Madeiran Snails: Faunal Differentiation on a Small Island

Land molluscs have been collected in the west and south-west coastal regions of Madeira, the main island of the Madeiran archipelago. These are compared with material already reported on from other coastal regions, and from forested areas inland. The inclusion of new data confirms the pattern, previ-ously identified, of low local but high regional diversity. It also demonstrates 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 species replace each other without obvious ecological divergence. By contrast, the faunal difference between forest and coast is a product of much more profound differentiation, involving different families. The presence of non-endemic species has no observable negative effect on the endemic fauna; there is no evidence that local diversity is directly constrained by competition. Species proliferate on islands like this because historic changes fragment ranges, while rare migration events allow unoccupied patches to be invaded. It is argued that isolates differentiate to the level of species over periods of the order of 105–106 years, while colonization occurs less frequently.