Interactions entre la végétation épigée d’une luzernière et des populations enclose ou non enclose de Microtus Arvalis Pallas

Abstract

From June 1st to October 15, 1966 the net alfalfa production and the densities and structures of two natural populations of Common Voles have been studied in Vendée, Western France. The areas compared, all located in the same area, were : (1) an enclosed alfalfa field harboring a vole population whose density was 150 voles/ha at the beginning of the experiment, and (2) a second open alfalfa field where a vole population of the same density lived. Another field (Al-4) free of voles was used as a control area. The increase in population density was greater in the enclosed quadrat than in the non-enclosed field, due to emigration impediment. In the enclosure the seasonal increase of sexually active males was much greater than in the control area, while sexually inactive males remained much scarcer. Similarly, the number of lactating females increased continuously in the enclosure, whereas their numbers dropped in early autumn in the control field. However the production of young voles remained smaller in the enclosure than in the control area throughout the summer. The causes of this reproductive failure are discussed. The trophic impact of the vole population upon the alfalfa production is small in the non-enclosed field — as shown by the comparison of the standing crop biomasses with those of the vole-free enclosures. But the voles exert a strong influence on the quality of the vegetation : when present the alfalfa stems are less numerous but thicker, and the ratio leaves/stems is smaller. The ecological efficiency of the Common vole is poor. Out of 350.000 g/ha of alfalfa eaten during 4 months within the enclosure, only 12.000 g/ha were «invested» into the vole population at the end of the experiment.

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