Environmental influences at the habitat scale

Each species tends to live within a certain environment; that is, it has a preference for a combination of environmental factors (a niche), such as the substratum, temperature, salinity and hydrodynamic conditions that it is able to live within. The tolerance to different environmental conditions varies between species; it can be rather broad for some very common species but much more tightly defined for others. The niche occupied by a species may vary both temporally and spatially and is influenced not only by its physiological requirements and tolerance to change but also by the interactions between species, i.e. competition and predator-prey relationships.
 
In any particular place on the shore or seabed, a suite of species will occur, each adapted to the particular environmental conditions of that place, such as the conditions of an intertidal mudflat. Where such a suite of species occurs in other locations under similar environmental conditions, it can be defined as a community (or association or assemblage) of species which is occurring within a particular habitat type. The collective term biotope is now in common usage to encompass both of these biotic and abiotic elements.
 
Shore and seabed habitats are colonised primarily by seaweeds (on the shore and in shallow water) and by marine invertebrates from a wide range of phyla. Lichens (in the splash zone), higher plants (especially in saltmarshes) and fish contribute to a lesser degree. In contrast to terrestrial habitats, it is commonplace for marine habitats to be characterised, i.e. dominated, by animals rather than plants, and for the substratum to provide the main structure to the habitat (rather than plants such as in a forest).
 
Only a proportion of habitats have obvious dominant species (e.g. kelp forests, mussel beds, maerl beds). Many, particularly in deeper water, support a mosaic of species, none of which is particularly dominant, which may exhibit a degree of patchiness over the seashore or seabed and, in some cases, vary markedly with time. In these respects the species offer a much less robust mechanism for structuring a classification system than does the physical habitat in which they occur.

 

In the marine environment, there is a strong relationship between the abiotic nature of the habitat and the biological composition of the community it supports. Most communities appear to occur within a recognisable suite of environmental factors, although some occur within a more tightly-defined set of factors (habitat). One of the most important factors influencing species composition is the type of substratum present, which can be broadly divided into rock and sediment (the latter is closely linked to the hydrodynamic regime) whilst in estuaries salinity is an important factor. Community structure is additionally modified by biological factors such as recruitment, predation, grazing and inter-species competition. Species may modify habitats by their boring, accretion and bioturbation. The most important habitat attributes which appear to influence community composition are described in Table 1.  In addition to habitat factors, biological and anthropogenic influences affect community composition. Some aspects of anthropogenic influence are outlined in Table 2.