Fróðskaparrit - 01.01.1992, Side 53

THE EXOTIC LAND PLANARIAN .. .
57
indeed, a nearby garden was found to be
infested.
Once the land planarian had arrived in
Kvívík, however, its spread could have
occurred by both active migration (crawl-
ing) and passive migration (e.g. with
exchanged crop produce and garden plants,
with run-off water trickling down the slop-
ing infields, and with the hooves of live-
stock wandering across infields after the
growing season). Its obvious colonisation
success in Kvívík is undoubtedly related to
the special conditions existing in the potato
fields whereby a concentrated supply of
earthworms occurs beneath the grass
turves.
In contrast to cultivated areas such as
potato fields, less fertile land tends to have
only sparse earthworm populations. In the
Faroe Islands, for example, outfields have
poorer soils and a lower density of earth-
worms than the enriched infields (Enckell
and Rundgren, 1983). These areas probably
represent sub-optimal habitats for A. trian-
gulata but may well be able to sustain low
density populations of the land planarian.
Concluding remarks
In Scotland and Northem Ireland, A. trian-
gulata is found most commonly in gardens
and other cultivated areas. Similarly, most
records of the land planarian in the Faroe
Islands come from these types of habitat.
For all three countries, although wide-
spread, the species is by no means ubiqui-
tous.
Cultivated areas in the Faroes, such as
infields and gardens, are often charac-
terised by a relatively rich earthworm fauna
plus the input of foreign plant material.
These areas are therefore considered to be
primary sites for the land planarian’s intro-
duction, population growth and subsequent
colonisation of new areas by both active
and passive migration. However, as habitat
suitability is related to food availability, the
land planarian population can only be
maintained at these sites as long as an ade-
quate supply of food remains, though sur-
vival without feeding maybe possible for
some months. A lowering of habitat
suitability due to food depletion appears to
induce emigration and thereby colonisation
of surrounding areas.
Less fertile land, such as outfields, may
prove to be important secondary sites in
relation to the land planarian’s survival, as
these areas may be able to support low den-
sity reservoir populations which can
recolonise adjacent infields and gardens
once earthworm populations begin to re-es-
tablish after having been depleted earlier on
in the predator/prey interaction.
Certain habitats in the Faroes, such as
potato fields, are remarkable in having very
high densities of the land planarian and a
concomitant lack of earthworms. As
infields are important for the cultivation of
crops, and as earthworms have a beneficial
effect on soil structure and fertility, we con-
sider that the potential threat to the earth-
worm fauna in such habitats should be
taken seriously.
All signs indicate that the distribution of
A. triangulata is expanding, and there is
growing concem that the land planarian
may eventually spread throughout northem