Glacial survival or tabula rasa? The history of North Atlantic biota revisited

The possibility that northern refugia for arctic and boreal biota existed in geographic regions other than Beringia during the ice ages has stimulated continuous debates among botanists, zoologists, and geologists. A voluminous literature has accumulated presenting biogeographic and other evidence to propose numerous high-latitude refugia, such as nunataks protruding above the ice caps and exposed coastal shelves. Similar discussions have addressed possible “intraglacial” refugia in southern mountain regions, for example the European Alps (reviewed by Stehlik, 2002, 2003; Tribsch & Schönswetter, 2003). In this paper, we revisit the evidence proposed to support the hypothesis of “in situ glacial survival” (the “nunatak” hypothesis; originally formulated by Blytt, 1876, 1882; Warming, 1888; and Sernander, 1896) or the alternative “tabula rasa” hypothesis stating that postglacial immigration is responsible for the entire presentday biota in various North Atlantic regions. Up to the 1960s, there was virtually complete consensus among biogeographers that the occurrence of endemics and disjunct distributions in this area could not be explained without postulating in situ survival, at least during the last glaciation. In the concluding remarks for the Reykjavik Symposium on the North Atlantic Biota and their History, the Icelandic botanist Áskell Löve (1963: 391) stated that the theory of survival of plants within the glaciated areas replaces “the now merely historical tabula rasa idea”. In Scandinavia, many mountain plants were thought