Functional Response of a Small Mammalian Herbivore: The Disc Equation Revisited

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Abstract
(1) A laboratory study of bank voles feeding on willow shoots during the winter showed a decelerating (Type II) functional response of the voles to increasing shoot density. (2) The ''disc'' equation showed the best fit to the data with respect to the number of shoots attacked. (3) The disc equation and the ''random parasitoid'' equation showed equally good fit to the data with respect to the biomass consumption. (4) In no case could the ''random predator'' equation produce realistic estimates of the handling time. It is argued that the predator mode of foraging is an inappropriate way of modelling many herbivore foraging patterns. (5) The parasitoid mode of foraging by vertebrate herbivores and its implications for plant-herbivore dynamics is briefly discussed. In particular, the possible effects on plant mortality rates should be considered.