Grazing by sheep Ovis aries reduces island populations of water voles Arvicola amphibius

Abstract

The population of water voles Arvicola amphibius was surveyed on 21 islands in the Solvær archipelago, northern Norway, in August 2012; 11 islands with semi-wild domestic sheep Ovis aries and 10 islands without sheep. Signs from water voles are very easy to detect and were used as a measure of the population (on a scale 0-10), and the numbers of sheep were counted. The ranking of signs on islands with and without sheep was compared, and a significant difference was found. Islands with sheep had, with one exception, only very small and fragmented populations of water voles, the one exception being a fairly large Carex swamp that was not grazed by the sheep and where a moderate-sized population of voles was found. Islands without sheep had much larger populations of water voles, giving a ranking about four times higher. One reason for the devastating effect of sheep on water voles is probably the fact that the sheep are living year-round on these islands with no supplemental food.