnacle-fucoid mosaics is rapid, the time scale for recovery of rocky shore communities following mass mortalities caused by oil dispersants used in the Torrey Canyon oil spill clean-up was at least 10 years. However, when considering limpet population structure and barnacle densities then the time to return to levels of spatial and temporal variation normally seen on barnacle-fucoid shores was closer to 15 years. (Hill et al., 1998).
Moderately exposed rocky shores are often made up of a mosaic of communities, each cycling through a number of successional stages and structured by a number of positive and negative interactions between the main species but with fluctuations generated by recruitment variation. These communities are each dominated by a particular group of species, which may give way to others and sometimes to bare rock over time so that the MLR.BF biotopes may represent one stage in a progression of biotopes.
This review can be cited as follows:
Barnacles and fucoids (moderately exposed shores).
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=33&code=1997>