Sheep herbivory within grassland patches: The potential cost of food item discrimination

Abstract

Large mammalian herbivores are key drivers of grazed community structure especially through their selective grazing which requires animals to discriminate between food items. The impact of local plant diversity on selective grazing is however largely unknown. We investigated the ability of sheep to discriminate between plant species within a patch (feeding station level) using sown plant mosaics involving up to four grassland species: perennial ryegrass (Lolium perenne), tall fescue (Festuca arundinacea), narrow-leaf plantain (Plantago lanceolata) and yarrow (Achillea millefolium). This made it possible to vary two factors likely to decrease sheep discrimination ability: plant species richness and degree of resemblance. Patch species richness only slightly decreased sheep selectivity for ryegrass. Sheep preferred plantain and avoided yarrow relative to ryegrass, confirming that they can discriminate these species at a fine scale. Contrary to previous surveys, sheep did not select ryegrass over fescue, which suggests that species resemblance impaired within-patch discrimination. The degree of species similarity would thus interact with the local distribution of resources so that either positive or negative effects of neighbouring plants can be observed on selection for a focal plant species. This highlights the importance of improving our knowledge of how grazers discriminate plant species for predicting their susceptibility to herbivory.