Fróðskaparrit - 01.01.1987, Síða 47

WHEN, HOW, AND WHENCE?
51
about 100 km distant from the mainland.
The degree of isolation is mirrored in the
depauperate insect fauna: only about 950
insect species are known from the islands,
while the number in the British Isles is
about 20000, in Norway at least 12000, and
in Denmark more than 13000 (Bengtson
1982).
The terrestrial invertebrates in the
Faroes have immigrated after the last gla-
ciation (see below). The immigration pro-
cess can be divided into stages, implying
that taxa with different dispersal abilities
and/or different habitat requirements have
colonized the islands during different time
periods (or at least have started to do so).
To take just one example: anthropocho-
rous species have been able to invade the
islands later than, e.g., anemochorous
species.
The Post-Glacial period
The Faroes were completely ice-covered
during the last Pleistocene glaciation (the
Weichselian). The islands were covered by
a local ice sheet (or at times isolated ice
sheets covering the single islands) - the
striae are directed from the centre of the is-
land group (Rasmussen 1982). The ice cov-
er reached far outside the present outline
of the islands and (based on geological evi-
dence) no floral or faunal refugium can
have existed (Rasmussen 1982). Nor is
there any evidence of a land-bridge con-
nection between the Faroes and the British
Isles. The geology of the Faroes is sum-
marized in Rasmussen (1982).
The polar front (or convergence) separa-
ting arctic and subarctic waters from sub-
tropical waters passed rapidly (on a geo-
logical timescale) from its position west of
Portugal northwards past the west coast of
the British Isles at about 13500 BP (Ruddi-
man et al. 1977) (Fig. 1). This caused (or
was caused by) a rapid expansion of the
currents bringing subtropical waters to the
seas of western Europe. This current, in
contrast to conditions today, was a restric-
ted, anticlockwise gyre (Jansen et al. 1983,
Buckland 1988). Coope (1979) has argued
that during this period the North Atlantic
islands and part of Scandinavia were invad-
ed by certain taxa but this does not seem
likely (Buckland 1988, see also below).
During the Younger Dryas Stadial the
polar front advanced southwards, reaching
the SW coast of Ireland at about 10200 BP
(Fig. 1), and the circulation of the ocean
current in the North Atlantic returned to
glacial conditions. The subsequent retreat
northwards of the polar front seems to
have been just as sudden (Ruddiman et al.
1977: Tab. 6), and the front reached a posi-
tion just south of Greenland by 9300 BP
(Fig. 1). The North Atlantic gyre then re-
turned to its former anticlockwise pattern
and was still at that time restricted to the
seas off northwestern Europe (Fig. 2).
Fossil finds from Iceland indicate that
the pre-Landnám fauna was a cool tempe-
rate rather than an arctic fauna (Buckland
1988). The few subfossil finds from the
Faroes indicate that similar conditions ob-
tained there; the single taxon of the 11 taxa
reported by Jessen and Rasmussen (1922)
that does not occur in the Faroes today, the
carabid beetle Calathus micropterus Duft.
is not an arctic species but a forest-dwelling
one, wich occurs both in Scotland and wes-
tern Norway today.
The oldest known flora in the Faroes is
of Preboreal age, although in composition