Fróðskaparrit - 01.01.1987, Side 60

64
WHEN, HOW, AND WHENCE?
logical properties of the individual species
must form the background for an estimate
whether the occurrence of these species on
distant islands is probable or improbable.
This is, of course, not true only for these
four invertebrate groups; biological com-
mon sense argues that such basic data must
be taken into account when dealing with
any animal (or for that matter plant)
group.
Differences between groups
As discussed above, different species (in
some cases higher taxa) probably experien-
ced peaks during different times in their co-
lonization of the Faroes, depending upon a
number of factors. This ought to be reflect-
ed (at least to some extent) in their present
distribution in the North Atlantic area. The
four groups used here to illustrate the pre-
sent pattern (Fig. 4) have different disper-
sal abilities. The spiders, for example, inc-
lude a large proportion of species which re-
adily disperse by air (ballooning, and in the
case of small-sized species also drifting by
currents). (For differences between fami-
lies of spiders in this respect, see Ashmole
1979: Tab. 6.) This is true also for beetles,
where many species are capable of disper-
sal by flight, though a number of them are
also readily transported over the sea (sali-
nity tolerant) or with man (anthropocho-
rous). In contrast, earthworms and terre-
strial gastropods are not able to cross water
without aid. It is striking, therefore, that
the last two groups are qualitatively very
alike on the island groups compared. In
fact, the few lacking records of Faroese
snails and slugs on the other island groups
in the area only refer to Deroceras agreste
not having been recorded from Shetland
and Orkney (and three small species of
snails not recorded in Iceland). In the case
of earthworms the discrepancies between
the island groups can be explained by the
find of one species in the Faroes (Allolo-
bophora similis Friend, previously only
found in Kew), and a few other species not
recorded in one or two of the other areas
(see Enckell and Rundgren 1983). In sum-
mary, then, the groups depending on Man
for their dispersal show a more homoge-
neous distribution pattern in this area than
groups with better dispersal abilities (Enc-
kell et al. 1986).
The general high degree of similarity in
faunal composition between the Faroes
and the surrounding areas permits various
interpretations as to the source areas of the
different species. A purely taxonomic com-
parison leads nowhere; it is possible, how-
ever, that comparisons of genetic variation
in species occurring in the Faroes and in
other areas could give some clues to the
origin of these species.
Last, it should be pointed out that a stat-
ic or equilibrium state has probably not
been reached as concerns the invertebrate
fauna of the Faroes. Certain species have
no doubt reached the islands only recently
and have still not colonized all the sites (or
islands) where they would be able to exist.
It is not known how rapid the colonization
of an island group is, once a species has ob-
tained a beach-head. In view of the fact
that the inner colonization often takes
place from a source of individuals which is
rather scarce (the population of another is-
land in the island group) it seems intuitive-
ly true that invasion of an island group may
initially be rapid (large-scale invasion from