Chalk grassland is found on the steep south-facing scarp slopes and the more gently graded north-facing dip slopes, where woodland is absent. The alkaline soils are thin and nutrient poor, which prevents deeper-rooted lush grasses (with a high water demand) from dominating. Each square metre of chalk downland may support up to 40 different species. This semi-natural habitat is maintained through sheep, cattle and rabbit grazing which prevents scrub encroachment. Chalk grassland to the west of the Medway Valley is dominated by upright brome and fescue, whilst grassland to the east is dominated by tor-grass. Owing to the close proximity of the North Downs to the European continent, the warm climate and the south-facing escarpment, several plant species survive on the chalk grassland which are scarce or not found elsewhere in the British Isles.