LAND SNAILS OF EASTERN MADEIRA - SPECIATION, PERSISTENCE AND COLONIZATION

The Madeiran archipelago has an exceptionally rich land snail fauna (over 250 taxa), consisting mostly of endemic species with affinities with pre-Pleistocene Europe. Two surveys have been done to examine the distribution of species on the main island of the group, Madeira. The first shows that there is a faunal discontinuity between the eastern peninsula and the rest of the island. The second analyses the relation of the peninsula to the rest of Madeira. The peninsula has several distinctive species of its own. It also includes species usually from damp woodland, which appear to have a relict distribution there, and taxa nearly confined to it on Madeira but abundant on the Deserta islands and Porto Santo. Examination of a fossil sequence shows that damper conditions have occurred on the peninsula in the past. Variation in Discula polymorpha, Heterostoma, Steenbergia spp. and Amphorella spp. may be interpreted as showing local adaptations that perhaps indicate parapatric evolution. The general pattern of distribution and the geological history of the islands suggest, however, that the present distributions result mostly from allopatric speciation in a geologically unstable region. As a whole, the archipelago is very species rich, but individual samples show low diversity. The possible contribution of interspecies competition to this pattern is considered.