Fróðskaparrit - 01.01.1987, Side 59

WHEN, HOW, AND WHENCE?
63
Species richness
It is immediately apparent that the Faroes
is the most species-poor area in the tempe-
rate North Atlantic. There is a decrease in
the number of species in all groups from
the Scottish mainland northwards and
particularly between Shetland and the Fa-
roes as concerns beetles and molluscs (it is
less pronounced in spiders). A number of
factors contribute to make the Faroes so
depauperate: 1) It is the most isolated area
in the North Atlantic; the distance to the
nearest mainland is 320 km (as compared
with Shetland’s 100 km). 2) The area of the
Faroes is small; it is about equal to Shet-
land. Iceland, for example, is 75 times lar-
ger. 3) It has been available for coloniza-
tion a shorter time than both the Hebrides,
Orkney, and Shetland, and it is probable
that its biota are not yet saturated. 4) The
plant cover of the islands is relatively ho-
mogeneous (in itself partly a result of the
low species number of plants, but also of
the intensive grazing over practicaliy the
whole vegetated area). Thus the structural
diversity of the plant cover is also low. 5)
Man reached the Faroes only about 1300
years ago, while for example Orkney and
Shetland were inhabited already in Neolit-
hic times, about 3000 BP. Human commu-
nications with the Faroes have also been
rather restricted for a large part of this pe-
riod as compared with e.g. the Outer He-
brides (Waterston 1981). Thus, the anthro-
pochorous species are proportionally fewer
than for these areas. 6) The climate of the
Faroes is distinctly more oceanic than for
the other areas (perhaps excepting Shet-
land), which means that continental spe-
cies immigrating to the islands may survive
less well even if they establish themselves
for a short period (one example is Betula
nana, see above).
Similarity with neighbouring areas
The majority of the Faroese species are to
be found in most of the neighbouring are-
as. However, this similarity (expressed in
Fig. 4 as the percentage of the Faroese spe-
cies found in the respective area) is higher
towards the south and towards western
Norway than between the Faroes and Ice-
land. Among the areas included Iceland
contains by far the largest numbers of high-
boreal and arctic species (including a num-
ber of Nearctic species) and exerts a dou-
ble-filter effect on the composition of the
Faroese fauna. Several species do not ex-
tend their distribution ranges northwards
beyond the Faroes, while some northern
species occur, e.g., in higher mountainous
regions in the British isles. Thus, while the-
re is a drop in the number of species betwe-
en Shetland and the Faroes there is a more
pronounced change in the taxonomic com-
position as one proceeds from the Faroes
to Iceland.
As to the most likely source areas for the
Faroese fauna, present-day faunal lists are
of limited use. The source species pools in
Fennoscandia and Scotland do not simply
represent a numerical measure from which
a certain fixed percentage of species has di-
spersed to the Faroes (or even, to the more
close-lying Orkney and the Hebrides), ac-
cording to some popular statistical formula
in current use. The source pools are made
up of assemblages of species with different
habitat requirements and different disper-
sal abilities. A statistical approach to the
numerical data is thus not feasible; the bio-