The Effect of Voles (Microtus Agrestis) on Vegetation

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Abstract
Two localities were selected for the expts., namely, the Newcastleton State Forest, Roxburghshire, Scotland, and the Corris State Forest, Merionethshire, Wales. The trees surrounding the exptl. quadrats were 3-6 yrs. old when the work began in 1932. Selected areas in the rides between the trees were fenced round with fine mesh wire 41/2 ft. high and sunk 11/2 ft. in the earth (the cages); controls, consisting of similar areas but without the wire, were also marked out. These quadrats were placed mainly in communities of Molinia caerulea but at Newcastleton also in those of Deschampsia caespitosa and Holcus mollis. By means of a method of "pinch" sampling an analysis of the vegetation in the cages and controls was made annually from 1932 to 1939, during which period 6-monthly estimates were made of the vole abundance over the area in which the quadrats were established. The most definite results of enclosure were the decrease of angiosperms other than the dominants in many of the cages and the almost total disappearance of mosses in all the cages. This is probably due to an increase in vigor of the dominant species following the removal of vole attack, and the consequent increase in competition between the dominants and the subordinate spp. The presence of voles with their feeding and tunnelling activities tends to keep the community more open and thus to create a habitat favorable to the continued existence of a greater number of species. To assess more accurately the effect of voles on Molinia numerous measurements were made; these show that, at any rate at the time of maximum vole abundance, tillers are considerably longer in the cages than in the corresponding controls; this is due partly to actual biting off of the ends of the tillers, and partly to the weakening of the plants following general burrowing and nibbling activities. The gradual succession following enclosure and drainage tends to mask the effects caused by the voles, as also does the growth of the planted trees. The influence of the former is probably more evident at Corris, that of the latter more obvious at Newcastleton.

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