The ACE surveys will detect gross changes in the abundance of the target species. The ad hoc assumption that a change of more than one abundance grade indicates ‘real’ change is the only guideline available. It must be suspected that such a change indicates a non-natural level of variation, though for most species there is little data on which to base such an assumption. Adopting a precautionary approach, such a change should be regarded as an index that management measures should be triggered or modified - an element of Compliance Monitoring.

The fixed quadrat monitoring will detect quite small changes in abundance within the quadrats - changes of 20% in abundance or percentage cover should be reliably detected (Hitchcock, 1988b). However, with current knowledge the interpretation of such changes is difficult: there is not sufficient knowledge regarding natural variation in CFT communities, and there will be uncertainty regarding the applicability of quadrat changes to the biotope as a whole. The long-tern cyclical changes described in section III.C.1 indicate the problem posed by the scale of natural variation. At this stage fixed quadrat monitoring must be considered primarily as a data gathering exercise, but a very necessary one for the formulation of improved future management policies. Large changes in species abundance within quadrats must be considered as warning signs, but it is not possible at this stage to propose compliance limits.

A key feature to look for in monitoring results is an increase in r-selected opportunistic species, which usually indicates that a perturbation (either natural or anthropogenic) has taken place. Increased mortality of the normal long-term dominants in a biotope creates free space which is rapidly occupied by the quickly colonising opportunists. Species of this type include the tube worm Pomatoceros triqueter, the mussel Mytilus edulis, and annual ascidians such as Ciona and Ascidiella.

There will be benefits from conducting monitoring to common protocols in the different SACs. When species or communities are found to vary synchronously in a number of separate sites the interpretation must that that some diffuse natural or anthropogenic factor (more probably the former) is responsible, rather that a purely local phenomenon which is more likely to be anthropogenic. Examples of synchronous changes at a number of sites, attributed to climatic factors, were discussed in section III.C.1 for both Britain (Fowler & Pilley, 1992) and Scandinavia (Lundälv, 1985).

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