It is also important to realise that successional changes may have vastly different rates in different places and that the observed zonation may not have resulted from the orderly replacement of one species, or group of species, by another with changing elevation. For example, some of the saltmarshes behind barrier islands along the north Norfolk coast may have changed very little in hundreds of years. Indeed the positions of the marshes and of major features such as channels appear to have been stable for at least 4000 years (Pye, 1992) Whilst these ancient marshes change extremely slowly, those to seaward of the shingle barriers are often highly dynamic. They are known from one study of permanent plots near Wells-on-Sea to have developed rapidly from pioneer marsh through to mid marsh in less than a decade, before being buried, at first by sandy sediments and later by mudflats with pioneer glasswort species (Pye, 1992). Clearly the high marshes above the barrier are, in successional terms, disconnected from the lower ones; the one zone is not evolving into the other.

A similar conclusion was drawn from long-term studies of permanently marked plots on a newly accreting mudflat in a part of the Wadden Sea. Changes in these plots between 1953 and 1980 showed little evidence, except locally, of zonation being generated by successional changes. Several separate sets of changes occurred depending on elevation and sediment type, so that the final zonation did not represent a complete sequence of changes from pioneer to upper marsh.

The best example in which zonation does not reflect succession is the presence of a pioneer Spartina grass marsh around much of the English and Welsh coastline. In most areas this species invaded mudflats below pre-existing saltmarsh and, because it is able to tolerate greater tidal submergence, actually added a zone on to the marsh. Although, particularly in the north west, Spartina is being replaced in a successional sequence to low marsh vegetation, over much of its distribution in the south east it arrived, spread and is now dying back without successional change to a different type of vegetation.