Madeiran snails: Faunal differentiation on a small island

Land molluscs have been collected in the west and south-west coastal region s of Madeira, the main island of the Madeiran archipelago. These are compar ed with material already reported on from other coastal regions, and from f orested areas inland. The inclusion of new data confirms the pattern, previ ously identified, of low local but high regional diversity. It also demonst rates more conclusively that microgeographical differentiation between site faunas is much more marked in coastal habitats than in the forests inland. At least four coastal regions are discernible in which similar sister spec ies replace each other without obvious ecological divergence. By contrast, the faunal difference between forest and coast is a product of much more pr ofound differentiation, involving different families. The presence of non-endemic species has no observable negative effect on th e endemic fauna; there is no evidence that local diversity is directly cons trained by competition. Species proliferate on islands like this because hi storic changes fragment ranges, while rare migration events allow unoccupie d patches to be invaded. It is argued that isolates differentiate to the le vel of species over periods of the order of 10(5)-10(6) years, while coloni zation occurs less frequently.