Fróðskaparrit - 01.01.1992, Side 52

56
THE EXOTIC LAND PLANARIAN .. .
Evidently, Faroese potato fields provide a
special kind of habitat which allows the
build-up of populations of these soil inver-
tebrates.
Potato cultivation in the Faroe Islands
incorporates a special technique (“reima-
velta”), first introduced in 1925, which
allows farmers to grow potatoes even in
infields with a relatively thin layer of soil
(Joensen, 1980, 1987). The technique
involves cutting the grass sward into alter-
nate narrow and wide strips; the narrow
strip of turf is left in place and seed pota-
toes plus a little fertiliser are placed on top,
whereas the wide strip of turf is cut free in
conveniently sized pieces (approx. 50 x 30
cm) and folded over the adjacent narrow
strip so covering the potatoes (Fig. 1). After
potato harvest, the turves are retumed to
their former position.
The reimavelta technique thus creates a
particularly favourable habitat for earth-
worms since a rich food supply (decom-
posing grass material and associated micro-
organisms) and moist shelter exist under
the turf mosaic. Such conditions promote
the build-up of an earthworm population
with a high density, biomass, and species
diversity. This, in tum, makes the potato
field an attractive habitat (virtually a
“larder”) for an earthworm predator such as
A. triangulata which has been accidentally
introduced into the area, with a rapid
growth of the predator population to fol-
low. Thus, it is evident that in situations
where a food supply is readily available,
the introduction of A. triangulata can have
a dramatic impact on the earthworm
population, and consequently on soil stmc-
ture, drainage and fertility.
The situation in the Kvívík potato fields
highlights important aspects regarding the
A. triangulata/earthworm interaction in
cultivated infields in the Faroes; (a) once
introduced to an area, the land planarian
population can build up to high densities
within a year; (b) a high planarian density
brings about the decline and eventual dis-
appearance of the local earthworm popula-
tion; (c) a diminishing food supply pro-
motes colonisation of adjacent areas where
food is more abundant; (d) depletion of the
food supply in the original area eventually
brings about a collapse of the planarian
population; (e) the area previously infested
with the planarian requires several years for
the earthworm population to re-establish,
assuming that colonisation of earthworms
from adjacent areas is possible.
The question remains as to how A. trian-
gulata came to be in the potato fields in
Kvívík in the first place. The farmer whose
field was infested in 1990 claimed that his
seed potatoes were his own produce,
though some neighbouring farmers
obtained their seed potatoes from outside
the village, for example from Scotland and
Denmark. Introduction of the land planari-
an via potatoes from these countries, how-
ever, seems less likely as the flatworm is
rarely found in agricultural fields in
Scotland (e.g. in a survey of randomly
selected farms, only 1 in 351 farms sam-
pled so far had the planarian present;
BRISC, 1992) and it has not been recorded
from Danish farmland. Introduction via
garden plant material is more likely and,