Sites on land (including freshwater sites and coastal sites down to low water mark⁶ ) are normally notified as SSSI or ASSI, and, where appropriate, the same boundary has been used to simplify administrative arrangements and to assist in identification of the boundary on the ground. However, SSSI/ASSI are often notified for features which are of national importance but which are not Annex I habitats or Annex II species. Consequently, SSSI/ASSI may be larger than the SACs that are contained within their boundaries. As a general principle, SAC boundaries have been drawn closely around the qualifying habitat types or the habitats of species for which the sites have been selected, taking into account the need to ensure that the site operates as a functional whole for the conservation of the habitat type(s) or species and to maintain sensible management units.

Buffer zones have generally not been included as part of SACs. Measures are provided in the UK Habitats Regulations to control, through the planning system, adverse impacts on a qualifying feature arising outside the site. Some sites straddle the land/sea divide or are entirely marine. In these situations the seaward boundaries of the site have been drawn as straight lines, to ensure ease of identification on charts and at sea.

Some SACs are in fact clusters of geographically discrete sites. This has been appropriate when qualifying interests are ecologically interdependent or were geographically contiguous before being divided by human activity, as, for example, has happened in a number of cases with heathland and woodland. In some cases, such as the North Pennine Dales Meadows, a number of relatively small SSSIs in the same geographical area have been clustered into one site. Such clusters may contain a range of habitat types. However, the argument for clustering of sites is strongest where the fragments support the same habitat types or species. Since the area of the cluster is larger than an individual fragment, it will often support more species characteristic of the habitat type, simply because of the species-area relationship. This is well-established for a variety of habitats (see, for example, Dawson 1994). In addition, a cluster is likely to span a wider range of conditions for a single habitat type than a single fragment. This will increase the total species-richness and to some degree buffer the habitat resource against the uncertain effects of climate and other changes. Where the sites in the cluster are close together and species have relatively mobile patterns of distribution over time, there will be a higher probability of maintaining species diversity, as opportunities for successful dispersal and establishment will be more frequent.

 

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⁶ Generally, Mean Low Water in England and Northern Ireland; Mean Low Water of Spring tides in Scotland. In Wales, the limit is Mean Low Water for SSSIs notified before 2002, and, for more recent notifications, the limit of Lowest Astronomical Tides, where the intertidal features extend down to LAT. There is no provision for marine SSSIs/ASSIs beyond low water mark, although boundaries sometimes extend more widely within estuaries and other enclosed waters.