Fróðskaparrit - 01.01.2008, Síða 180

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DISTRIBUTION AND THE IMPACT OF OUTFIELD DRAINAGE ON CARABIDS
(COLEOPTERA, CARABIDAE) IN NORTH WESTERN EYSTUROY, FAROE ISLANDS
tudes in the ND area, N. rufescens domina-
ting the more stony areas while N. salina is
dominating areas with less gravel and stones
and more grass, which is in good accordance
with results from Danielsen and Hansen
(2000), Sadler and Dugmore (1995) and de-
scribtions by Lindroth (1986).
The reason for Nebria salina being sig-
nificantly more abundant at the higher alti-
tudes in UD area - even though the lower
areas are suitable - might be competition
from Calatbus fuscipes at the lower altitudes.
These two species are quite similar in size Ca-
iathus fuscipes being a bit larger (10-14.4
mm) than Nebria salina (10-13.5 mm) (Lin-
droth, 1985; 1986). General principles of co-
existence predict that species that are equal
in size do not share the same habitat if they
use the same resourses. Bengtson (1982)
working with habitat utilisation and niche
breadths found that there was very little
overlap between Nebria salina and Calathus
fuscipes. This, together with our results point
to C.fuscipes excluding N. salina from areas
where C.fuscipes dominates. An explanation
for Nebria rufescens being more abundant at
the higer altitudes in the UD area can be re-
lated to the riversides and banks having
more stones and gravel in the higher altitude
areas, this is one of Nebria rufescens' pref-
ered habitats (Lindroth, 1985) and also cor-
responds well with results from Danielsen
and Hansen (2000).
The upper altitude areas in the OD area
contain more heather, while grasses become
increasingly more dominant in the lower al-
titudes, which could make the lover altitudes
better suitable to Nebria salina and Nebria
rufescens, compared to the upper altitudes
(Lindroth, 1985). The OD area shows sign of
drying up (Fosaa et al., 2008); therefore dif-
ferences in distribution according to alti-
tudes that might have been are cancelled be-
cause the upper heather influenced areas
have become drier (Fosaa et al., 2008), which
makes them better suited to Nebria salina
and Nebria rufescens (Lindroth, 1985). Also
one of the rivers that pit-fall traps were
placed along in this area did not have so
much heather at the upper altitudes com-
pared to the lower altitudes, which mini-
mizes differences in habitat structure.
Hansen (2006) also found more Trechus
obtusus at higher altitudes; the present find-
ing of higher abundance of T. obtusus at the
higher altitudes in the UD area might be re-
lated to the fact that the lower elevations in
the UD area are a lot steeper than the higher
altitudes, therefore the lower elevation
might be drier as a whole. Also the lower al-
titudes in UD area appeared to be heavily
grazed because of the very short vegetation
and high presence of sheepdroppings; and
therefore minimizing shelter in the vegeta-
tion. All this reduces the favourable condi-
tions to Trechus obtusus at the lower alti-
tudes (Lindroth, 1985).
The association between vegetation and
carabidae in the 3 areas is summarized in
Table 5.
Typically Nebria salina, Nebria rufescens,
Patrobus septentrionis, Patrobus atrorufus
and Trechus obtusus were among the domi-
nating species. Carabus problematicus was
only among the dominating in the OD area
and in the lower altitudes in the UD area and
Calathus fuscipes dominated in the lower al-
titudes in the OD and UD area (Tables 2, 3
and 4). The higher abundance of Carabus
problematicus in the OD area is related to