Environmental influences on community structure


Table 1 Environmental factors which influence community structure


Rocky habitats
Sediment habitats
Varies from bedrock, through boulders to stony plains, often mixed with sediment. The degree of stability of the rock is important, with algae and animals increasingly able to colonise smaller stones in more sheltered stable conditions.
Ranges from shingle (mobile cobbles and pebbles), through gravel and sand to very soft mud and muddy gravels. The type of sediment, mainly determined by the dynamics of water movement at the site, is highly important in structuring community composition, although salinity may become more critical in upper estuarine conditions.
Zonation: emersion / immersion on the shore (desiccation); depth in the subtidal (illumination)
(see Figure 1 and Table5 for further details)
A major factor, related to the length of time the rock is exposed by the tide, which leads to very marked horizontal bands of zonation on most rocky coasts. Supralittoral and littoral fringe zones on the extreme upper shore are lichen dominated. The main eulittoral zone is characterised by barnacles, mussels or fucoid algae, the infralittoral by kelps and the circalittoral by animals.
Much less obvious than on rocky coasts, but with a zone of drying on the upper shore and a more water-logged/saturated zone on the lower shore. With increasingly finer sediments the saturated zone extends further up the shore. Very sheltered areas often support saltmarsh vegetation at extreme high water level. Shallow subtidal sediments reflect a high degree of wave disturbance and high temperature/salinity fluctuations, with increasingly more stable conditions with depth.
Exposure to wave action
Marked differences result due to different wave exposures. Exposed shores are usually animal (mussel and barnacle) dominated, whilst sheltered shores are fucoid algal dominated. Such differences can occur over only 10's of metres at certain sites, such as opposite sides of a headland. In the subtidal a similar pattern is exhibited, but is increasingly more masked by tidal-current influence with depth.
Principally expressed by the resultant grade of sediment, with coarse sands on exposed coasts and fine muds on sheltered coasts. Areas subject to periodic (seasonal) wave action may exhibit sub-climactic communities.
Strength of tidal currents
Strong offshore currents affect many coasts and have a particularly marked influence on circalittoral communities, with lessening effects in shallow water and on the shore (where the influence of wave action predominates). However constricted sections of some inlets, particularly the narrows in sealochs, can have very strong currents which affect both the shallow subtidal and the lower shore zones, significantly increasing species richness.
Contributes, with wave action, to determining sediment grade and consequent community type. In estuaries and sealochs this can lead to coarser sediments than would normally be expected in wave-sheltered areas. The lower shore of some inlets by the main channel can have tide-swept sands and gravels with distinctive species-rich communities.
The majority of rocky coasts are subject to full salinity, but within marine inlets are subject to increasing freshwater influence. Variable salinities (in estuaries) lead to species-poor examples of open coast communities whilst the very limited areas of rock in permanently reduced salinities (in lagoons) may support quite distinct communities. Localised freshwater influence often results in the growth of ephemeral green algae on the shore.
Variable and reduced-salinity conditions are typical of sediment shores within inlets, especially estuaries, and play an important role, alongside sediment type, in determining community type. Salinity eventually becomes the more important structuring factor in the upper reaches of estuaries and in lagoons.
Temperature (relates to biogeography)
National differences in water temperature give more species-rich communities in the south and west and more species-poor communities in the north and east.
Topography has a marked influence on the variety of communities which may occur. Variations in topography (resulting from a particular rock type) which lead to vertical faces, overhangs, gullies, caves and rockpools all increase habitat and micro-habitat diversity compared with uniform areas of rock.
Variations in the slope of the beach can indicate differing degrees of saturation, whilst drainage channels may be subject to increased freshwater influence or currents. In the subtidal, variation in slope has little influence on community type, although the presence of dunes can affect small-scale community structure. Larger features such as sandbanks usually indicate coarser sediments compared with surrounding sediment plains.
The rock type is significant in two respects, affecting overall topography (see above) and the surface texture for colonisation. Soft limestones and chalks have a pitted surface which can affect species composition, whilst these types, plus peats and clays, are soft enough to be bored by piddocks and other species.
Not applicable.
Not generally applicable, as most rocky habitats are subject to full oxygenation. Severe deoxygenation can lead to reduction in species and the presence of bacterial growths.
More sheltered fine sediments tend to become anoxic below the surface, giving a distinct black layer. Severe deoxygenation significantly reduces species richness.
Wave surge
On exposed coasts gullies subject to wave surge have distinct animal-dominated communities. Wave surge on vertical rock tends to give communities typical of more exposed sites (e.g. Alaria esculenta occurring on moderately exposed vertical rock).
Influences sediment grade and result in highly-mobile species-poor habitats.
Scour, turbidity and siltation
Sand scour and sediment in suspension can encourage growth of ephemeral algae and sometimes mussels (Mytilus spp.) and tube-worms (Sabellaria spp.). Siltation in sheltered areas often restricts the growth of algae.
A high degree of scour and turbidity may result in species-poor communities.
Shaded faces on the shore encourage the growth of species intolerant of desiccation.
Not applicable.
Organic carbon
Not applicable.
Significant in many sediment communities. Organic enrichment can alter community structure and lead to increased numbers of opportunist species e.g. capitellid worms.
Hydrographic regime (residual currents); water quality
The overall hydrographic regime and water quality characteristics of an area play an important role in determining community composition. Key aspects of these factors are discussed above. In addition to these, residual current flow is also very important, as it may affect larval distribution and water quality aspects such as nutrient levels as well as water temperature, salinity and turbidity.