Interpretation of results of monitoring programmes is likely many instances to be difficult, because many of the biogenic reef biotopes under consideration (with the probable exception of most Modiolus biotopes) undergo natural fluctuations in populations which are either remarkably wide (many Mytilus, Sabellaria alveolata and probably many annual S. spinulosa reefs), so that even almost complete loss of reefs could be regarded as ‘normal’, or relatively unknown (the more apparently stable S. spinulosa reefs at the mouth of the Wash; serpulid reefs). The abundance and diversity of the associated fauna and flora will inevitably have their own sources of variation in recruitment, growth and survival superimposed upon the variations in the ‘supporting’ reef populations; in general terms one can expect richer and more diverse communities on older and more stable reefs than on younger or less stable ones, but determining acceptable limits of change will again be very difficult in most cases. In the early years the primary benefit of many surveys will be in giving information about typical levels of natural variation, assuming that there are no major human influences.

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