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BIODIVERSITY REFERENCE
 
British habitats
 
   Chalk Grassland and Downs   
 
(page 1)


British distribution: southern, south-eastern and eastern England.
World distribution: no data, but in view of the localised distribution of carbonate rocks and the unusual nature of the Chalk formation compared with carbonate rocks in general, the habitat discussed here may be a feature mostly of England, France and the Netherlands.


Chalk downland near Royston, Hertfordshire, 2003


 

THE NATURE OF CHALK AND LIMESTONE ROCKS

Limestone is a sedimentary rock composed of calcium carbonate, formed from deposits of varying degrees of purity, particularly in warm, shallow seas. Many organisms, particularly many types of microorganism, secrete calcium carbonate, CaCO3, as a by-product of metabolic activity or for formation of protective coverings, and their remains may make up a significant proportion of the resultant limestone rock. The crystalline calcium carbonate mineral is calcite, or, in an alternative crystalline form, aragonite. Some limestones also contain significant amounts of dolomite, CaMg(CO3)2, and may be termed 'dolomitic limestones'. Where the mineral dolomite predominates, the rock itself is referred to as a 'dolomite'. Limestones may contain sand particles and when the calcium carbonate is primarily a cementing material between the sand grains, this will be a calcareous sandstone, though outcrops often support species assemblages similar to those of limestones.

Limestones vary in hardness, porosity and pore-size. Those with the lowest porosity and smallest pore size are termed 'marble', a valued rock in building and sculpture. The massive limestones of the Carboniferous era, forming many limestone pavements, also have low porosity and small pore-size. Passage of water through these rocks is mostly via joints and fissures. Limestones composed substantially of oolites – tiny, spherical, concentric concretions – are softer, with greater porosity and larger pores and consequent greater permeability to water, and limestones derived from corals or consisting substantially of shell and coral fragments tend to be highly porous and with large pores.

Chalk is a soft, relatively pure limestone laid down in massive deposits in the Cretaceous era. It has high porosity, though small pore size, and hence is permeable to water, though rates of flow are slow. To the naked eye it is an amorphous material, readily crumbling and eroding but generally without distinct joints and bedding planes. In fact, while some is precipitated calcium carbonate from the warm Cretaceous seas, electron microscope studies reveal that a very substantial amount of the chalk material is composed from coccoliths, the secreted outer walls of minute planktonic algae, coccolithophores. The secreted exoskeletons of planktonic protozoans, Foramenifera, also have contributed to chalk deposits and the microfossils were at one time highly collectable as microscope preparations (and preparations from Victorian times are still sought after by specialist collectors).

Chalk sediments contain various inclusions, including occasional seams of marl, localised fossiliferous beds (especially ammonites and other molluscs) and, most conspicuously, layers or nodules of flint – the hard, silicaceous material that was worked in prehistoric times for knives, axes, arrow-heads and similar tools and weapons.

Chalk, like other limestones, is dissolved by rainwater, especially when this has been acidified by dissolved carbon dioxide or organic acids.
Calcium carbonate has low solubility in pure water, though some dissociation to calcium and carbonate ions occurs:

CaCO3  →  Ca2+ + CO32–
Carbonate ions are then hydrolysed to bicarbonate and hydroxyl ions:
CO32– + H2O  →  HCO3 + OH
When the water is acidified, i.e. hydrogen ions are present, then there is direct dissolution of the calcium carbonate to calcium and bicarbonate ions:
CaCO3 + 2H+  →  Ca2+ + 2HCO3

Soils formed over chalk are generally either brown earths, where the soil has formed under original deciduous forest cover, or rendzinas, thin soils directly over the chalk and containing fragments of bed-rock. There are high levels of free calcium and bicarbonate and a high soil pH, but there may be deficiencies in iron and other essential elements. Plants on these soils are, for the most part, calcicoles, demanding or tolerating high calcium levels (or perhaps tolerant of high levels of bicarbonate that can inhibit root respiration in other species). Since many common, highly competitive species tend to be absent on these soils, there can be a high diversity of other plant species, many generally local or rare and poor competitors.


 

DISTRIBUTION

Chalk forms many well-known landscape features, especially on the coast, including the White cliffs of Dover in Kent, the Seven Sisters, including Beachy Head, in Sussex, and the Needles at the western point of the Isle of Wight. Inland there are three principal ranges of hills in southern England, the South Downs, the North Downs and the Chilterns. In East Anglia, much of the Chalk is overlain by glacial and other drift deposits, but in Cambridgeshire it forms the Gog Magog Hills. The large expanse of Salisbury Plain in Wiltshire consists largely of chalk grassland – an area that has been protected from other developments by its use for military training. Further north, the chalk forms Flamborough Head in Yorkshire.

Since chalk, while easily eroded, is still a harder rock than many other sediments in southern England, it forms the characteristic rounded hills that have become known as "downs". The thin turf, when removed, exposes the white chalk beneath, and certain downs have ancient designs cut into their slopes, famously (or infamously) the unambiguously explicit Cerne Abbas Giant in Dorset and the White Horse at Uffington in Oxfordshire.

Chalk cliffs, the Needles, Isle of Wight, 2004
 
Ancient pasture and trackway on chalk downland at the western end of Tennyson Down, above the Needles, Isle of Wight, 2004


 

HABITATS

This page deals primarily with grassland, though it should be remembered that the chalk downs may be covered with scrub or woodland or be under cultivation.

The classic chalk downlands are pastures (grazed grasslands), having been continuously grazed by sheep for many hundreds of years, some perhaps even since Roman times when sheep were first introduced. Grazing by cattle must date even further back to Neolithic times, when much of the forest was first cleared. Consequently a landscape has been created that can be regarded as artificial but, nevertheless, one that is now vitally important for British wildlife. Short turf is essential for many species that would not be able to compete with rank vegetation or which are annuals or short-lived perennials needing germination sites for their seeds. Chalk turf is dominated, of course, by various grasses, such as Koeleria macrantha, the Crested Hair-grass.

The Rabbit (Oryctolagus cunniculus) is also an important grazing species on the downs, necessary for creating very fine, short turf, even though the rabbit itself was originally introduced to Britain. In the 1950s there was the ill-considered, deliberate introduction of the rabbit disease, myxomatosis, into Britain, in a bid to control the rabbit as a pest species. Consequent collapse of rabbit populations has had many adverse ecological effects and resulted in reduction or loss of many short turf habitats of conservation importance. On the downs, this affected, for example, two of the rarest butterflies of the chalk, the Adonis Blue (Polyommatus bellargus) (illustrated on page 2) and the Silver-spotted Skipper (Hesperia comma). Both species, already rare, suffered further major decline with the spread of myxomatosis and have also shown partial recovery as rabbit populations themselves recovered (Asher et al., 2001). The status of both species remains substantially dependent on the status of rabbit populations.

Chalk, being white or nearly so, reflects solar radiation. Consequently, south facing slopes are warmed not only by direct sunlight but also by any reflection from exposed bedrock. Many plant and invertebrate species are thermophilic, benefiting from these conditions, and they often have a southern or Mediterranean range in Europe. Chalk slopes are also freely draining, both by surface run-off and by water soaking into the bedrock, and ability to survive periodic drought conditions is a requirement for survival in this habitat.

Chalk has been much quarried, and so old and more modern chalk pits are a feature of areas where chalk outcrops or is near the surface. Where the chalk is covered by other sediments or drift, quarries may provide the only areas of chalk habitat. Far older artificial disturbances of the chalk are various ancient earthworks. Such areas can provide valuable areas of open ground and the walls of abandoned quarries provide a chalk cliff habitat. Orchid species often form strong, if sometimes transient, populations on quarry floors. However, such workings have to be very old to provide a turf habitat comparable with the ancient sheep walks.

One such ancient earthwork is the Devil's Dyke that runs for many miles through Cambridgeshire – a rich refugium for many species that no doubt characterised the now vanished chalk pastures of the surrounding area.

 
Old chalk pits with rough grass and developing scrub, on the edge of Blows Downs (eastern end of the Chilterns), above Luton, Bedfordshire, 2004. The slope shown has a population of Marbled White butterflies, though some of the rare plant species of the adjacent ancient downland appear not to have recolonised this area.
 


A somewhat different habitat is 'chalk heath' that forms very locally when a shallow layer of sand or acid drift material overlies the chalk. Heather (Calluna vulgaris) and other calcifuges, plants that normally avoid calcareous soils, occur in an anomalous mixture with calcicolous species. Some of the heaths of Breckland in East Anglia are of this character, though most chalk heath in this area has been ploughed for agriculture and remaining heathland is mostly formed over much deeper sand and shows little influence of the underlying chalk.

Where grazing has been abandoned, scrub develops and eventually forest. Beech (Fagus sylvatica) is native on the chalk of southern England and certain ancient Beech 'hangers' formed on steep slopes are of high conservation importance. Forest on the upper parts of chalk hills is often not on the chalk itself but on 'clay with flints', heterogeneous material probably derived from glacial deposits plus solution products from the chalk. Further notes on scrub and forest are provided in the next section, but detailed consideration is outside the intended scope of this webpage.

Where the chalk forms hills, it is a necessary consequence that there must also be valleys. The porous nature of chalk means that these valleys are often dry, but where valleys cut down to underlying strata, or where valleys are fed by groundwater that has passed through the chalk, 'chalk streams' can form. When unaffected by agricultural run-off and other sources of pollution, chalk streams and rivers have high floristic and faunal species diversity and may be noted for trout fishing. Many, however, are damaged by pollution, physical modification or adverse fisheries management and they are a priority habitat under the UK Biodiversity Action Plan.


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© A.J. Silverside
e-mail: alan.silverside@uws.ac.uk
Page first hosted at www-biol.paisley.ac.uk/bioref/, November 2004; transferred to lastdragon.org October 2009
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