Structure of the classification
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.
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:
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.
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.
Distinguishing and defining types
To ensure consistency across the classification in how types
are defined, a working definition as to what constitutes a biotope,
enabling its distinction from closely-related types, has been
developed. The following criteria are applied:
1. The entity
can be distinguished on the basis of a consistent difference in
species composition based on:
- different dominant species, some of which (e.g. mussels and
kelps) may be structurally important; and
- the co-occurrence of several species characteristic of the
particular habitat conditions (even though some of these may occur
more widely in other combinations).
A combination of both the presence and abundance of the most
'obvious' species in a community is used. Sub-biotopes are often
defined using less conspicuous species.
2. It occurs
in a recognisably different habitat (but acknowledging that
distinct communities may develop in the same habitat through change
with time). Sub-biotopes are often defined on the basis of more
subtle habitat differences. Some highly subtle differences may be
critical in determining community structure (e.g. water
circulation/exchange patterns in sealoch basins, oxygenation levels
in the water column/sediment, sediment structure other than grain
size composition). The separate divisions of habitat factors used
in field recording are not necessarily be reflected in the end
division of types.
3. It is a
recognisable entity in the field, i.e. it is not an artefact of
data analysis.
4. The
assemblage of species recurs under similar habitat conditions in
(at least several) widely-separate geographical locations.
Associations of species confined to a small geographical area are
considered unlikely to represent a recurrent community (unless the
habitat is considered unique), but should rather be treated as a
variation of a more widely occurring type.
5. As a
working guide the biotope extends over an area at least 5 m x
5 m, but can also cover many square kilometres, such as for
extensive offshore sediment plains. For minor habitats, such as
rockpools and overhangs on the shore, this 'minimum size' can be
split into several discrete patches at a site. Small features, such
as crevices in rock or the biota on kelp stipes, are described as
features of the main biotope rather than biotopes in their own
right. Some entities, by virtue of their extent around the coast,
may warrant description despite showing only minor differences in
species composition; such types are often treated as
sub-biotopes.
6. It is a
single entity in the field, although there may be some spatial
variation or patchiness from one square metre to the next.
Therefore each area of shore or seabed should correlate to only one
biotope defined in classification (a 1:1 relationship of field
units to classification units). Whenever possible, the surface
species characteristics of sediment habitats (their epibiota) are
described in association with the sediment infauna as a single
entity, rather than treated as separate communities. Note however
that the nature of available data has severely restricted the clear
association of these two aspects in the classification as they are
typically derived from differing survey techniques. Thus in the
present classification there remain units defined primarily on the
basis of their epibiota or their infauna but which, given further
research, will be shown to be the same biotope. Epibiota-derived
biotopes may also 'overlay' a number of infaunal biotopes, which
are differentiated by more subtle environmental differences, and
thus need to be referred to a higher unit in the
classification.
The following considerations are also taken into account in
deciding whether to establish a biotope:
- There is a need to recognise that it is commonplace to have no
distinct boundary between two different 'types', but a gradual
transition, such that distinction of types is somewhat arbitrary at
particular reference points or nodes along a continuum.
Additionally, some communities may be largely transitional (in a
temporal sense) in nature and whilst recognisable in the field
represent a stage between two or more 'stable' biotopes. In some
areas, e.g. due to periodic disturbance, a community may be held in
a transitional or sub-climactic state for prolonged periods and
certain habitats may be so variable that the position of a biotope
along a gradient cannot be accurately defined. These factors are of
critical importance when assessing typicality of a site to a
particular type or its quality or conservation importance.
- Where different associations are shown to occur within the same
habitat, they may be spatial or temporal mosaics caused by factors
such as grazing, disturbance or chance recruitment. These should be
linked together in the classification as, for conservation
purposes, it is important to manage or protect the habitat in which
several communities may occur over time.
- To produce a practicable working classification it has been
necessary at times to be general rather than specific in splitting
different types, so that an excessively and unnecessarily complex
classification is not developed (bearing in mind the end units that
are necessary for practical use).
- Separation of communities can be related to conservation value
- does the type add variety (of habitat or species) to a particular
stretch of coast. This relates to natural habitats and excludes
artificial, polluted or disturbed habitats which should not be
considered of high conservation value although they may support
distinct communities.