Frequency-Dependent Grazing by Slugs and Grasshoppers

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Abstract
(1) Frequency-dependent grazing was investigated in the slug, Deroceras reticulatum, feeding on mixtures of Dactylis glomerata and Trifolium repens, and in the grasshopper, Omocestus viridulus, feeding on five grass species in paired combinations. (2) Deroceras reticulatum grazed disproportionately on the rarer species and showed a marked preference for Trifolium repens. (3) Omocestus viridulus grazed disproportionately on the commoner species between some pairs of grasses but not others. (4) It is speculated that these types of grazing behaviour arise because Deroceras reticulatum seeks a varied diet and Omocestus viridulus becomes conditioned onto the most abundant food. (5) The relative palatabilities of grasses to O. viridulus from species-pair trials did not conform to a consistent ranking species palatabilities. The palatability of a species appeared to be influenced by the alternative species. (6) The results are discussed in relation to invertebrate herbivory and vegetation species richness.