Long-term stability of morph frequency and species distribution in a sand-dune colony of Cepaea

In general, the land snails Cepaea nemorolis and C. hortensis do not coexist on sand dunes, and the distributional data that are available suggest that, where their ranges overlap, C. nemoralis usually tends to exclude its congener from this type of habitat. However, populations at Seaton Sluice, Northumberland, constitute an exception to this general rule: here the two species coexist over a considerable extent of the dune system. Not only is this unusual coexistence of interest from an ecological perspective but, because of a striking correspondence between a genetic change in the C. nemoralis population and the presence of C. hortensis, it is also of evolutionary interest as a possible case of competitive selection. This colony of Cepaea has been subjected to an intensive study over 16 years, with this paper presenting the most recent data set deriving from a survey conducted in 1991-1992. The paper also considers the inferences that may be drawn from the combined data set about the stability and causality of the observed genetic and ecological patterns.