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.