Abstract:According to classical exploitation theory, an increase of primary productivity should result in increased herbivore grazing pressure, thus maintaining a low plant standing crop. However, field data obtained from a salt marsh revealed a maximal grazing pressure by hares, rabbits, and geese at intermediate levels of plant standing crop. Grazing pressure was relatively low in the more productive parts. We argue that this pattern is due to a low foraging efficiency of these herbivores in dense vegetation. In order to investigate this hypothesis, we examine a plant-herbivore model where grazing becomes less efficient in dense vegetation, and analyze the behavior of this model along a gradient of primary productivity. In systems of intermediate productivity, the model predicts that a plant-herbivore system may have two stable states. In one state, the herbivore maintains a low standing crop. The other state is dominated by a dense vegetation unsuitable for herbivore grazing. In systems of high productivity, the herbivore is unable to keep plant growth in check and a dense vegetation develops. Thus, in line with our field data but in contrast to classical exploitation theory, our model does not predict "top-down" control in productive environments.