Biodiversity & Conservation

LR.FLR.CvOv

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum will remove the associated community. The majority of species characteristic of the community have planktonic larvae that are produced once a year so that settlement will be rapid. However, successional change will occur before the community reaches a dynamic stability similar to that before destruction.
Smothering
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Smothering on overhangs and in caves is likely to occur as a result of overgrowth by dominating species (and not by siltation). Some overgrown species will survive (cf. Turner 1988) but a thick growth of Dendrodoa grossularia is likely to kill encrusting organisms below. Assuming that some living individuals remain and allowing for settlement of planktonic larvae, recovery is likely to be fairly rapid.
Increase in suspended sediment
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The vertical and overhanging nature of surfaces means that siltation is unlikely to occur. Therefore, although component species may be highly intolerant of siltation, the biotope as a whole is not.
Decrease in suspended sediment
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Desiccation
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Species present under overhangs and in intertidal caves are emersed and subject to some drying for significant lengths of time and so withstand some desiccation. Changes of time immersed equivalent to one zonal height may not be significant providing that the communities are shaded. Recovery would occur from surviving colonies and individuals and new settlement from larval sources.
Increase in emergence regime
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The main threat to species is if emersion time increases, in which case, intolerance is similar to that for desiccation, i.e. changes of time immersed equivalent to one zonal height may not be significant providing that the communities are shaded. However, during emersion they will be unable to feed so that some decline is probable. Recovery would occur from surviving colonies and individuals and new settlement from larval sources.
Decrease in emergence regime
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Increase in water flow rate
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Where an overhang or cave is subject to flowing water (for instance, where it is in a tunnel or gully open at both ends), the character of the community is likely to change as species characteristic of stronger or weaker water flows predominate. However, many of the key characteristic species occur in a broad range of water flow types and the biotope will remain the same. Recovery would occur from surviving colonies and individuals and new settlement from larval sources.
Decrease in water flow rate
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Increase in temperature
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In the case of higher temperatures, some species that are predominantly sublittoral are likely to suffer higher rates of desiccation and drying. In the case of lower temperatures, some species may be subject to exposure to cold conditions although this is uncertain. Crisp (1964) recorded some adverse effects on compound ascidians in North Wales following the severe winter of 1962-63.
Decrease in temperature
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Increase in turbidity
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The communities that occur under overhangs are similar in areas where high turbidity occurs (e.g. the entrance to estuaries) to those on the open coast, suggesting a tolerance to changing or different conditions of turbidity. Moore (1973b) found that Umbonula littoralis, one of the characterizing species, was ubiquitous in relation to a turbidity factor. Recovery would occur from surviving colonies and individuals and new settlement from larval sources.
Decrease in turbidity
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Increase in wave exposure
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The communities that occur under overhangs are different on wave exposed and wave sheltered coasts - generally having a wider range of species on wave exposed sites compared to wave sheltered sites. However, many of the key characteristic species occur in a broad range of wave exposure types and the biotope is likely to remain the same even with a change of two exposure categories. Change in species richness has to be identified as "not relevant" because it is likely to increase with greater wave exposure and decrease with decline in wave exposure. Return to a previous species composition would occur from surviving colonies and individuals and new settlement from larval sources.
Decrease in wave exposure
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Noise
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The characteristic species of the biotope are unlikely to detect noise.
Visual Presence
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The characteristic species of the biotope do not have visual organs and will not therefore detect visual presence.
Abrasion & physical disturbance
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Communities under overhangs and in caves which extend to a seabed that is of mobile substrata demonstrate a zonation from bare (abraded) rock adjacent to the bottom, through fast settling and growing species to abrasion tolerant species to the typical overhang community (that is nevertheless probably subject to abrasion and damage during storms). Whilst whole communities are destroyed by abrasion from coarse sediments including especially pebbles and cobbles, recovery would occur from surviving colonies and individuals and new settlement from larval sources. Abrasion by human activities might include anchoring and dredging (fisheries).
Displacement
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The most important species in characterizing this community are sessile and will be unable to reattach and killed if displaced. Recovery would occur from surviving colonies and individuals and new settlement from larval sources.

Chemical Factors

Synthetic compound contamination
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Most sessile invertebrates have not been shown to have a high susceptibility to synthetic chemicals such as anti-fouling compounds. However, some mobile species that are a part of the community, such as Ocenebra erinacea and Nucella lapillus (Gibbs et al. 1990 & 1991) have been shown to be intolerant of some synthetic chemicals. More difficult-to-detect effects may occur to larval stages of a wide range of the species that occur under overhangs. The species of mollusc that have been shown to be adversely affected lay eggs on the shore and have a short or no planktonic phase so that recovery of those minority of species will be slow.
Heavy metal contamination
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Insufficient information.
Hydrocarbon contamination
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Ryland & de Putron (1998) found no detectable damage to under-boulder communities, which are similar to some overhang communities, in Watwick Bay, Pembrokeshire following the Sea Empress oil spill. Part of the resistance to effects might be because oil does not settle onto overhanging surfaces. However, some species, especially gastropods are likely to be narcotised and killed and some damage is likely. Return to a previous species composition would occur from new settlement from larval sources although some gastropods have no or only a short dispersal phase so that recovery will be slow.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Similar overhang communities occur where nutrient levels are high (for instance at the entrance to estuaries) or normal (near to the open coast).
Increase in salinity
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Overhang communities occurring near to the entrance to estuaries are probably subject to occasional lowered salinity and so appear tolerant of some reduction in salinity at least for short periods. However, some species characteristic of overhangs on the open coast do not extend into estuaries whilst the sea squirt Dendrodoa grossularia often becomes dominant into conditions of variable or low salinity. Thus change is likely to occur if salinity is lowered for a significant length of time (the benchmark is one salinity grade for one year). Recovery would occur from surviving colonies and individuals and new settlement from larval sources.
Decrease in salinity
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Changes in oxygenation
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Decaying marine life may be observed in the areas of overhangs and caves most sheltered from water movement. The species characteristic of the community therefore do seem to be adversely affected by de-oxygenation. Recovery would occur from surviving colonies and individuals and new settlement from larval sources.

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information.
Introduction of non-native species
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Any effects are dependant on the biology of the non-native species. Whilst those non-native species currently present in Britain and Ireland have not been observed to significantly affect overhang or cave biotopes, effects of future imports are unpredictable.
Extraction
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Crabs (Cancer pagurus) and lobster (Homarus gammarus) are taken from deep recesses at the backs of overhangs often using a metal hook. However, none of the species indicative of sensitivity are likely to be extracted and the removal of these crustaceans will not affect the recognisable biotope. Not relevant has been suggested.

This review can be cited as follows:

Hiscock, K. 2002. Overhangs and caves. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 16/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=242&code=2004>