Fróðskaparrit - 31.12.2000, Blaðsíða 106

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DOMINANT SPECIES ABUNDANCE RELATED TO ENVIRONMENTAL
FACTORS ON ROCKY SHORES IN THE FAROEISLANDS
Fig. 1. Canonical Correspondence Analysis (CCA) for
rocky shore communities in the Faroe Islands: ordina-
tion diagram of species and environmental variables
for canonical axes 1 (horizontal) and 2 (vertical), dis-
playing 31% ofthe inertia (= weighted variance) in
species abundances and 92% ofvariance in the
species—environment -relation. The eigenvalues of
axes 1 and 2 are 0.26 and 0.02, respectively. The envi-
ronmental variable vectors are pointing in the direc-
tions of increased wave exposure, substrate category
(stones/rocks>boulders>bedrock), fjord index (in-
creasing into fjords), tidal currents and tiđe (tidal am-
plitude). The data comprise 159 sites with tidal ampli-
tude larger than 0.4 m. Species names are given infull
in Table 1.
variables, Detrended Correspondence Ana-
lysis (DCA) was used. Detrending-by-seg-
ments (Hill and Gauch, 1980) was done due
to the “arch effect” if Correspondence
Analysis (CA) was used. The first DCA
axis reflects the main trend in the data.
Species response curves, showing the
abundance of each species along this axis,
were then constructed. Abundance values
were plotted against site scores on the axis
and the response curves were fitted using a
Generalised Linear Model, assuming Gaus-
sian distributions in the species data. These
curves were compared to the response
curves obtained by Bruntse et al. (1999b)
using Expon. Nine of the sites, with tidal
amplitude 0.4 m or less, exhibited a unique
pattern that differed from the other stations
and, thus. were explored separately with
DCA. DCA was chosen as a method in or-
der to compare the results with those ob-
tained for the other sites.
The analyses were carried out with the
Canoco for Windows 4.0 Package (ter
Braak and Smilauer, 1997-1999).
Results
Effects of environmental factors at sites
with tidal amplitude larger than 0.4 m
In the Canonical Correspondence Analysis
(CCA, Table 3), the first axis explained
29% of the species variation, and each of
the consequent axes explained only 2.1, 1.0
and 0.8% of the species variation, respec-
tively (eigenvalues of the two, first axes
were 0.26 and 0.02). The focus of the inter-
pretation, therefore, will be on the first axis.
In the forward selection of variables (Table
3), wave exposure appeared to be the most
important variable, explaining 19% of the
species variance, followed by substrate and
fjord index. Each of these variables had
marginal effects explaining more than 10%
of the species data. The unique effects of
wave exposure and substrate explained 7-
8% of the species data, while the unique ef-
fect of the fjord index only explained 2%.
Each of the following variables - current,