Fróðskaparrit - 01.01.1987, Side 52

56
WHEN, HOW, AND WHENCE?
terms, a sweepstake; see also the discus-
sion in Diamond and Gilpin (1983) about
the origin of the Philippine avifauna, which
shows certain essential similarities to this
scenario.
During such a dispersal process island
size probably played a small role, if any.
The magnitude of this passive dispersal
would have ensured that a species sooner
or later would have reached all areas
available for colonization regardless of
their size (provided, of course, that it took
part in the dispersal process; see below).
That a successful dispersal resulted in a
waif colonization was, of course, not given.
And even when it did, persistence in such
areas was partly a result of later climatic
changes. Periods like the Small Ice Age,
for example, probably meant floral and
faunal impoverishment [see also the discus-
sion in Lindroth et al. (1973) on the diffe-
rence between dispersal and colonization].
It must be remembered that the condi-
tions outlined above do not prevail today.
They did exist for a (geologically) short
period at the end of the Weichselian (pro-
bably somewhere between 500 and 1000
years, Ruddiman et al. 1977) before the
Fennoscandian ice sheet had melted away
far from the coasts (when the meltwater
would be distributed over land instead of
running directly to the sea via fjords and
bays) and before the retreat of the polar
front northwards had enabled the North
Atlantic Drift to establish its »normal« in-
terglacial pattern.
It should also be noted that the source
areas for such a large-scale passive disper-
sal as envisaged here are not only low-lying
lands like Jaeren in Norway (Andersen
1980: Fig.l) and parts of the Hebrides, but
also fjord landscapes like the stretches
north and south of Bergen in Norway and
the mountainous region of western Scot-
land. Much of the meltwater entered the
sea in such mountainous regions for purely
topographic reasons, but the low-lying
lands (which had perhaps been colonized
earlier - for example Jaeren from the
British Isles) were probably flooded by the
meltwater, with the result that many
species were rafted to the sea.
This, however, also meant that species
with special habitat requirements (for ex-
ample those requiring deep soil layers or
luxuriant vegetation) could not have immi-
grated to the North Atlantic islands during
this period. A terminal date for this passive
colonization can be set at about 9000 BP
(Buckland 1988) but probably the process
was more rapid (see above).
After the re-establishment of the inter-
glacial ocean current pattern immigration
by this means ceased altogether (or nearly
so). Instead other possibilities opened up
for species immigrating over water (alth-
ough on a much smaller scale). The reesta-
blished North Atlantic Drift brought con-
siderable amounts of driftwood to the
Faroes and Iceland, especially from the
American continent. Certain amounts of
driftwood from the east (notably originat-
ing from the rivers Ob and Yenisej, Bjørk
1985) later started to run ashore. Drift-
wood has been (and still is) abundant in the
Faroes as indicated by, e.g., place names
like Viðareiði and the island name of Viðoy
(»viður« is Faroese for wood). It has been
reported (Bjørk 1985) that tree trunks of
Siberian origin which had run ashore at
Kirkjubøur (at the southern tip of Streym-
oy) still had soil attached to the roots. The