A framework for the classification (EUNIS levels 2 and 3)

Whilst the classification has been developed for nature conservation purposes and hence needed to be biologically driven, the dynamic nature of certain populations of species, and sometimes whole communities, meant it was essential to identify the habitat within which the community (of potentially varying composition) occurs to ensure types defined would be robust over time. Full use is also made of the habitat attributes to provide a structure to the classification which is both logical and easy to use. In this way much more significant use of habitat characteristics is made than for many terrestrial classifications, where vegetation alone is often the prime determinant of the classification's structure. The classification is presented in such a way as to allow access via either the habitat attributes through a series of habitat matrices or the biological community in a hierarchical classification of biotopes and higher types.

Each of the environmental gradients outlined in Table 1 can be considered to form an axis within a multi-dimensional matrix. Each community develops according to a suite of environmental conditions (and biological influences) which lie within such a multi-dimensional matrix, reflecting varying biological character according to its position along each particular gradient. Although the degree of importance of each habitat attribute varies for differing communities, the first two, namely substratum and the vertical gradient or zonation, appear to play a highly significant role in all communities. They are also the most easily and reliably recorded attributes in the field and are readily mapped. These factors combine to make the attributes of substratum and zonation the most appropriate for structuring the upper end of the classification.

The primary habitat matrix of substrata versus zonation (Table 3) illustrates the framework adopted for the classification. It represents EUNIS levels 2 and 3 in the hierarchical classification and has been developed to reflect the most significant changes in biology at a scale appropriate to an internationally applicable classification. Table 4 outlines the rationale behind the divisions adopted for these two levels in the classification.