Rock hardness

Slope and Complexity

Mixed Substrata

Rock microhabitats

Hard shores in the UK are composed of a large range of rock types, dependent on the geology of each region. Man-made surfaces such as coastal defences, harbour walls and pier pilings also provide a substratum for rocky shore species. The community present on a rocky shore will be strongly influenced by the structure of the substratum at several spatial scales.

Hard, igneous rocks such as basalt and granite along with fine sedimentary rocks such as siltstone and shale will drain and dry rapidly as the tide recedes. More porous rocks such as sandstone and chalk retain water for longer, reducing desiccation stress. Hard rocks provide a more secure anchorage for large plants and animals such as fucoids and limpets. Animals such as piddocks are able to burrow into chalk and other soft rocks as can endolithtic microalgae, particularly cyanobacteria. Many littoral organisms are more likely to settle on some rock types than others. Proposed explanations for this include the surface roughness of the rock and the ability of rocks of different types to sequester and partition solutes (Holmes et al., 1997).

Rock type influences the slope and topographical complexity of the shore, and slope determines the area available for littoral species. Barnacles and limpets are successful on steep shores, while mussels and seaweeds are more common on gently sloping or horizontal shores. The colonisation of shores by animals is an active process in those species with motile larvae, whereas algae rely on the retention of passive spores. Therefore, algae are better able to colonise more horizontal surfaces. Increasing topographical complexity also increases space and the number of microhabitats available. These factors have a strong influence on biodiversity.

Numbers of species and abundance increases with topographical complexity (Kostylev et al., 1996). Greater abundance results from a larger surface area while the number of species present is influenced by the range of microhabitats available. Tiny pits in the rock may provide shelter for juvenile littorinids and other small species. Larger cracks and crevices provide shelter for mobile predators like dogwhelks. Crevices also collect sediment and trap air. This provides a favourable environment for a highly specialised fauna of burrowing worms and air-breathing arthropods. Air-breathing animals include centipedes, millipedes, beetles and pseudoscorpions, and pulmonates such as Onchidella celtica, foraging only during emersion and taking refuge in air pockets during tidal immersion. Crevices also provide a daytime retreat for nocturnally active invertebrates of marine origin, such as members of the genera Lineus, Eulalia, Ligia and Orchestia. Hypoxic silt accumulating in deep crevices allows ‘soft sediment’ biota such as Cirratulu cirratus to live on rocky shores.

Rocky shores might consist mainly of bedrock or they may be a mixture of bedrock and boulders, cobbles or pebbles. Boulders and smaller rock fragments can cover the whole shore or they can occur in lower densities on sediment shores. Bedrock itself is rarely smooth. Usually, there will be some form of three dimensional structure. Conglomerates have rounded humps where pebbles are present in the rock with troughs between and hollows where hard pebbles have been removed. Basalt forms smooth, regular columns which can collapse to form boulders. Limestone can form extensive ledges dotted with eroded pools. Soft rocks like chalk are usually rounded by erosion while slate shores consist of slabs with angular gullies. Each type will have its own degree of topographical complexity according to the three dimensional features present: cracks, crevices, overhangs, folds, rock fragments and pools.

Rock pools are a well known feature of many rocky shores although their ecology is less well understood than that of bedrock (see Chapter VII). Pools provide an intertidal habitat for obligate water dwellers including many species of fishes. They also support low-shore organisms, such as seaweeds and anemones, at higher levels than they would otherwise be found. However, those pools at higher shore levels are subject to physical and chemical fluctuations which would not be experienced in the sea (Morris and Taylor, 1982). Pools at about the level of neap tide high water will be separated from the sea for continual periods of about 11 hours. Higher up the shore, pools experience several days without inundation (Naylor and Slinn, 1958). Temperatures in these small bodies of water change in response to air temperatures more rapidly than those in the sea. Water evaporates from pools and rainwater and freshwater runoff collects in them, causing salinity fluctuations. Fluctuations in oxygen content and pH also occur with a diurnal cycle. During the daytime, plants photosynthesise, saturating the water with oxygen and supersaturation is not uncommon. During the night, a net uptake of oxygen and production of carbon dioxide occurs. Oxygen concentration drops and pH falls as a result.

Boulders have an upper surface similar to bedrock and, if stable, the tops are essentially the same as bedrock. Larger boulders will suffer less impact from waves on their shoreward sides. These sides may therefore be used by species at risk of dislodgement on more exposed surfaces. Boulders also have interstitial spaces below. Like crevices, these provide shelter for grazers and foragers. Mobile animals, including moulting crabs take refuge under rocks. Sediment will collect in the interstitial spaces and, often, the boulders rest on a sediment substratum. So, while communities on the tops of boulders are similar to bedrock shores, communities characteristic of sediment shores can be found below.

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