Fróðskaparrit - 01.01.1987, Page 51

Fróðskaparrit - 01.01.1987, Page 51
WHEN, HOW. AND WHENCE? 55 The process of the first immigration »It is suggested that the creatures are likely to be car- ried by currents while hiding in chinks of drift-timber, and more rarely with icebergs, thus situated they may sometimes be protected, or partially protected, for a time from contact with sea-water, and may possibly be safely carried during calm weather to great distan- ces, so that arrival of shells, still alive, on the shores of a foreign country or distant island may not be a very rare event.« Harry Wallis Kew 1893 How, then, did the immigration to the northern islands occur? Let us return to the scenario at that time. The Scandinavian and Loch Lomond ice sheets were rapidly melting. The lowering of the sea level meant that a large part of the present North Sea was dry (e.g., Coope 1979). Thus, distances across water where much shorter than today. Moreover, the short- ened course of the North Atlantic Drift, which in addition ran anti-clockwise, would have facilitated dispersal across the water (Fig. 2). If we assume that species invading the western part of the Scandinavian peninsula at least partly originated from the British Isles as suggested by Coope (1979), the obstacles that had to be overcome can not have been great (Fig. 2). The deep trench off the southern coast of Norway probably had a surface layer of fresh (or brackish) water from the melting Scandinavian ice sheet. For species capable of flight its cros- sing cannot have posed any great problem. For flightless taxa this freshwater trench was probably not an insurmountable bar- rier, although the northwards progress was in all probability slower than for the flying ones [as has been discussed by, e.g., Coope (1969, 1979)]. The migration process for flightless species probably involved a transport on flood debris on the surface of the floating ice washed out to the sea at the time of the spring floods. Such floods were probably immense in areas where the ice sheets were melting and freshwater could have floated on the denser marine water for consider- able distances. The species dispersing in this manner would then have been protec- ted from the seawater both by debris layers, ice rafts, and a layer of fresh (or brackish) water (Coope 1979). It couid be mentioned here that, e.g., certain carabid beetles are able to endure for surprisingly long times even when drifting or swimming in seawater (Thiele 1977). The short course of the then anticlockwise North Atlantic Drift would then have assisted the dispers- ing species in reaching habitable areas be- fore their transport disintegrated. Species from the British Isles could have reached the west coast of Norway in this way. The North Atlantic islands, however, were probably invaded from two directions during this period - from the west coast of Norway and from the west coast of Scot- land (including the Hebrides); in both cases rafting on ice carried by the fresh water from the melting Scandinavian and Loch Lomond ice sheets (Fig. 2). In the case of Scotland and the Hebrides the sea- borne flood debris was problably swept northwards by the North Atlantic Drift (for a different view see Buckland 1988). In this way we would have a passive dispersal, with increasing losses of species, from both western Norway and the western Scotland/Hebrides area, over Shetland, the Faroes, Iceland and on to Greenland (Fig. 2). This was, in Simpson’s (1940)
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