When, How, and Whence?

Abstract

An attempt is made to distinguish between different phases in the immigration history of terrestrial invertebrates to the Faroe Islands and to estimate their relative importance. It is shown that a phase of large-scale immigration over the sea occurred some time between 10000 BP and 9000 BP, and that immigration by this means later became less important. Immigration by driftwood has probably occurred now and then; it probably started earlier from the American continent than from the northern parts of Eurasia. Aerial immigration has been important continuously since the islands became available for invasion; this process has increased in importance over time because of increased species richness in the main source areas in NW Europe. Immigration by agency of Man has taken place only during the last 1100 or so years; source areas have been western Norway, Denmark, and the northern part of the British Isles. Chance immigration of species with special habitat requirements has occurred during the human occupancy of the islands; examples are e.g. species that require deep soil layers, and forest-dwelling species in the recently established forest plantations.
 The place of the Faroes in the North Atlantic biogeographic context is shown by a comparison between the occurrence offour groups (Coleoptera, Araneae, Lumbricidae, and Gastropoda) in the Faroes and the surrounding areas. Differences inspecies numbers between the areas are small for anthropochorous groups (Lumbricidae, Gastropoda); they are considerablygreater for groups where the main immigration has been by other means than by Man (Coleoptera, Araneae). For thesegroups isolation, island area, and plant cover are important, in addition to the respective species' dispersal ability.