Fróðskaparrit - 01.01.1987, Side 53

WHEN, HOW, AND WHENCE?
57
report of Jessen and Rasmussen (1922)
mentioned above also included a find of a
piece of Picea (or Larix in a subfossil layer,
which was interpreted as originating from
Siberia. That driftwood is common and has
been of economic importance is clear; it
was included as an asset in the Taxation
report of 1873 (Anon. 1973) and for
example Kirkjubøur, mentioned above,
was taxed with 25 Rigsdaler annually for
the value of driftwood regularly running
ashore.
Just as with the drifting ice floes, the
driftwood could have brought only certain
kinds of species to the Faroes. These may
have included a number that survived for a
time but later went extinct (perhaps species
like C. micropterus mentioned above,
which is a forest-dwelling species, or wood-
boring species of various kinds).
Aerial dispersal
In addition to this short waterborne immi-
gration an aerial sweepstake has influenced
the fauna during the whole of the Holocene
(Gíslason 1981, Buckland 1988) and is still
going on. It seems probable that this aerial
dispersal mainly stems from NW Europe,
although the winds over the Faroes are
mainly westerly. However, winds from the
south, southeast and east are not uncom-
mon; according to Sestoft (in Trap 1968)
they amount to 27.6% of prevailing winds
(S 9.3%, SE 9.8%, E 8.5%). Among the
species that may have immigrated by air is
a considerable number of spiders. It is per-
haps significant that reports on the spider
founa of the Faroes from two collecting
periods 50 years apart (Braendegaard
1928, later added to in Ashmole 1979, and
Bengtson and Hauge 1979) yielded about
the same number of species, but that over-
lap of the species lists was only about 50%.
This might indicate that spiders are con-
stantly immigrating to the islands by aerial
dispersal (ballooning) and that a number of
the immigrants after a time go extinct.
Other taxa that have been reported to
disperse by air are, e.g., freshwater
sponges (Porifera); the gemmulae attach to
the feathers of birds. There is only one
species of Porifera on the Faroes, the
amphiatlantic Heteromeyenia ryderi Potts.
In addition to its American distribution it
has been found in Ireland, on the island of
Mull, in the Outer Hebrides, and in the
Faroes (Arndt 1928, Waterston 1981) (see
the discussion above of the transport along
the western coast of Scotland, including the
Hebrides, northwards by the North Atlan-
tic Drift).
Other anecdotic evidence exists. During
our investigation in the Faroes (see Bengt-
son and Hauge 1979) we found, on the is-
land of Sandoy, the sand-dune living lepi-
dopteran Photedes stigmatica Ev. This
species was found in Iceland for the first
time in 1937 (a single record); in the 1970’s
a population was found in SE Iceland
(Ólafsson and Bjornsson 1976). The near-
est known occurrence of the species is in
the Ural area in Siberia; in the Faroes it
was found in the only sand dune in the
islands.
Dispersal by birds is perhaps not uncom-
mon. Pedersen (1971), intrigued by the
occurrence of same nearctic species of Dip-
tera in Iceland, fed robins (Erithacus rube-
cula) egg-bearing females of Tipula autum-
nalis. About a third of the eggs that were
collected from the faeces were still viable.