Introduction to the guidance manual
13. Habitat mosaics and transitions




13.1 Habitat mosaics

The SSSI selection guidelines allow habitat mosaics to be identified as interest features in their own right. The component habitats may, or may not, be individually qualifying SSSI features. In practice, with the exception of upland SSSIs, it appears that very few sites have been notified specifically for their habitat mosaics.
Where a habitat mosaic is a notified interest feature, it is expected that a conservation objective will be drawn up for the mosaic as a whole. This could include various attributes which reflect:
  1. the overall extent of the mosaic (targets should generally be set to avoid any loss of area);
  2. the diversity of component habitats of the specific mosaic (the range of semi-natural habitats should usually be at least maintained at current levels);
  3. the extent of selected component habitats, where these are felt to be of particular importance (taking into account the dynamic nature of many mosaics, and also any targets for management-controlled habitat expansion/reduction);


These attributes could all be assessed from vegetation maps or aerial photographs.
One or more components of the mosaic will often be notified interest features in their own right, and, in these circumstances, attributes should be selected and targets set using the relevant habitat guidance. Even where component habitats are not individually qualifying features, it may still be appropriate to select a restricted number of additional habitat condition attributes, over and above those listed previously. This will need to be judged on a case by case basis, balancing the desire for simplicity against the need to collect sufficient information to judge the overall condition of the mosaic. On large and diverse upland sites it would usually be pointless to undertake detailed monitoring of component habitats such as acid grassland which are of limited conservation value. But for smaller mosaics (e.g. comprising two habitats, neither of which are extensive enough to qualify individually) some measure of quality of the separate habitats may be essential to reach an informed decision regarding the condition of the mosaic feature.

13.2 Transitions between habitats
Vegetation types are not always sharply delimited from each other and different forms of vegetation may grade into one another in response to environmental gradients, and there may be quite extensive transition zones. Transitions between habitats are often of interest in their own right; their importance is explicitly acknowledged in the SSSI selection guidelines, and, in some cases, transitions contribute to the reasons for site designation. Habitat transitions and stands of intermediate character may also be important factors in the context of SAC designation.
Where transition zones are limited in extent but are of significant conservation interest, they should be incorporated within the site's conservation objectives. This can generally be done by including the transition as one of the 'indicators of local distinctiveness' (see section 15.5) chosen to reflect locally distinctive elements of the site. Targets could then be set for the location, extent and composition of the transition, as appropriate.
Transitions between habitats may be relatively extensive, and sometimes entire stands of vegetation may be intermediate in character. In such cases, it is possible that use of the generic habitat guidance would result in an inappropriate choice of attributes and targets. For example, a stand of vegetation may be transitional between lowland heathland and acid grassland. This does not necessarily lessen its conservation value, but rigid application of the monitoring guidance for either lowland heathland or acid grassland may lead to a judgement that it is in unfavourable condition when it is actually favourable. In these circumstances it is recommended that an integrated set of attributes and targets is chosen, based on both sets of guidance, and taking into consideration the salient characteristics of the vegetation. Advice from specialists will be required to ensure that appropriate targets are chosen.