Summer grazing by voles and lemmings upon subarctic snow-bed and tall herb meadow vegetation – an enclosure experiment

Ther hypothesis of the exploitation of ecosystems predicts that plant communities in productive habitats should experience a low grazing pressure, while communities in less productive habitats should be structured by intense grazing. Thus, if a high population density of rodents was to be established in these communities and they could graze without predation, the plant community in the productive habitat should be more affected than the community in the less productive habitat. This was tested by building enclosures for grey-sided voles Clethrionomys rufocanus and lemmings Lemmus lemmus in a productive tall herb meadow and a less productive snow-bed on Finnmarksvidda, northern Norway. The simulated rodent population densities varied from 150 to 750 ind. ha^-1, and the rodents were allowed to graze for 18 to 55 d. The shoot mortality of relatively abundant plant species were monitored together with plant community structure, and the above-ground biomass were harvested at the end of the experiment. The shoot mortality were generally low, the significantly grazed species were either known preferred species or species with a grazing-sensitive morphology. The community structure did not show any major changes. The aboveground biomass of woody plants was significantly lower in the enclosures on the snow-bed, but not in the tall herb meadow. The total biomass did not differ significantly. These results are somewhat ambiguous with reference to the predictions from the food chain hypothesis, and the conclusion were drawn that intense grazing by voles and lemmings during 18 to 55 d during the growing season does not seem to be sufficient to greatly influence shoot mortality and structure of the studied plant communities.