Often, especially in
lowland, agricultural country, the limits to a site are clearly
defined on the map and on the ground by sharp, man-made boundaries which give a
division between ground of high scientific interest and that with low or negligible
scientific value. This is perhaps most obviously true of woodlands surrounded by
farmland, but applies to most other formations. Such boundaries, defined by hedges,
fences, walls or ditches are the obvious limits to draw for many key sites.
Sometimes, however, there are no ready-made boundaries of this kind, especially in
uplands, where areas of high scientific interest merge imperceptibly into less
important ground.
In these cases, boundaries
must usually be drawn arbitrarily though they may follow
natural features such as streams, or artificial ones such as walls, fences, and roads.
The principle followed here, though very largely subjective, is to define an area which
is large enough to include the full range of diversity shown by the formation in the
particular locality, but not so large as to give an unnecessarily extensive
representation of certain features. This principle has some analogy with the concept
of 'minimal area' in the plant community, i.e. the smallest area in which a particular
community attains its full number of constant and characteristic species.
Sometimes it happens
that an area is not of uniform quality, but the omission of the
less valuable parts would create an unsatisfactory boundary, and perhaps cause
later problems of management and protection. In such instances, areas which
appear as natural units are often best treated as such in defining key sites. For some
sites it is desirable to have a marginal or adjoining buffer zone to protect a vulnerable
ecosystem, e.g. the hydrology of a valley or basin mire is particularly affected by
changes in the catchment, and a measure of control over the latter is desirable.
Similarly, it may not be possible to protect mobile creatures, notably birds, simply by
safeguarding the place where they breed, e.g. species such as the red kite and
osprey depend for their food supply on quite a large area beyond the confines of their
nesting woods.
Where there is actual
geographical continuity between high-quality examples of
different formations, a boundary is drawn to include these within a single site. A
single geographical unit containing examples of more than one major formation is
known as a composite site. Where examples of the same formation lie close
together but are separated by different land (especially farmland) of much lower
quality, it is convenient to regard these as a single geographical and ecological unit,
termed an aggregate site. Decisions on the particular clusters of sites which should
be so treated are necessarily arbitrary. Conversely, within certain sites (mainly
upland) which form single, continuous units, some parts are much less valuable than
others, or certain vegetation types may be unnecessarily extensive. The
implementation stage of the Review should consider how best to deal with these
situations.
One of the disadvantages
of carrying out the Nature Conservation Review according
to subdivision into major formations is that geographical entities are often broken up
and artificially compartmentalised, instead of being treated as integrated ecological
units. As a result, the particular importance given by their diversity does not always
emerge clearly from the separate assessments and descriptions. This applies
especially to the New Forest, which is not simply an aggregation of important
woodlands, acidic heaths and valley mires, but gains extra status as the largest
continuous area of undeveloped land remaining in the whole of lowland England, i.e.
its total value is greater than the sum of the individual parts. Some areas which once
had this quality, e.g. Breckland and the Isle of Purbeck, have partly lost it through
recent dissection and depletion by human activities, though their importance is still
outstanding. In certain other districts, there are unusual concentrations of important
sites, often covering a variety of formations, and these should be regarded as having
special importance to nature conservation on this large scale.
