Nature of the marine environment

The habitat scale in characterising the marine environment

The marine environment can be described or characterised at a number of different scales, ranging from ocean-level processes through to those that occur at species and genetic level (Connor et al. 2002). The scales of relevance here are marine landscapes, habitats and species; their inter-relationship can be expressed as follows:
 
  • Species provide the globally accepted original classification of biological diversity, with well-established rules of taxonomy to distinguish between different types. Their classification is arranged in a hierarchy of genera, families, orders, classes and phyla.

 

  • Habitats comprise suites of species (communities or assemblages) that consistently occur together, but which are derived from different parts of the taxonomic hierarchy (e.g. kelps, molluscs and fish in a kelp forest habitat). Their classification can also be structured in a hierarchy (biotopes, biotope complexes, broad habitats), reflecting degrees of similarity.

 

  • Marine Landscapes comprise suites of habitats that consistently occur together, but which are often derived from different parts of the habitat classification hierarchy (e.g. saltmarsh, intertidal mudflats, rocky shores and subtidal mussel beds in an estuary).

 

The approach to classification or characterisation at each scale differs, each adopting differing factors to suit the requirements at that scale. Whilst the classification (taxonomy) of species, and to a lesser degree habitats, is now well established the seascapes concept and their characterisation is a more recent approach to characterisation of the marine environment (Laffoley et al. 2000, Day & Roff 2000). The marine landscape concept was applied to the seabed and water column of the Irish Sea as part of the Irish Sea Pilot project.
 

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.
 

Terminology: the terms biotope, habitat and community

A biotope is defined as the combination of an abiotic habitat and its associated community of species. It can be defined at a variety of scales (with related corresponding degrees of similarity) and should be a regularly occurring association to justify its inclusion within a classification system.
 
A habitat is taken to encompass the substratum (rock, sediment or biogenic reefs such as mussels), its topography and the particular conditions of wave exposure, salinity, tidal currents and other water quality characteristics (e.g. turbidity and oxygenation) which contribute to the overall nature of a place on the shore or seabed.
 
The term community is used here to mean an association of species which has particular species, at certain densities, in common.
 
Although communities are influenced by biological interactions (e.g. predation, recruitment processes) and by interference from certain human activities, their overall character is very strongly determined by the nature of the surrounding abiotic conditions. This consistent relationship between the biotic and abiotic elements is fundamental to the structure of the classification system. Types can be defined at a variety of scales, enabling the development of a hierarchical classification of types. The degree of similarity varies depending upon the scale considered, with more broadly defined types (e.g. sheltered rocky shores) having a lower level of similarity compared with more finely defined types (e.g. a lower shore sheltered rocky biotope).
 
Whilst the term habitat, as used here, is its more accepted scientific meaning, the term is more widely used, for instance in the EC Habitats Directive, to also include the community of species living in the habitat; the common use of the term is, therefore, synonymous with the term biotope.