J. Vandekoppel et al., PATTERNS OF HERBIVORY ALONG A PRODUCTIVITY GRADIENT - AN EMPIRICAL AND THEORETICAL INVESTIGATION, Ecology, 77(3), 1996, pp. 736-745
According to classical exploitation theory, an increase of primary pro
ductivity should result in increased herbivore grazing pressure, thus
maintaining a low plant standing crop, However, field data obtained fr
om a salt marsh revealed a maximal grazing pressure by hares, rabbits,
and geese at intermediate levels of plant standing crop. Grazing pres
sure was relatively low in the more productive parts. We argue that th
is 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 mo
del 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 graz
ing. In systems of high productivity, the herbivore is unable to keep
plant growth in check and a dense vegetation develops. Thus, in line w
ith our field data but in contrast to classical exploitation theory, o
ur model does not predict ''top-down'' control in productive environme
nts.