Defining a habitat

Charles Darwin (1859) defined a habitat as the locality in which a plant or animal naturally lives. This classical definition still holds true today and essentially refers to the environment in which a single species lives. However, for the purposes of habitat mapping, it is usual to expand the concept to refer to a habitat as the place where multiple species occur together under similar environmental conditions, such that a habitat can be distinguished from surrounding habitats on the basis of both its species composition and its physical environmental characteristics (e.g. type of seabed, tidal currents, salinity, etc). In this context the species are often considered to be associated together in a community and the combination of species and their environment is referred to as a biotope. On land, it is thus possible to distinguish a forest from a meadow or grassland, and to map these on the basis of their differing physical and biological characteristics. This concept is equally applicable in the marine environment, and leads to the description and mapping of habitats such as mudflats, kelp forests and seagrass beds.
It is this multi-species use of the term habitat which forms the basis of marine habitat mapping and which is the focus of this MESH Guide. Mapping of habitats for a single species (the original Darwin concept) is often more useful to consider in relation to larger more mobile species, such as fish and mammals, which can occupy a wide range of environments; this type of mapping is not considered in this Guide.
It is possible to also define habitats for the water column or pelagic environment, although plotting their geographic distribution becomes more difficult because we have to include the third dimension of depth in any map. The MESH Project only considered seabed mapping and so these water column habitats are not considered further in the MESH Guide.
Unfortunately the term ‘habitat’ is commonly used generically in many scientific circles, as well as in management, policy and legal arenas where it often has a much broader definition. For example, the OSPAR Convention has defined a list of ‘rare and threatened and/or declining habitats’ that includes features such as ‘seamounts’ that are much more akin to a physical feature rather than the narrower definition used in the MESH Guide. Such uses of the term habitat are further explained in the sections 'Related scientific terms' and 'Legal and policy use of the term habitat'.
The MESH Project and hence this MESH Guide have adopted the more widely used term ‘habitat’ but we define it in its ‘biotope’ form to mean both the physical and biological characteristics of an area of the seabed.
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