Chance and necessity: Land-snail faunas of Sao Miguel, Azores, compared with those of Madeira

The land-snail faunas of 23 sites from forest, disturbed and open habitats were sampled oil Sao Miguel, Azores archipelago. Collectively, the samples contained nearly all the known fauna of the island. Forest sites were tire richest; while the number of non-endemic species differed little between habitats, forests held more endemic, species, many of which were widely distributed. Open habitats have 110 unique endemic species. These results arc compared with similar data from Madeira. Although the two islands arc of the same size, and share the same Laurisilva forest cover, their fialmas differ in important aspects of diversity. While many non-endemic species arc common to both islands, and occur in similar numbers, endemic species arc more numerous On Madeira, even though the numbers at individual forest sites arc, oil average, fewer than oil Sao Miguel. Many endemics are confined to open, coastal habitats. These differences are discussed in terms of the islands' histories, the range of habitats available and the case of dispersal from continental sources. Age, accidents of colonization and the range of natural habitats available must have major effects oil the faunas but present endemic faunal diversity is more a product of spcciatiou and extinction than of immigration.