Chance and necessity: land-snail faunas of São Miguel, Azores, compared with those of Madeira

The land-snail faunas of 23 sites from forest, disturbed and open habitats were sampled on São Miguel, Azores archipelago. Collectively, the samples contained nearly all the known fauna of the island. Forest sites were the 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 no unique endemic species. These results are compared with similar data from Madeira. Although the two islands are of the same size, and share the same Laurisilva forest cover, their faunas differ in important aspects of diversity. While many non-endemic species are common to both islands, and occur in similar numbers, endemic species are more numerous on Madeira, even though the numbers at individual forest sites are, on average, fewer than on São 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 ease of dispersal from continental sources. Age, accidents of colonization and the range of natural habitats available must have major effects on the faunas, but present endemic faunal diversity is more a product of speciation and extinction than of immigration.