Fróðskaparrit - 01.01.1998, Page 26

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A BIOGEOGRAPHIC ANALYSIS OF THE PLANTS IN THE FAROE ISLANDS
Similarity Index
If island no 1 contains x species and island
no 2 contains y species, the islands ‘ought’
to have the areas A, resp. A2, defined by
formula 1) :
N(At) = x and N(A2) = y. The two areas
are found by numerical calculation.
The calculated, expected number of com-
mon species in the two islands is now:
v = x + y - k
in which k is the calculated number of
species in an island of an area of Aj + A2
If we compare the observed number of
common species, u, we shall see that u-v is
negative for 77 pairs of islands and positive
for 76 pairs of islands.
The sizes of u-v, however, cannot be ap-
plied as similarity index because the vari-
ance of v differs from one pair of islands to
another. As we have been unable to find
Var(v) we have applied the following equa-
tion as an approximation for every single
pair of islands:
“Var(v)” = VarL ■ Var ,
Var, + Var2
in which Var, is defined by formula 2), as
we sum up over the 294 - x species which
are not found on island no 1, and the area is
A-2.
Var, is defined correspondingly. In simple
cases “Var(v)” is controlled by direct calcu-
lation, and it seems that it is slightly small-
er than the actual variance.
As similarity index we apply:
u - v
v/“Var(v)”
Example of a result:
Skúvoy, number ofspecies x = 112, is compared to the other islands:
Name of Number of Total number of Number of Calculated ‘dispersion’
island species species y common number of
observed on species z on species (obs) common Í “Var(v)”
the island the two u = x + y- z species:
islands (obs) v = x + y- k
Index
( u - v )/ sp
Suðuroy 23S 239 111 110.5 1.3 0.4
Stóra Dímun 65 121 56 51.9 2.7 1.5
Sandoy 222 224 110 109.6 1.9 0.2
Mykines 107 142 77 79.4 2.9 - 0.8
Vágar 211 213 110 108.7 1.6 0.8
Hestur 118 142 88 85.2 2.9 1.0
Koltur 108 136 84 80.0 2.9 1.4
Nólsoy 115 160 97 96.1 2.8 0.3
Streymoy 221 222 111 109.5 1.4 1.1
Eysturoy 207 212 107 108.3 1.7 -0.8
Kalsoy 139 158 93 94.0 2.7 -0.4
Kunoy 151 173 90 97.9 2.7 -2.9
Borðoy 163 177 98 101.1 2.6 - 1.2
Viðoy 136 157 91 93.0 2.8 -0.7
Svínoy 128 150 90 89.8 2.8 0.1
Fugloy 126 145 93 88.9 2.8 1.5
Lítla Dímun 12 113 11 10.1 1.4 0.6