Contrasting patterns of variation in urban populations of Cepaea (Gastropoda: Pulmonata): a tale of two cities

The shell colour and banding polymorphisms in urban populations of Cepaea snails were studied in Sheffield (England) and Wrocław (Poland), which are two cities of similar size. Both Cepaea nemoralis and Cepaea hortensis were found in Sheffield, but only C. nemoralis was found in Wrocław. In neither city was variation with habitat apparent, nor was there evidence of large-scale geographical patterns. Spatial autocorrelation analyses revealed only vaguely defined local clusters. There was a greater degree of among-population variation in Sheffield, and more cases of extreme frequencies and low levels of polymorphism. Wrocław populations were more uniform and highly polymorphic. These differences can be related to the history of these snails in each city. Sheffield has been colonized mainly in the last 20 years, and some parts of the city with apparently suitable habitat remain unoccupied. Leptokurtic dispersal of small propagules followed by local spread appears likely. Values of FST resemble those seen in districts elsewhere with 'Area Effects'. Although there are ancient woodlands within the city, they have been colonized too recently for selection to achieve a new balance. Wrocław has held Cepaea populations for at least 100 years, and they are denser and more continuous. Shaded habitats are recent and temporary; the overall pattern of variation matches that seen in open habitats in districts elsewhere in which variation with habitat is apparent, and values of FST also correspond. Population history and the character of dispersal clearly affect the patterns of variation observed.