Svensk Botanisk Tidskrift. 1'.)21. Bd. 15, H. 1. 



VOLES AS A FACTOR IN l^LANT ECOLOGY. 



BY 



LARS-GUNNAU ROMELL. 



In Ihe Journal of Ecology, 1916, Fahuow has described a case 

 of heath turning into grassland because of rabbils röding, Ihe 

 injured heather plants being replaced by Leucobryiim glancnm 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 Ihe 

 recognized facls may be of interest as a parallel to the slateinents 

 of Farrow. 



The observations refer to the liltle island Svartlöga, of the ouler 

 skerries olY Stockholm. It is situaled at 59" 34 ^' X. lat., 19" 5' loiig. 

 E. from Greenwich, and has a surface of some 3 scjuare 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 wilh birch 

 and small peat mosses, and of shallow valleys ^vilh Ilat botlom, 

 where the vegetation clothing the sandy ground has the character 

 of meadows or bright meadow-woods consisting mostly of ålder. 



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 lield-voles 

 {Arvicola terrestris and agrestis) and the viper [Vipera hems). This 

 seems to be a consequence of the relative isolation of Ihe islands 

 which makes everyone of them in some respect a liltle world in 

 itself, so that for instance the voles during some years may mul- 

 tiply in the absence of their worst* eneniy, the viper, who may 

 then happen to come in and increase abundantly al the cosl of 

 the voles, and so on. On the whole, on everyone of Ihese litlle 

 skerries, the greater deviations from the average equilibrium seem 

 to be far more frequent than on the conlinent. As a consecjuence, 

 common rules of ecological equilibrium oflen seem to be susponded. 



