Area effects in Cepaea on the Larkhill Artillery Ranges, Salisbury Plain

A survey of morph frequencies in Cepaea nemoralis and hortensis in a district of about 7½ by 7½ miles (12 by 12 km.) on the chalk downland at the northern edge of Salisbury Plain has shown area effects in both species similar to those described elsewhere on the Chalk by Cain & Currey (1963). In C. nemoralis the principal morphs involved are the pink, mid-banded and spread-banded forms. High frequencies of mid-banded are found in area effects elsewhere, but in the present district there is no fixation and the five-banded form occurs widely. As in other districts on the Chalk, there is no correspondence of morph frequencies with type of background although predation by thrushes is widespread; visual selection is either absent or ineffective. There is also no correspondence between morph frequencies or areas and obvious features of the environment. The size of the populations involved makes it improbable that random genetic processes are responsible for the area effects. Some abnormal populations have been found in which variation is great and seems to include in its range characters of both nemoralis and hortensis; these are considered to belong to C. hortensis. A description is given of the spread-banded condition as expressed in both mid-banded and five-banded shells.