Biodiversity & Conservation

LR.MLR.BF.Fser.Fser.Bo

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Substratum removal will result in the loss of the entire MLR.Fser.Fser.Bo community and, therefore, intolerance has been assessed as high. Although mobile species including the long- and broad-clawed porcelain crabs may survive, they are not, in isolation, representative of MLR.Fser.Fser.Bo. Recoverability is likely to be high (see additional information).
Smothering
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Many of the underboulder species are low-lying encrusting forms that cannot escape smothering and are, therefore, especially vulnerable. Over the course of one month, feeding in suspension feeders is likely to be inhibited as a result of the clogging of the feeding apparatus. In addition, deoxygenation will occur due to the decomposition of smothered matter under the boulder. Balanus crenatus can withstand covering by silt provided that the cirri can extend above the silt layer but smothering by 5 cm of sediment would prevent feeding and could cause death. It is likely that many of the important species including the bryozoans and colonial ascidians will experience mortality and accordingly, intolerance has been assessed as high. However, smothering by sand is part of the natural dynamics of some boulders (Foster-Smith, pers. comm.) and the fact that the majority of underboulder communities are downward facing means that the effects of smothering are likely to be relatively short lived. Recoverability is expected to be high (see additional information). (This assessment is for smothering by sediment - some typical underboulder species can survive overgrowth by other species (c.f. Turner, 1988)).
Increase in suspended sediment
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Underboulder communities face downwards so that silt is unlikely to settle but may clog the feeding structures of some species such as hydroids, bryozoans and ascidians thereby reducing total ingestion over the benchmark period. Umbonula littoralis for example, is expected to have a limited ability to clear itself of silt. Rich underboulder communities are known to occur in turbid waters, for instance, the Menai Strait. However, increased suspended sediment, in combination with areas of low wave energy or water movement may lead to siltation (see water flow rate) and therefore, intolerance has been assessed as intermediate. Recoverability is likely to be high (see additional information).
Decrease in suspended sediment
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A decrease in suspended sediment is likely to be beneficial to most of the underboulder community. The suspension feeders may become more efficient as there would be fewer inorganic particles to clog and interfere with feeding apparatus. Assuming that the decrease in suspended sediment refers to inorganic particles, a reduction in total ingestion in the suspension feeding community is not expected. Therefore, tolerant* has been assessed.
Desiccation
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Underboulder communities are generally damp due to the fact that they are mostly unaffected by the drying influences of wind and insolation. Furthermore, many underboulder communities are in contact with flowing water. Underboulder species on boulders which are turned so that the undersurface community ceases to be shaded and damp are likely to be killed. The number and diversity of species likely to be killed be dependent on the size of the boulder since larger boulders are less likely to be turned over and so will have more developed community (see Ecology). Small rocks frequently turned over (either through natural energy, e.g. by wave energy, or humans) will have fewer species and species that, nevertheless, are opportunistic species characteristic of disturbed environments. Balanus crenatus, for example, were reported to have a mean survival time of 14.4 hours in dry air (Barnes et al., 1963). The community will eventually re-develop on the new underside and therefore, recoverability is expected to be high (see additional information).
Increase in emergence regime
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A one hour change in the time not covered by the sea for a period of one year is unlikely to adversely affect the majority of the MLR.Fser.Fser.Bo community since the habitat is likely to remain shaded and damp. Mobile species such as Pisidia longicornis and Carcinas maenas, because of their mobility, may be able to escape the effects of increased emergence by crawling to damper areas further down the shore.

On balance, however, MLR.Fser.Fser.Bo has been assessed as being of intermediate intolerance to changes in emergence to reflect the likelihood that species at the limits of their tolerance to emergence might be killed. Recoverability is likely to be high (see additional information).

Decrease in emergence regime
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A decrease in emergence would reduce the influence of desiccation on the community which would be beneficial to the biotope. However, this benefit may be counteracted by the fact that the more submerged boulders may be subject to increased disturbance through wave energy. Larger boulders previously undisturbed may move around more, potentially leading to an increased species diversity (see Ecology).

On balance, MLR.Fser.Fser.Bo has been assessed as tolerant to a decrease in emergence.

Increase in water flow rate
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The richest underboulder communities develop in areas subject to strong tidal flows and, therefore, at the benchmark level, MLR.Fser.Fser.Bo is likely to be tolerant*.
Decrease in water flow rate
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A decrease in strength of tidal flow will lead to loss or reduction in abundance of some species and this would most likely be a result of increased siltation. Species including Pisidia longicornis and Umbonula littoralis thrive in habitats that are in areas of moderate to strong water movement. A decrease in water flow rates where wave action is also weak would be likely to result in mortality in, for example, some bryozoans, colonial ascidians and sponges. This is most likely as a secondary effect from siltation but possibly also due to a reduction in food source. Barnes & Bagenal (1951) found that the growth rate of Balanus crenatus epizoic on Nephrops norvegicus was considerably slower than animals on raft exposed panels and this was attributed to reduced currents and increased silt loading of water in the immediate vicinity of Nephrops norvegicus. Intolerance is, therefore, assessed as intermediate. However, recoverability will be high (see additional information).
Increase in temperature
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The shaded and damp conditions found in underboulder communities may serve to protect the MLR.Fser.Fser.Bo community from extremes of temperature. Nevertheless, the important species found in this biotope have varying levels of tolerance to changes in temperature at the benchmark level and some species living under boulders are normally subtidal species and may be unable to withstand large changes in temperature.
  • Pisidia longicornis occurs in a wide range of temperature regimes from Norway to Angola and it is unlikely that they would be adversely affected by an increase in temperature at the level of the benchmark.
  • The British Isles are at the centre of geographical range for Umbonula littoralis, Botryllus schlosseri and Halichondria panicea suggesting that colonies are likely to be tolerant of both an increase and decrease in temperature at the benchmark level.
  • Balanus crenatus is a boreal species that is likely to be intolerant of increases in water temperature. In Queens Dock, Swansea, where the water temperature was on average 10 °C higher than average due to the effects of a condenser effluent, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite. After the water temperature cooled Balanus crenatus returned (Naylor, 1965). Balanus crenatus was unaffected during the severe winter of 1962-63, when average temperatures were 5 to 6 °C below normal (Crisp, 1964a).
  • Gamete release in Dendrodoa grossularia decreases at 15 degrees and is suppressed at 20 degrees and below about 8-11 degrees (Millar, 1954). It is likely to be sensitive to an increase and decrease in temperature at the benchmark level.
On balance, it is likely that overall intolerance to an increase in temperature will be low.
Decrease in temperature
(View Benchmark)
The shaded and damp conditions found in underboulder communities may serve to protect the MLR.Fser.Fser.Bo community from extremes of temperature. Nevertheless, the important species found in this biotope have varying levels of tolerance to changes in temperature at the benchmark level and some species living under boulders are normally subtidal species and may be unable to withstand large changes in temperature.
  • Pisidia longicornis were adversely affected by the 1962-63 winter in Britain. Crisp (1964a) records that many hundreds were found dead on the strandline at Oxwich, south Wales. In other locations, they were not found on the shore (although could have migrated offshore).
  • The British Isles are at the centre of geographical range for Umbonula littoralis, Botryllus schlosseri and Halichondria panicea suggesting that colonies are likely to be tolerant of both an increase and decrease in temperature at the benchmark level.
  • Gamete release in Dendrodoa grossularia decreases at 15 degrees and is suppressed at 20 degrees and below about 8-11 degrees (Millar, 1954). It is likely to be sensitive to an increase and decrease in temperature at the benchmark level.
On balance, it is likely that overall intolerance to a decrease in temperature will be low
Increase in turbidity
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Rich underboulder communities are known to occur in turbid waters, for instance, the Menai Strait. Therefore, it has been suggested that MLR.Fser.Fser.Bo is tolerant to an increase in turbidity at the benchmark level.
Decrease in turbidity
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A decrease in turbidity may stimulate phytoplankton production which would be beneficial to the suspension feeding community associated with MLR.Fser.Fser.Bo. Therefore, it has been suggested that MLR.Fser.Fser.Bo is tolerant to an increase in turbidity at the benchmark level.
Increase in wave exposure
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Many of the species likely to be found in MLR.Fser.Fser.Bo communities are probably tolerant of very wave exposed conditions. However, increases in wave exposure may cause more boulders to become mobile and abrade underboulder communities. Increased mobilization of boulders may result in patches of sponges, bryozoans and barnacles being crushed on impact with other boulders. For example, Umbonula littoralis has a hard calcareous skeleton which is likely to be broken through contact with hard surfaces such as cobbles moving around during storms. Crabs and other fragile mobile species are also at risk from being crushed. Furthermore, many of the stable boulders are fused together by algal growth (especially corallines) and breaking up this matrix would adversely affect the community (Foster-Smith, pers. comm.). The release of sediment between boulders may serve to interrupt suspension feeding (see Suspended Sediment above).

MLR.Fser.Fser.Bo is found on shores ranging from wave sheltered to moderately wave exposed and as a result the communities in the biotope between each of these locations will vary anyway and. Therefore, different sites are likely to have varying tolerances with respect to changes in wave exposure. On balance, MLR.Fser.Fser.Bo has been assessed as being of intermediate intolerance to a change I wave exposure since some species may experience mortality although even frequently disturbed boulders with a few pioneer species may still represent MLR.Fser.Fser.Bo. Recovery is expected to be rapid (see additional information).

Decrease in wave exposure
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A decrease in wave exposure may facilitate sedimentation which will smother underboulder species resulting in mortality (see Smothering above).

MLR.Fser.Fser.Bo is found on shores ranging from wave sheltered to moderately wave exposed and as a result the communities in the biotope between each of these locations will vary anyway and. Therefore, different sites are likely to have varying tolerances with respect to changes in wave exposure. On balance, MLR.Fser.Fser.Bo has been assessed as being of intermediate intolerance to a change in wave exposure since some species may experience mortality although even frequently disturbed boulders with a few pioneer species may still represent MLR.Fser.Fser.Bo. Recovery is expected to be rapid (see additional information).

Noise
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The characteristic and permanent members of the fauna are invertebrates unlikely to detect or be affected by noise.
Visual Presence
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The characteristic and permanent members of the fauna are invertebrates unlikely to detect or be affected by visual presence.
Abrasion & physical disturbance
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In addition to disturbance caused by wave energy, intertidal boulder communities are often disturbed by, for example, bait collectors, inquisitive school groups and field researchers. Boulders left overturned place the organisms on the now upward facing part of the boulder at great risk of desiccation (see Desiccation above). Furthermore, many stable boulders are fused together by algal growth (especially corallines) and breaking this matrix would be very harmful (Foster-Smith, pers. comm.). Furthermore, this disturbance and habitat degradation could change a stable boulder field to an unstable field on a long-term basis (Foster-Smith, pers. comm.). Movement of the boulder surface against other hard surfaces (for instance, during extreme storm events) is likely to cause significant damage to encrusting fauna that is characteristic of the community. Recoverability is expected to be high (see additional information).
Displacement
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Due to the fact that the majority of species likely to be found in the MLR.Fser.Fser.Bo community are permanently attached to the substratum, displacement will have the same effect as substratum removal and, therefore, intolerance has been assessed as high.

Chemical Factors

Synthetic compound contamination
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Some members of the community, particularly crustaceans and molluscs, may be intolerant of chemicals that may have an adverse effect on reproduction. Chemicals developed as anti-fouling paints have been developed to counter fouling communities which are similar to underboulder communities. Barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved (Holt et al., 1995). Hoare & Hiscock (1974) found that Balanus crenatus survived near an acidified halogenated effluent discharge where many other species were killed, suggesting a high tolerance to chemical contamination. Little information is available on the impact of endocrine disrupters on adult barnacles or on the effects of synthetic chemicals on the other important species. However, intolerance has been suggested as intermediate to reflect the likely effects of antifouling chemicals. Component species generally have planktonic larvae and reproduce frequently so that re-colonization will be rapid, providing the environment is clean of any chemicals that were having an adverse effect ob the community.
Heavy metal contamination
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Barnacles accumulate heavy metals and store them as insoluble granules (Rainbow, 1987). Pyefinch & Mott (1948) recorded a median lethal concentration of 0.19 mg/l copper and 1.35 mg/l mercury, for Balanus crenatus over 24 hours. Barnacles may tolerate fairly high level of heavy metals in nature, for example they are found in Dulas Bay, Anglesey, where copper reaches concentrations of 24.5 µg/l, due to acid mine waste (Foster et al., 1978). However, insufficient information was available on the remaining important species in MLR.Fser.Fser.Bo and sensitivity has not been assessed.
Hydrocarbon contamination
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Little evidence was found. Ryland & de Putron (1998) found no detectable damage to underboulder faunas during oil pollution in Watwick Bay, Pembrokeshire. However, fresh oil is likely to narcotize and kill Decapoda and some Gastropoda. Component species generally have planktonic larvae and reproduce frequently so that re-colonization will be rapid, providing the environment is clean of any chemicals that were having an adverse effect on the community.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Underboulder communities occur where nutrient concentrations are high in enclosed coastal areas, for instance, the Menai Strait. The underboulder area is shaded and therefore any risks of thereby reducing the likelihood of smothering by ephemeral green algal species that are likely to flourish in the event of nutrient influx. Tolerant has been suggested.
Increase in salinity
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Underboulder communities occur in full to variable salinity habitats although it might be that higher salinity occurs at the outflow of some basins. At the levels expected, MLR.Fser.Fser.Bo is likely to be tolerant to an increase in salinity.
Decrease in salinity
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Rich underboulder communities occur in outflows from areas of variable salinity (for instance, in the Menai Strait, North Wales and in sea lochs). The communities therefore have some tolerance to at least short-term reduced salinity. However, some of the component species may be intolerant of reduced salinity. Pisidia longicornis and Umbonula littoralis both occur in full salinity and are likely to be intolerant of an acute reduction in salinity. Other species would be very tolerant. Balanus crenatus, for example, can tolerate salinities down to 14 psu if given time to acclimate (Foster, 1970). On balance, an intolerance of intermediate has been suggested to reflect the possibility that some species may experience some mortality. Component species generally have planktonic larvae and reproduce frequently so that re-colonization will be rapid.
Changes in oxygenation
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Underboulder habitats may be subject to lowered oxygen levels due to restricted water flow in calm periods. Also, organic debris that becomes trapped under the boulders may rot and cause de-oxygenated conditions. Some tolerance of low oxygen levels is therefore expected in some situations. However, the richest underboulder communities occur where water flow is strong and almost continuous and might suffer in de-oxygenated conditions. Component species generally have planktonic larvae and reproduce frequently so that re-colonization will be rapid.

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information.
Introduction of non-native species
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Insufficient information.
Extraction
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Species that are extracted from underboulder communities include edible crustaceans which, as scavengers, are not of key importance in the functioning of the community. None of the important species are likely to be targeted for extraction although the collection of other creatures including crabs and shrimps may result in increased physical disturbance, to the detriment of the community (see Physical Disturbance).

Additional information icon Additional information

Recoverability
The community associated with MLR.Fser.Fser.Bo will very greatly depending on various factors including the size of boulder, wave exposure and the presence or absence of flowing water under the boulder. In addition, it is difficult to identify a ‘climax’ community per se because the extent of community succession will vary greatly between boulders of different sizes etc. Furthermore, because there are no key functional, structural or characterizing species, any combination of the important species could, theoretically, determine the biotope community. Nevertheless, the recolonization of fauna typically associated with MLR.Fser.Fser.Bo will occur within a year or two and recoverability is expected to be high. However, the development of a mature community characteristic of seldom disturbed boulders dominated by e.g. Halichondria panicea and Dendrodoa grossularia may take longer although many boulders will never mature to this stage.

In the study of recolonization of vertical rock wall in Maine (Sebens, 1986), epifaunal and algal crust species were shown to re-colonize cleared areas quickly. For example encrusting bryozoans, tubeworms, tubicolous amphipods and worms, erect hydroids and bryozoans were reported to cover cleared areas within 1-4 months in spring, summer and autumn (Sebens, 1986). Sebens (1985) reported that Halichondria panicea had reached previous cover within two or more years. It was slow to recolonize the cleared areas, only appearing after about a year, although it is relatively fast growing. Balanus crenatus is another important early colonizer of sublittoral rock surfaces (Kitching, 1937) and it heavily colonized a site that was dredged for gravel within 7 months (Kenny & Rees, 1994).


This review can be cited as follows:

Hiscock, K. 2005. Underboulder communities. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 20/11/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=371&code=1997>