INTENSE GRAZING BY VOLES (MICROTUS-PENNSYLVANICUS) AND ITS EFFECT ON HABITAT QUALITY

Meadow voles (Microtus pennsylvanicus) were housed in fenced plots to simulate the effects of crowding on qualitative and quantitative changes of vegetation. Within one summer they reached density levels equivalent to 1000-1300 voles/ha and were removed from the plots during the fall. Biomass yields were estimated during fall and the following spring to assess the immediate effects of grazing and plant recovery before the new growth period. Significant reductions in yields of green biomass were registered in fall (15%) and in the following spring (52%) in the grazed plots. Dead matter was also reduced by intense grazing (>7%). Biomass quality was estimated from measures of protein, total phenolics. and total nonstructural carbohydrates (TNC). Protein and phenolics in green biomass samples did not vary in the fall and spring collections but TNC varied significantly between grazed and ungrazed plots during both sampling periods. Intense grazing by voles during one summer of high density imposes quantitative changes on meadows that are still noticeable the following spring while most of the nutritive constituents of vegetation are affected little by grazing. These results support the idea that voles do not induce defensive mechanisms in heavily grazed plants.