THE POST-GLACIAL COLONIZATION OF THE NORTH ATLANTIC ISLANDS

The paper discusses the nature and origins of the present-day insect faunas of Greenland, Iceland, and the Faeroes in relation to those of North America and Europe. The markedly warm-adapted faunas of the Early Tertiary were modified or eliminated as the climate cooled from the Oligocene onward to the Pleistocene glaciations. The Wisconsinan glaciation peaked about 20 000 years ago, and then gave way rapidly to the arctic and cool temperate climates of the present, and the North Atlantic islands thus became habitable again but separated by wide expanses of northern seas. At most only a few strongly arctic-adapted species could have persisted through the Pleistocene and no land bridges from the continents have existed since the Early Miocene, 20 million years ago.Southern Greenland, Iceland, and the Faeroes have been colonized across sea passages from the adjacent continents, mainly by air but partly by sea, during the postglacial period (ca. 10 000 years). The faunas are all young, with no endemic species among about 2000 in all; the faunas are not arctic but distinctly subarctic, mainly of the High and Low Boreal life zones, and derived from these life zones of North America or Europe. The naturally established faunas are small or very small, less than 14% of the corresponding continental faunas, and are obviously disharmonic, with some groups absent across the North Atlantic, e.g. Culicidae, Tabanidae, Tachinidae, Papilionoidea, aculeate Hymenoptera (except Bombus sp.). This indicates a severe "sweepstakes" route. The lack of Tachinidae is noteworthy because their hosts are plentiful, and indicates dispersal by air, with adult Tachinidae, unlike adult Lepidoptera, unable to make the journey; dispersal by a land bridge would offer parasites and hosts an equal opportunity. Aerial transport is indicated also by the high proportion of migrant species (of Lepidoptera) in the island faunas, and the arrival in Surtsey (a new volcanic island) of almost 25% of the Icelandic fauna in 12 years. The Surtsey observations suggest that the Icelandic fauna is preadapted to aerial dispersal, by selection during its journey from Europe.The fauna of southern Greenland is derived partly from boreal America and partly from boreal Europe. The North American moiety becomes vestigial in Iceland and the Faeroes and does not reach Europe. Iceland and the Faeroes have been populated from northwestern Europe, especially Britain and Scandinavia. A few species extend to southern Greenland and thence, or even directly, reach North America, and have thus completed a post-glacial traverse of the North Atlantic.The fauna of North Greenland differs fundamentally from all the above. It is a high arctic fauna, nearly identical with the high arctic fauna in Canada, and thus complete, not disharmonie, though very small by virtue of its high arctic nature. It has encountered no "sweepstakes" dispersal. North Greenland is separated from High Arctic Canada only by a narrow channel which permits winter dispersal by wind across unbroken sea ice. Biologically, North Greenland is part of the North American High Arctic, and although certain species (e.g. mosquitoes and butterflies) may extend somewhat into southern Greenland, it has not contributed to the basic faunas of the North Atlantic islands.Among other problems, the extreme variability in wing pattern of many Lepidoptera in Iceland, the Faeroes, and Shetland is also commented on.