Biogeography of the Islas Revillagigedo, Mexico

The Islas Revillagigedo consist of four volcanic, oceanic islands located 400 miles west of Colima, Mexico. The islands in order of increasing size are: Roca Partida, San Benedicto, Clarion, Socorro. The terrestrial fauna of the islands is depauperate and disharmonic and consists of birds, three species of reptiles, and flying and rafting arthropods. Many of the species of sea birds and shore birds are cosmopolitan or cosmotropic, while the plants, reptiles and land birds show their greatest affinity to the mainland Sonora-Sinaloa area of Mexico. Prevailing currents in the Gulf of California favour chance dispersal to the islands. The percentage of the biota which is endemic is high. The degree of endemism of birds seems to be favoured by the size of the island and the diversity of habitats, while endemism and success of establishment by reptiles seems favoured by the age of the island. This seems consistent with methods of dispersal (flying versus rafting). There is no evidence of adaptive radiation of the terrestrial vertebrates within the islands. Recent introductions of exotic species have upset the apparent stability of the fauna and flora of these islands.