Studies of the interplay of immigration, emigration and extinction in shaping the fauna of islands (McArthur & Wilson, 1967) have influenced the terrestrial ecologists view of the creation and maintenance of diversity. Although in the deep ocean, hydrothermal vents have been the subject of a number of biogeographic studies (Tunnicliffe, 1991), there have been few attempts to validate theories of island biogeography in the shallow marine environment. To rectify this situation, a study comparing the fauna of the mainland of Cornwall, with that of the Isles of Scilly, which lie 45 km from the mainland and were separated from it ~0·3 my BP has been undertaken.

Evidence for some comparative impoverishment of the Isles of Scilly fauna was provided by Crisp & Southward (1958) who noted that a small number of cirripedes and molluscs with planktonic larvae, living close to their geographic limits of distribution in Cornwall, were unable to bridge the gap to the islands. They suggested that although species with long-lived planktonic larvae can be widely dispersed, excessive dispersal can lead to their loss from the fauna of small islands. This is clearly demonstrated on the remote island of Rockall where both Crisp (1956) and Moore (1977) noted that only animals with direct development occur. On the Island of Lundy, four species of gastropod were estimated to be less abundant than on the mainland which lies only 18 km away (Hawkins & Hiscock, 1983). However, such studies only deal with a small number of conspicuous intertidal species, and there is a requirement to expand observations to the community level. The present study attempts to question the following hypotheses: (1) α - diversity (that of single samples) is lower on the islands than the mainland; (2) the diversity of species with planktotrophic larvae is lower on the islands than on the mainland.