The ecology of Mollusca in ancient woodland. 3 Frequency of occurrence in west Cambridgeshire woods

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Ancient woods in the west Cambridgeshire area fall into three categories: 1. those which retain their management structure and a rich flora, 2. Forestry Commission plantations recently cleared and replanted with conifers and 3. elm dominated woods with a thick ground flora of grasses. Mollusc fauna is richest in woods of the first group and poorest in those of group three, hut the fauna of the recent plantations will decline seriously soon. Modern woodland is distinguished as adjacent to, or isolated from, ancient woodland.
Ancient woodland mollusc fauna falls into four groups: 1. freshwater species found in streams and pools, 2. almost ubiquitous species found in a wide variety of habitats, 3. common species generally restricted to woodland in East Anglia but more widely dispersed elsewhere in Britain and 4. relatively rare species which in East Anglia are confined to woodland. The original Atlantic mollusc fauna is likely to have been composed of elements from all four groups. Modern woodland adjacent to ancient has a richer mollusc fauna than isolated modern woods since some ancient woodland species colonize new areas of woodland relatively rapidly. Even so, only species of group four are conspicuouslv absent from isolated modern woods.