Development of a hierarchical classification

It was considered essential to develop a hierarchical classification structure in which broader, higher types in the classification could be more finely divided to support more detailed use. The development of the hierarchy comes from both a top-down and a bottom-up approach:
 

Top-down classification

Taking the marine environment as a whole, it can be sub-divided into a series of broad habitat categories, based largely on their physical character as described here. At the very broadest level, differentiation can be made between rock and sediment habitats, and between those on the shore (intertidal) and those in the subtidal or deep ocean. These high-level divisions can be further subdivided on the basis of different types of sediment (e.g. gravel, mud), different degrees of wave exposure on rocky coasts (exposed, sheltered) and varying depth bands below the low water mark (e.g. shallow water where light penetrates, deeper water with little light). Such broad-scale differences in habitat character are readily understood by non-specialists and provide classification types that are easily mapped; however, they also have ecological relevance as they reflect major changes in habitat character upon which species depend (see above).
 
The top-level types depicted in the primary habitat matrix (Table 3) show levels 2 and 3 in the hierarchical classification. It is important to note that these top-level categories were developed after consideration of how best to classify biological data at the lower end of the classification.
 

Bottom-up classification

Field survey, whether on the shore or in the subtidal, reveals that different places support different communities of species. The precise combination of species and their relative abundance varies from place to place and is dependent both on environmental characteristics and upon interactions between species. Visits to different sites that have similar environmental characteristics, such as sediment type and depth, show certain levels of similarity in their species composition. Multivariate analysis of the data from field surveys (e.g. grabs, diver observations) groups these data into clusters that have similar character – this forms the basis of defining the types at the lower end of the classification (levels 5 and 6). These can themselves be grouped into higher types with similar character (level 4), thus forming the basis for the bottom-up approach to development of the classification based on real field sample data.
 
The two approaches have been merged together into a single hierarchy, thus catering for broad-scale application in management and mapping and fine-scale application for detailed survey, monitoring and scientific study. The levels can be differentiated in relation to their degree of biological distinctiveness, to the ability to discriminate types by various methods of remote and in situ sampling, to the ease of recognition by workers with differing skill levels and to the end use of the classification for conservation management at various scales.
 
Six levels in the hierarchy have thus been developed, equating directly to the levels in the EUNIS classification:
 
Level 1: Environment (marine) – A single category is defined within EUNIS to distinguish the marine environment from terrestrial and freshwater habitats.
 
Level 2: Broad habitats - These are extremely broad divisions of national and international application for which EC Habitats Directive Annex I habitats (e.g. reefs, mudflats and sandflats not covered by seawater at low tide) are the approximate equivalent.
 
Level 3: Main habitats - These serve to provide very broad divisions of national and international application which reflect major differences in biological character. They are equivalent to the intertidal Sites of Special Scientific Interest (SSSI) selection units (for designation of shores in the UK) (Joint Nature Conservation Committee 1996) and can be used as national mapping units.
 
Level 4: Biotope complexes - These are groups of biotopes with similar overall physical and biological character. Where biotopes consistently occur together and are relatively restricted in their extent, such as rocky shores and very near-shore subtidal rocky habitats, they provide better units for mapping than the component biotopes, better units for management and for assessing sensitivity than the individual biotopes. They are relatively easy to identify, either by non-specialists or by coarser methods of survey (such as video or rapid shore surveys), thereby offering opportunities for data collection by a wide range of people and without recourse to specialist species identification skills.
 
Level 5: Biotopes - These are typically distinguished by their different dominant species or suites of conspicuous species. On rocky substrata, most should be readily recognised by workers with a basic knowledge of marine species, although quantitative sampling will be necessary in many of the sediment types. The vast majority of available biological sample data are attributable to this level (or the sub-biotope level), which is equivalent to the communities defined in terrestrial classifications such as the UK National Vegetation Classification (e.g. Rodwell ed. 1995). Intertidal and subtidal sediment biotopes may cover very extensive areas of shore or seabed.
 
Level 6: Sub-biotopes - These are typically defined on the basis of less obvious differences in species composition (e.g. less conspicuous species), minor geographical and temporal variations, more subtle variations in the habitat or disturbed and polluted variations of a natural biotope. They will often require greater expertise or survey effort to identify.
 
The primary habitat matrix (Table 3) provides an overview of levels 2 and 3 in the classification. Matrices also been created for each broad habitat, showing the relationship of biotopes and sub-biotopes to key environmental factors (click here for more). For each broad habitat, a hierarchy structure diagram showing the relationship between units at the higher and lower hierarchical levels has been created in Excel™ (click here for more). A fully expandable hierarchical list of the whole classification system, from broad habitats to sub-biotope level, is available here. To make navigation through the hierarchical classification structure easier, a standard colour scheme is used throughout this website, with a single colour for each hierarchical level.