Food availability, food utilization patterns and levels of some nutritional factors in plants were studied in 1984-85 in an old-field community supporting low to moderate densities of meadow voles (Microtus pennsylvanicus). Food choice and preference indices were positively related to levels of proteins, and negatively related to levels of total phenolics and ADL fibers in plants. High quality resources for voles were resources that had the highest amount of proteins, and the lowest level of total phenolics and ADL fibers among available plant species. There were only two plants species among the available ones that possessed these characteristics, Festuca rubra and Vicia cracca and they represented 25 to 50% of the available biomass during summer. Vole densities of up to 64 animals/ha were therefore not limited by the availability of high quality resources. Fecal analyses performed on 267 animals in 1984 and 269 voles in 1985 showed that all voles ate high quality resources. Meadow voles appear to choose foods with high protein content and low levels of digestion inhibitors, and not on the basis of caloric content or availability. These observations contradict predictions arising from current antiherbivore hypotheses and lead us to reconsider these models in light of this new evidence.
It is thought by many (see Ims 1987 for review; Desy and Batzli 1989) that high quality food regulate population processes, territoriality and mating systems among small herbivores like meadow voles (Microtus pennsylvanicus). We thought that comparisons of nutritive components from selected plants eaten by sexually active and inactive voles, as well as between territorial and non territorial sexually active females would bring some light into these theoretical considerations. Sexually active females did have a higher diet quality over inactive ones and over active and inactive males. Nutritive components of selected species from territorial reproductive females did not vary significantly from those of the non territorial females the year of higher crowding conditions but they varied significantly the following year when population density of voles was much lower. This decline in food quality coincided with a switch in food selection. Since there were only eight plant species involved in such processes, we think that crowding condition and availability of high quality food are two factors involved concurrently in space management and territoriality among voles.
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.
Meadow voles (Microtus pennsylvanicus) were housed in fenced plots over the winter to establish whether intense grazing in the summer could affect their winter diet and thereby reduce their chances of survival. Intense grazing in the summer was simulated by removing 14% of the fall biomass. In November 1992, voles were introduced into two clipped plots and two unclipped plots. Two other plots were used to follow changes in biomass not associated with intense grazing. In January 1993, population densities and chemical constituents of fecal samples were significantly reduced as a result of experimental and winter environmental conditions. Over time, the vole feces contained increasing proportions of phenolics and neutral detergent solubles, and their diet became more diversified. These effects were more evident in the clipped plots, suggesting that the removal of fall biomass caused a rapid decline in vole numbers and the increased use of low-quality food during winter.RCsumC : Nous avons gardC des Campagnols des champs (Microtus pennsylvanicus) dans des arknes cldturCes pendant l'hiver dans le but de ddterminer si un broutement intense au cours de 17CtC peut affecter le rCgime alimentaire hivernal et par le fait m2me diminuer les chances de survie des animaux.Les effets du broutement d7CtC ont Ct C simulCs par retrait de 14% de la biomasse i l'automne. Les campagnols ont Ct C placCs en novembre 1992 dans deux arknes tondues et deux arknes non tondues. Les changements de biomasse ont Ct C suivis aussi dans deux arknes tCmoins non associies au broutement. En janvier 1993, les densitds de population et les composantes chimiques des fkces se sont avCrCes significativement rdduites i cause des conditions expdrimentales et des conditions du milieu durant l'hiver. Au cours de 17Ctude, les fkces des campagnols se sont mises i contenir des proportions progressivement plus importantes de phCnols et de matikres solubles dans les dktergents neutres, ce qui semble indiquer que le retrait de la biomasse i l'automne a entrain6 une diminution rapide du nombre de campagnols et une augmentation de la consommation d'aliments de mauvaise qualit6 durant l'hiver. [Traduit par la RCdaction]
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