
Grazing by domestic livestock can benefit heathland in a number of ways. It can reduce the amount of scrub that develops, as animals graze off seedling trees. Grazing can promote diversity of flora and fauna by reducing the dominance of grasses such as purple moor-grass and wavy hair-grass. Grazing can promote 'structure' in the vegetation to the great benefit of heathland wildlife. It can also benefit heathland invertebrates that make use of dung. Different grazing animals - cattle, ponies, sheep and goats - will have different effects on the vegetation. The nature of the site - its size, topography and the make-up of its vegetation - and the level of stocking are also important determinants on the effects of grazing.
http://www.surreycc.gov.uk/?a=206832