Fróðskaparrit - 01.01.1987, Blaðsíða 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)