Fróðskaparrit - 01.01.1998, Side 16

22
A BIOGEOGRAPHIC ANALYSIS OF THE PLANTS IN THE FAROEISLANDS
Why are there more species
on bigger islands?
According to Begon et al. (1990), there are
at least two possible explanations to the
above island biogeographical correlations.
The first explanation is pretty obvious: the
number of (plant) species on an island re-
flects the conditions of life, the habitats of
the island. The bigger the island, the more
habitats and the more species will be found
on the island.
The second explanation has been deliv-
ered by MacArthur and Wilson (1967). It is
based on the assumption that there is a con-
tinued immigration to the island and that
this process is neutralised by the fact that
other species will constantly become ex-
tinct from the island. There will always be
an exchange of species, but the total num-
ber for the individual island will remain at
the same level.
In the case of a group of islands such as
the Faroes, the immigration of species is
probably the same for all islands, given the
fact that the distances from the individual
islands to the different sources of disper-
sion (Norway, Iceland and the British Isles)
are approximately the same. On the other
hand, the areas of the islands vary conside-
rably and species become extinct much
more often on smaller than on larger is-
lands. Consequently, the island theory pre-
dicts a correlation between the number of
species on a island and its size.
Bengtson (1982) expounds the theory
very well and offers illustrative examples
from the Faroese invertebrate fauna.
Some sets of data will support the first
explanation whereas others will support the
second one, see e.g. Williamson (1981) and
Begon et al. (1990). The latter writes that
the two explanations are in fact compli-
mentary; they are both valid. We have stu-
died two consequences of the Faroese ma-
terial which both seem to be incompatible
with the island theory.
Two consequences of the island theory
The z value of the Faroese plants is, as al-
ready pointed out, 0.20. If the Faroe Islands
were moved closer to the continent, we
would experience a larger immigration
from the continent to all islands according
to MacArthur and Wilson (1967). Hereby
the z value would drop to a value below
0.20.
The Shetlands and the Orkneys are
groups of islands which have roughly the
same areas, number and variety of habitats
and similar location in the same plant geo-
graphical region as the Faroe Islands.
Johnson and Simberloff (1974) have list-
ed area and number of plant species for all
major islands in the British group of islands
wherever they have been available. As re-
gards the Shetlands, the study comprises:
Shetland, Yell, Unst, Fetlar, Bressay, Foula
and Whalsay. Fair Isle is a small island ly-
ing between the Shetlands and the Orkney
Islands. Without Fair, the Shetlands will
have a z value of 0.20. With Fair, z will be
0.14. For the Orkneys (Orkney, Hoy, S.
Ronaldsay, Westray, Sanday, Stronsay and
N. Ronaldsay) the z value is 0.41; if Fair is
included the z value will drop to 0.24.
The material is limited, and just as Lítla
Dímun altered the picture completely as re-
gards the Faroes, we observe that the small