Summary

UK and Ireland classification

Description

Moderately exposed rocky shores characterized by a mosaic of fucoids and barnacles on bedrock and boulders, where the extent of the fucoid cover is typically less than the blanket cover associated with sheltered shores. Other species are normally present as well in this habtat including the winkle Littorina littorea, the whelk Nucella lapillus and the red seaweed Mastocarpus stellatus. Beneath the band of yellow and grey lichens at the top of the shore is a zone dominated by the wrack Pelvetia canaliculata, scattered barnacles, while the black lichen Verrucaria maura covers the rock surface (PelB). Below, on the mid shore the wrack Fucus vesiculosus generally forms a mosaic with the barnacle Semibalanus balanoides and the limpet Patella vulgata (FvesB). Finally, the wrack Fucus serratus, dominates the lower shore, while a variety of red seaweeds can be found underneath the Fucus serratus canopy (Fser). A number of variants have been described: lower shore bedrock and boulders characterised by mosaics of Fucus serratus and turf-forming red seaweeds (Fser.R); where the density of Fucus serratus is greater (typically Common to Superabundant) and the abundance of red seaweeds less Fserr.FS should be recorded. The presence of boulders and cobbles on the shore can increase the micro-habitat diversity, which often results in a greater species richness. Although the upper surface of the boulders may bear very similar communities to Fserr.FS there is often an increase in fauna (crabs, tube-forming polychaetes, sponges and bryozoans) and Fser.Bo should be recorded. Sand-influenced exposed to moderately exposed lower shore rock can be characterized by dense mats of Rhodothamniella floridula (Rho). (Infromation from Connor et al., 2004; JNCC, 2015).

Depth range

Upper shore, Mid shore, Lower shore

Additional information

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Listed By

Habitat review

Ecology

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.

Productivity

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.

Recruitment processes

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).

Additional information

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.

Preferences & Distribution

Habitat preferences

Depth Range Upper shore, Mid shore, Lower shore
Water clarity preferences
Limiting Nutrients Nitrogen (nitrates), Phosphorus (phosphates)
Salinity preferences Full (30-40 psu)
Physiographic preferences Open coast
Biological zone preferences Eulittoral
Substratum/habitat preferences Bedrock, Large to very large boulders, Small boulders
Tidal strength preferences
Wave exposure preferences Moderately exposed
Other preferences None found

Additional Information

Changes in the relative abundance of the cold-water barnacle Semibalanus balanoides and its warm-water counterparts Chthamalus stellatus and Chthamalus montagui show strong links with climatic conditions (Southward et al., 1995).

Species composition

Species found especially in this biotope

    Rare or scarce species associated with this biotope

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    Additional information

    No text entered.

    Sensitivity review

    Sensitivity characteristics of the habitat and relevant characteristic species

    Fucus vesiculosus and Semibalanus balanoides are the characterizing species of this biotope. However Patella vulgata can play a very important role in structuring the biological community. The complex interactions between these species create the mosaics which are characteristic of this biotope. In mid shore locations, where this biotope is found, Fucus vesiculosus is the dominant fucoid species. The dominant barnacle species is Semibalanus balanoides. The limpet species Patella vulgata also plays an important role as a grazer on the shore and contributes to the regulation of algal patches. Ecological relationships within these biotopes are very complex resulting in dynamic and patchy communities.

    As ecosystem engineers fucoid algal canopies modify habitat conditions. This facilitates the existence and survival of other intertidal species and therefore strongly influencing the structure and functioning of intertidal ecosystems (Jenkins et al., 2008). The dominant grazing species is Patella vulgata, which strongly affects the distribution of the fucoid canopies. Other important grazing species include littorinid snails which can be prominent on algae fronds. The filter feeding barnacle Semibalanus balanoides can be very common within this biotope; its distribution can be highly patchy. Both the mussel Mytillus edulis and the anemone Actinia equina can be found in crevices and fissures in the bed rock and boulders. The most obvious predator in this biotope is the dogwhelk Nucella lapillus. A number of other algae species can be found within this biotope including; Corallina officinalis, Mastocarpus stellatus and Osmundea pinnatifida.

    Resilience and recovery rates of habitat

    Fucoids dominate sheltered intertidal rocky shores due to the lack of damage from wave action (Jonsson et al., 2006), good recruitment (Southward & Southward, 1978) and the limited recruitment of grazers (Jenkins et al., 1999).  An increase to moderate wave exposure destabilizes the balance between fucoids, limpets and barnacles (Hartnoll & Hawkins, 1985), and results in a dynamic equilibrium between fucoid and barnacle dominance, mediated by physical wave action, and natural variation in grazing and recruitment. For example, limpets graze on algae and prevent algal growth but fucoid patches encourage the recruitment of juvenile limpets.  Newly settled barnacles are reduced in number by limpets but are able to settle due to the removal of fast growing, and competitively superior ephemeral algae.  Barnacles reduce limpet foraging efficiency allowing algal escapes. Dogwhelks thin-out barnacles, allowing limpets to more effectively reduce algal cover. The sweeping by fucoid fronds reduces barnacle settlement (Raffaelli & Hawkins, 1996). Hence, these biotopes exist 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 biotopes are subject to change and may cycle between different biotopes or sub-biotopes.  In addition there is also natural variation, and patchiness within intertidal rocky shores (Burrow & Lodge, 1950, Raffaelli & Hawkins, 1996).

    Hartnoll & Hawkins (1985) found that within test areas on a moderately exposed intertidal rocky shore a natural cycling of species on the mid shore took 6 – 7 years. Southward (1956) recorded a similar cycle taking five years.

    Since the 1940s major declines in the distribution of Fucus vesiculosus (Kautsky et al., 1986) and even local extinctions (Nilsson et al., 2005) have been observed in the Baltic Sea where the species dominates the shallow hard-bottom areas. The decline was likely a consequence of increased anthropogenic stress. Large-scale disappearance of Fucus vesiculosus from an ecosystem can result in large scale changes in the community composition (Wikstrom & Kautsky, 2007). The canopy created by Fucus vesiculosus forms a microclimate for the understorey fauna and flora. Removal of the canopy exposes under lying fauna and flora to environmental conditions with which they would be intolerant of resulting in mortality events.

    Hartnoll & Hawkins (1985) reported that Fucus vesiculosus recruits readily to cleared areas of the shore and full recovery takes 1-3 years in British waters. Keser & Larson (1984) investigated the recovery of Fucus vesiculosus after clearance experiments where plots were scraped clean and burned with a propane torch. Fucus vesiculosus was the first perennial alga to colonize the experimentally denuded transects, even at sites and tidal levels that had been dominated by Ascophyllum or Chondrus beforehand. Recovery occurred at all sites between 3 to 21 months. The study found newly settled germlings of Fucus vesiculosus in most months, indicating a broad period of reproduction. When grazers are excluded from areas of intertidal shores fucoids have the ability to rapidly recolonize all areas of the shore, even those which in a balanced ecosystem they do not normally occur (Burrows & Lodge, 1950, Southward & Southward, 1978).  Fucoid distributions return to their recognized zones when grazers are re-established on a shore (Burrows & Lodge, 1950, Southward & Southward, 1978). Although intertidal shores can rapidly regain fucoids it can take considerably longer for ecosystem function to return if grazers have also been lost (Hawkins & Southward, 1992). If the whole community is removed, recovery is likely to occur at a much lower pace. Indeed, Hawkins & Southward (1992) found that, after the M.V. Torrey Canyon oil spill, it took between 10 and 15 years for the Fucus spp. to return to 'normal' levels of spatial and variation in cover on moderately exposed shores. Therefore, for factors which are likely to totally destroy the biotope, recoverability is likely to be low.

    Fucus vesiculosus growth rates can vary both spatially and temporally (Lehvo et al., 2001). Temperature, exposure, and light availability are some of the factors which cause these changes in growth rates (Strömgren, 1977, Knight & Parke, 1950, Middelboe et al., 2006). Strömgren (1977) investigated the effect of short-term increases in temperature on the growth rate of Fucus vesiculosus. It was found that the growth rate of the control sample kept at 7°C was 20 times lower than the sample introduced to temperatures of 35 °C (Strömgren 1977). When the effect of temperature was investigated on the shore, relative growth rates in summer were found to be as high as 0.7% / day in summer, compared to less than 0.3% / day in winter (Lehvo et al., 2001). For macroalgae the trend is for shorter individuals found in situations with greater wave exposure (Lewis, 1961, Stephenson & Stephenson, 1972, Hawkins et al., 1992, Jonsson et al., 2006). Fucus vesiculosus also comply with this trend, and growth rates mirror this difference in physiology. On Sgeir Bhuidhe, an exposed shore in Scotland, Fucus vesiculosus grew on average 0.31 cm / week. On a sheltered Scottish shore the average increased to 0.68 cm / week (Knight & Parke, 1950).

    The development of the receptacles takes three months from initiation until when gametes are released (Knight, 1947). On British shores, receptacles are initiated around December and may be present until late summer (Knight , 1947). The alga is dioecious, and gametes are generally released into the seawater under calm conditions (Mann, 1972; Serrão et al., 2000) and the eggs are fertilized externally to produce a zygote. Serrão et al. (1997) determined that the wrack had a short-range dispersal capacity. Under calm conditions in which eggs are released, most eggs fall in the immediate vicinity of the parent plants. The egg becomes attached to the rock within a few hours of settlement and adhere firmly enough to resist removal by the next returning tide and germling may be visible to the naked eye within a couple of weeks (Knight & Parke, 1950). Despite the poor long range dispersal, the species is highly fecund often bearing more than 1000 receptacles on each plant, which may produce in excess of one million eggs. On the coast of Maine, sampling on three separate occasions during the reproductive season revealed 100% fertilization on both exposed and sheltered shores (Serrão et al., 2000). Fertilization is thus not considered as a limiting factor in reproduction in this species (Serrão et al., 2000).

    Mortality is extremely high in the early stages of germination up to a time when plants are 3 cm in length and this is due mostly to mollusc predation (Knight & Parke 1950). While Fucus vesiculosus  may resist some degree of environmental stress, their long-term persistence depends on their reproductive ability as well as the survival and growth of early life history stages (germlings) that are generally more susceptible to natural and anthropogenic stressors than adults (Steen, 2004; Fredersdorf et al., 2009). It is therefore necessary to include early life stage responses in the assessment of effects of environmental changes on fucoid algae as only considering fully developed adult specimens may lead to false conclusions (Nielsen et al., 2014).

    Genetic diversity can influence the resilience of fucoids in particular when pressure persists over a long period of time. Genetically diverse population are generally more resilient to changes in environmental conditions compared to genetically conserved populations. Tatarenkov et al. (2007) determined a high level of genetic variation in Fucus vesiculosus and extensive phenotypic variation. They suggested this might explain why the species is more successful than most fucoid species in colonizing marginal marine environments such as low-salinity estuaries, showing a range of morphological, physiological and ecological adaptations (Tatarenkov et al. 2005). Pressures causing a rapid change will have a greater impact as the natural ability of the species to adapt is compromised.

    In addition to sexual reproduction, Fucus vesiculosus is also able to generate vegetative regrowth in response to wounding. McCook & Chapman (1992) experimentally damaged Fucus vesiculosus holdfasts to test the ability of the wrack to regenerate. The study found that vegetative sprouting of Fucus vesiculosus holdfasts made a significant addition to the regrowth of the canopy, even when holdfasts were cut to less than 2 mm tissue thickness. Four months after cutting, sprouts ranged from microscopic buds to shoots about 10 cm long with mature shoots widespread after 12 months. Vegetative regrowth in response to wounding has been suggested as an important mean of recovery from population losses (McLachan & Chen, 1972). The importance of regeneration will depend on the severity of damage, not only in terms of the individuals but also in terms of the scale of canopy removal (McLachan & Chen, 1972).

    Semibalanus balanoides is a small but long lived barnacle with a life expectancy of 3 –6 years depending on shore height. Individuals on the low shore typically die in their third year, whereas those found from mean high water neaps downwards may live for five or six years. Individuals are hermaphrodites and reach sexual maturity between 1 – 2 years. Fertilization occurs between November – December in the British Isles. Fertilized eggs are retained brooded over winter for dispersal in the spring plankton bloom. The planktonic stage of these organisms is 2 months long during which they can disperse up to 10 km. Reproductive success is affected by temperature, latitude, light, food availability, age, size, crowding, seaweed cover and pollution.  High shore Semibalanus balanoides breed first and low shore specimens last (up to 12 days difference) (Barnes, 1989).  Fertilization is prevented by temperatures above 10 °C and continuous light.

    Local environmental conditions, including surface roughness (Hills & Thomason, 1998), wind direction (Barnes, 1956), shore height, wave exposure (Bertness et al., 1991) and tidal currents (Leonard et al., 1998) have been identified, among other factors, as factors affecting settlement of Semibalanus balanoides. Biological factors such as larval supply, competition for space, presence of adult barnacles (Prendergast et al., 2009) and the presence of species that facilitate or inhibit settlement (Kendall, et al., 1985, Jenkins et al., 1999) also play a role in recruitment. Mortality of juveniles can be high but highly variable, with up to 90% of Semibalanus balanoides dying within ten days (Kendall et al., 1985).

    Successful recruitment of high number of Semibalanus balanoides individuals to replenish the population may be episodic (Kendall et al., 1985). After settlement the juveniles are subject to high levels of predation as well as dislodgement from waves and sand abrasion depending on the area of settlement. Semibalanus balanoides may live up to 4 years in higher areas of the shore (Wethey,1985). Predation rates are variable (see Petraitis et al., 2003) and are influenced by a number of factors including the presence of algae (that shelters predators such as the dog whelk, Nucella lapillus, and the shore crab, Carcinus maenas and the sizes of clearings (as predation pressure is higher near canopies (Petraitis et al., 2003).

    On rocky shores, barnacles are often quick to colonize available gaps. 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. Petraitis & Dudgeon (2005) also found that Semibalanus balanoides quickly recruited (present a year after and increasing in density) to experimentally cleared areas within the Gulf of Maine, that had previously been dominated by Ascophyllum nodosum However, barnacle densities were fairly low (on average 7.6 % cover), predation levels in smaller patches were high (Petraitis et al., 2003). The success of recruitment and settlement of Semibalanus balanoides to an intertidal shore can be affected by the components of the community itself (Beermann et al., 2013). Barnacles are gregarious and larvae settle within areas where adults are present (Knight-Jones & Stevenson, 1950). The mechanism by which they are able to sense adults is chemosensory (Knight-Jones, 1953). Adults exude a protein named arthropodin, which the larvae can sense when they are searching for suitable substrates to settle on (Crisp & Meadows, 1962). The mortality rates for larvae who settle within an area containing a mosaic of adults is less than those who settle in areas without adults (Jenkins et al., 1999). Macroalgae can have both positive and negative impacts on the success of barnacle larvae. Jenkins et al. (1999) investigated settlement and post settlement impacts of three macroalgaes on Semibalanus balanoides cyprid larvae. The investigation found that Fucus spiralis, Ascophyllum nodosum and Fucus serratus all have negative impacts on the ability of larvae to settle due to the sweeping action of their fronds. Larvae which had settled underneath Fucus serratus had a mortality rate of 82 – 97% within a single high tide (Jenkins et al., 1999). Fucus serratus also inhibited the settlement ability of larvae due to the dense low lying fronds. Although larvae which settle below a fucoid canopy have a low chance of survival, mortality of barnacle spat is significantly lower under fucoid canopies than in unprotected areas (Jenkins et al., 1999, Beermann et al., 2013).

    The life expectancy of Patella vulgata depends on location.  Those found under fucoid canopies may only live for 2 – 3 years. In contrast, those which are found on bare rocks and have slower growth rates due to food limitations can live for 15 – 16 years. Maximum life expectancies have been estimated at 20 years. This species is a protandrous hermaphrodite, male sex organs can mature at nine months. However in northern England, limpets reach sexual maturity in their second year (Blackmore, 1969) and thereafter reproduce annually. The female reproductive organs can mature most often between 2 – 3 years, but in some situations they never mature. In Robin Hood’s Bay, Lewis & Bowman (1975) observed spawning of Patella vulgata in the Autumn, with spatfall occurring in winter when desiccation pressures were lower.

    Patella vulgata is mobile and can relocate to avoid the negative impacts of a pressure. Lewis (1954) found that on particular shores seasonal variations in temperature induced Patella vulgata to migrate further down rocky intertidal shores in the warmer months and further up the shore in winter months. However the ability to relocate depends on the shore type and roughness. Patella vulgata individuals also create home scars these are areas of rock where the limpet returns to repeatedly to rest when not feeding. The shell of the organism slowly wears down the rock to create a home scar which can reduce the level of desiccation (Davies, 1969) and the level of predation (Garrity & Levings, 1983). Mortality of these species can increase if they are unable to return to a home scar.

    Re-colonization 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. However, although re-colonization was rapid, the alteration to the population structure (size and age class) persisted for about 15 years because of the complex cycles of dominance (see below) involving limpets, barnacles and algae (Hawkins & Southward, 1992, Lewis & Bowman, 1975). Hence the establishment of fucoids if Patella vulgata and other grazers are absent.

    Resilience assessment. If specimens of Fucus vesiculosus remain in small quantities it is likely that re-growth will occur rapidly due to efficient fertilization rates and recruitment over short distances. The ability of Fucus vesiculosus to re-grow from damaged holdfasts will also aid in recolonization. Recovery is likely to occur within two years resulting in a ‘High’ resilience score. Semibalanus balanoides exhibits episodic and patchy recruitment. The evidence suggests that the size of the footprint of an impact and the magnitude will influence the recovery rates by mediating settlement and post-settlement recruitment. Barnacles are attracted to settle in the presence of adults of the same species (Prendergast et al., 2009); so that the presence of adults will facilitate recovery. Resilience is assessed as ‘High’ (within 2 years) where resistance is ‘High’ (no significant impact). Recovery of Patella vulgata will depend on recolonization by larvae which have pelagic life stage. As Patella vulgata is a common, widespread species. Where the footprint of the impact is relatively small, larval supply from adjacent populations should support recolonization. Where source populations are very distant due to regional impacts or habitat discontinuities, larval supply and recovery could be affected.

    However, changes and recovery trajectories following the removal of key species are unpredictable and interactions between the key species may be positive or negative. Limpets may enhance barnacle settlement by removing algae (Little et al., 2009) or by depositing pedal mucus trails that attract larvae (Holmes et al., 2005), or they may crush and displace newly settled individuals (Denley & Underwood, 1979).  Barnacles may enhance survival of small limpets by moderating environmental stresses but they may also have negative effects on recruitment by occupying space and by limiting access to grazing areas. On the moderately wave exposed shores on which this biotope occurs, grazing may limit initial settlement of macroalgae but wave action will limit the presence of adults and larger species through, breakage and drag effects leading to loss.  Mrowicki et al., (2014) found that limpet and barnacle removal allowed ephemeral and fucoid macroalgae to establish on sheltered and wave exposed shores in Ireland.  Unlike the characteristic animal species macroalgae have short dispersal distances, over tens of metres (Dudgeon et al., 2001) and therefore rapid recovery will require the presence of adults.

    Overall, where populations of the characterizing species remain after disturbance, then recovery is likely to be rapid (Hartnoll & Hawkins 1985) within 1-3 years.  Similarly, if the natural cycle in species abundance (from fucoid to barnacle dominance) takes 5-7 years (Southward, 1956; Hartnoll & Hawkins, 1985) then resilience would be considered to be ‘High’ to ‘Medium’ depending on the degree of disturbance.  However, where the disturbance causes a severe decline in the characteristic species (resistance is ‘None’) then recovery is likely to be prolonged (resilience is ‘Low’). Southward & Southward (1978) recorded that after the M.V. Torrey Canyon oil spill recovery of intertidal shores to their previous ecosystem function recovery can take 10 –15 years.

    The resilience and the ability to recover from human induced pressures is a combination of the environmental conditions of the site, the frequency (repeated disturbances versus a one-off event) and the intensity of the disturbance.  Recovery of impacted populations will always be mediated by stochastic events and processes acting over different scales including, but not limited to, local habitat conditions, further impacts and processes such as larval-supply and recruitment between populations. Full recovery is defined as the return to the state of the habitat that existed prior to impact.  This does not necessarily mean that every component species has returned to its prior condition, abundance or extent but that the relevant functional components are present and the habitat is structurally and functionally recognisable as the initial habitat of interest. It should be noted that the recovery rates are only indicative of the recovery potential.

    Hydrological Pressures