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SVENSK BOTANISK TIDSKRIFT. 1921. BD: Los EG?
NOLESYAS A FACTOR IN: PLANT ECOLOGY,
BY
LARS-GUNNAR ROMELL.
In the Journal of Ecology, 1916, Farrow has described a case
of heath turning into grassland because of rabbits roding, the
injured heather plants being replaced by Leucobryum glaucum and
grasses. Quite a similar case was observed in 1915 by the zoolo-
gist, fil. lic. T. PEHRSON and the writer. A short account of the
recognized facts may be of interest as a parallel to the statements
of FARROW.
The observations refer to the little island Svartlöga, of the outer
skerries off Stockholm. It is situated at 599 34 1" N. lat., 190 5’ long.
E. from Greenwich, and has a surface of some 3 square km. It
is a low rocky island, as are all in that region, consisling of
gneiss rocks of some 10 m highest elevation, naked or with birch
and small peat mosses, and of shallow valleys with flat bottom,
where the vegetation clothing the sandy ground has the character
of meadows or bright meadow-woods consisting mostly of alder.
On the small isolated islands in the outer archipelago of Stock-
holm, such as Svartlöga, there seems to be remarkabel changes
in the frequence of certain animals, viz. the water- and field-voles
(Arvicola terrestris and agrestis) and the viper (Vipera berus). This
seems to be a consequence of the relative isolation of the islands
which makes everyone of them in some respect a little world in
itself, so that for instance the voles during some years may mul-
tiply in the absence of their worst enemy, the viper, who may
then happen to come in and increase abundantly at the cost of
the voles, and so on. On the whole, on everyone of these little
skerries, the greater deviations from the average equilibrium seem
to be far more frequent than on the continent. As a consequence,
common rules of ecological equilibrium often seem to be suspended,
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