The impacts of domestic ruminants fall directly on the vegetation and soils but they also influence birds, mammals and invertebrates and affect landscape value and water quality. They are a function of the density of individual ruminant species, even when expressed on a livestock unit basis, their foraging behaviour, i.e. where animals graze, and their ingestive behaviour, ie what animals choose from a particular grazing site. These need to be understood if the role of livestock in habitat management is to be quantified. Impacts of grazing biodiversity at the field scale Defoliation of a single plant can influence plant species composition by reducing the ability of the plant grazed to compete with other plants. Species vary in the ability to withstand grazing depending, inter alia , upon the position of their apical meristem.
Trees are less resistant to grazing than shrubs or forbs with grasses being the most resistant.
Within each of these vegetation types, there are differences between species with, for example, annual forbs being less resistant to defoliation than perennials and tussock grasses less resistant than tillering grasses. This combination does not occur frequently in grazed plant communities and hence is one reason why rapid shifts in species in a plant community, which has been grazed for some time, do not occur. High and low grazing pressures produce few plant species whilst intermediate grazing intensities lead to maximum species diversity. Such sward heights, especially the 4 cm sward heights, are equivalent to high grazing pressures in Figure 2. In a long term study at 3 sites Marriott et al .