The North Downs support several important habitats. The most distinctive of these is chalk grassland which is largely limited to steep escarpment and valley slopes. This semi-natural habitat is maintained through sheep, cattle and rabbit grazing which prevents scrub encroachment. It has been noted that chalk grassland to the west of the Medway Valley is dominated by Upright Brome (Bromus erectus) and Fescue (Festuca) whilst grassland to the east is dominated by Tor-grass (Brachypodium pinnatum). Because of the close proximity of the North Downs to the 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.
The nationally rare Late Spider Orchid (Ophrys fuciflora) is limited within the British Isles to chalk grassland between Folkestone and Wye. One of the two native British sites for the nationally rare Monkey Orchid (Orchis simia) is on the North Downs. The nationally scarce Man Orchid (Orchis anthropophora) is not uncommon on chalk grassland on the downs in Surrey and Kent. Other scarce plant species such as Early Gentian (Gentiana anglica ), Dwarf Milkwort (Polygala amarella) and Bedstraw Broomrape (Orobanche caryophyllacea) also occur on chalk grassland in the North Downs.
Chalk grassland also supports a rich fauna, particularly of insects. Notable butterfly species include Adonis Blue (Lysandra bellargus) and Silver-spotted Skipper (Hesperia comma) which may be found on warm, sheltered, south facing slopes. The chalk downland above the Stour Valley is the only British site for the Black-veined Moth (Siona lineata). Other notable moth species that occur on the North Downs include the Fiery Clearwing Moth (Bembecia chrysidiformis) and Straw Belle Moth (Aspitates gilvaria).
Woodland was far more extensive on the North Downs prior to human clearance 4000 to 5000 years ago. Fragments still remain particularly where the layer of clay with flints overlying the chalk has inhibited clearance. Where this is the case species such as Pedunculate Oak (Quercus robur) predominate although much woodland has been replanted with conifer and Sweet Chestnut (Castanea sativa). In Surrey, there are localised areas of chalk heath where heathland and chalkland plants grow alongside each other.
Calcaerous woodland occurs on thin soils where chalk is close to the surface. These conditions are most often found on the escarpment of the North Downs and on valley slopes but may also occur on the plateau of the dip slope. Calcaerous woodland is typically dominated by Beech (Fagus), Hornbeam (Carpinus betulus), Yew (Taxus baccata) and Ash (Fraxinus excelsior). Box Hill comprises one of the largest areas of native Box (Buxus sempervirens) woodland in England. One notable species characteristic of calcaerous woodland is the nationally scarce Lady Orchid (Orchis purpurea) which is found in more than 100 sites on the Kent stretch of the North Downs although it is confined to just two sites elsewhere in the UK.
On the summit of the cliffs between Deal, Kent and Folkestone the Early Spider Orchid (Ophrys Sphegodes) occurs in large numbers as well as the rare Oxtongue Broomrape (Orobanche artemisae campestris). Naturally exposed chalk is rare inland with the exception of the river cliffs formed by the Mole on the west face of Box Hill and at Ham Bank in Norbury Park. However, quarry lakes within chalk pits provide habitat for Great Crested Newt (Triturus cristatus). The scarce Musk Orchid (Herminium monorchis) has colonised disused chalk pits near Hollingbourne in Kent.
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