Fróðskaparrit - 31.12.2000, Blaðsíða 107

TITTLEIKIN AV VANLIGASTU FØROYSKU DýRA- OG TARASLØGUNUM
A KLETTASTROND IMUN TIL YMISK UMHVØRVISVIðURSKIFTI
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slope, tidal amplitude, aspect, and local
protection - was able to explain 1-2% of
the species data, if selected as the only fac-
tor (i.e. marginal effects). Of these, only
current and tidal amplitude had significant
unique effects, and were selected in the for-
ward selection. Slope was entered both as a
continuous (shown) and a categorical vari-
able (i.e. as three variables, not shown),
neither of which were significant in the for-
ward selection. The weighted correlation
between the variables and the fírst canoni-
cal axis (Table 4) confirmed this ranking of
the variables in importance. Wave expo-
sure, substrate, and fjord index were all
highly correlated with the axis (correlation
coefficients -0.64, 0.56 and 0.53, respec-
tively). These environmental variables,
however, correlated with each other (Table
4). In particular, wave exposure and fjord
index were highly correlated (correlation
coefficient -0.47), partly due to similarities
in the definitions, which may explain the
low unique effect of the fjord index. Due to
the correlations, it may be difficult to sepa-
rate the effect of each variable.
In the ordination plot of the CCA (Fig.
I), the species were mainly dispersed along
the first axis. The species at the left in the
plot were most likely found at high wave
exposure, bedrock substrate and/or at the
open coast. These species included Fucus
distichus ssp. anceps, Aglaothamnion se-
positum, Himanthalia elongata, Polysipho-
nia stricta, Alaria esculenta, Corallina of-
ficinalis, Porphyra umbilicalis, and Masto-
Table 4. Weighted correlation matrixfor environmental variables and axes in Canonical Correspondence
Analysis (CCA) and Detrended Correspondence Analysis (DCA)for 159 rocky shore sites with tidal amplitude
larger than 0.4 m in the Faroe Islands.
Exposure 1
Substrate -0.21 i
Fjord index -0.47 0.20 i
Current 0.18 -0.16 -0.32 i
Slope 0.15 -0.15 -0.03 -0.04 i
Tide 0.13 0.03 0.15 -0.17 0.03 1
Local protection -0.03 0.11 0.09 -0.17 -0.18 0.01 1
Aspect 0.14 0.08 0.05 -0.12 0.00 0.07 0.07 1
CCA axis 1 -0.64 0.56 0.53 -0.18 - 0.14 -
CCA axis 2 0.29 0.25 0.02 -0.07 - 0.20 -
CCA axis 3 0.03 0.02 -0.20 0.37 - 0.14 -
CCA axis 4 0.04 -0.13 0.28 0.20 - -0.02 -
DCA axis 1 0.68 -0.52 -0.53 0.16 - -0.12 -
DCA axis 2 -0.28 0.24 0.13 0.01 - 0.11 -
DCA axis 3 0.16 -0.15 -0.05 0.20 - 0.03 -
DCA axis 4 -0.04 0.00 0.15 -0.32 - -0.04 -
Expo. Substr. Fjord i. Current Slope Tide Loc. pr. Aspect