Applied Ecology/Case Studies/British Limestone Grasslands
Types of grassland
Britain’s lowland limestone grasslands share the common geochemical feature that they are composed mainly of rocks rich in calcium carbonate. Also, until recent times they were unenclosed and formed the principal sheep pastures of southern England. These grasslands, which may never have been cultivated or have escaped ploughing for hundreds of years, are often described as semi-natural grasslands. Their distribution in England rests on five main geological formations,
- Oolitic limestone,
- Carboniferous limestone,
- Magnesium limestone
- Devonian limestone.
The most important differences which influence the vegetation they carry appears to be physical rather than chemical. Limestones are harder and frequently form rocky outcrops on hills which may give rise to cliffs, often associated with screes. In some districts there are extensive areas in which landscape features produced through rocks being dissolved by running water are developed (karstic features). It has been postulated that the difference in hardness of the rocks may have been an important factor in determining the present-day flora, the softer limestone hills having been tree-covered in early post-glacial times whereas the harder rocks, especially on cliffs, gorges and similar exposed sites may have remained open throughout the period of forest maximum. This hypothesis suggests that until forest clearance by people in the Neolithic(3000-1850 B.C.) and Bronze Age (1850-550 B.C.) periods some of these sites may have served as refuges for plants which subsequently colonized the open grasslands. Lowland calcareous grasslands are derived from sedimentary deposits of chalk or other types of limestone, e.g. metamorphic mica schist and serpentine. Other base-rich substrates, such as ultrabasic igneous formations and heavy metal contaminated soil support Calaminarian grassland.
The most diverse communities, with characteristic flora including some important lower plant communities, characteristic and specialist invertebrates and birds, occur on shallow rendzina soils over calcareous bedrock. Other quality measures include the presence of short-lived disturbance patches, patches of developing and mature scrub, and areas with continuity of low-intensity grazing.
Large sites tend to occur along escarpments where modern agricultural techniques are not dominant and where soils derived from glacial deposits have not obscured the calcareous bedrock. Large sites, or smaller contiguous sites, are more likely to suppc characteristic species with large ranges and provide opportunities for recolonisation local extinctions occur.
The Lizard peninsula Cornwall's is Britain's most southerly point. Nowhere else in Cornwall can boast such a density of nationally recognised Sites of Special Scientific Interest (SSSI's) or regionally important county geology sites (formery known as RIGS). The rocks on the Lizard are totally different from the rest of Cornwall. The most extensive (20 square miles) is the serpentine which is largest outcrop.
Serpentine and gabbro produce magnesium or calcium rich soils and it is the resulting alkalinity of the soils on these parts of the Lizard that has enabled a large number of quite rare plants to thrive here, such as dropwort, salad burnet, bloody cranesbill and the rare Cornish heath which is only found on The Lizard.
Chalk is a particularly soft form of limestone, which weathers to form smooth rounded hills known as "downs" or "wolds", often dissected by flat bottomed valleys associated with prominent, rounded spurs. The rock formation was laid down in shallow seas some sixty million years ago. Chalk rarely outcrops and only forms cliffs at the coast (e.g. the "White Cliffs of Dover"). English downland is broadly divisible into four main regions:-
- the North Downs of Kent and Surrey;
- the broader South Downs of Sussex, Hampshire and the Isle of Wight;
- the Chilterns;
- the Wessex downs of Wiltshire, Dorset, Hampshire and Berkshire, in the centre of which lies Salisbury Plain.
Until the middle of the last century these downlands were devoted almost exclusively to sheep-rearing. In many places on the chalk, woodland occurs, and in some areas such as the Chilterns, there are substantial beech forests. Ungrazed chalk grassland will soon be invaded by bushes, and eventually by woodland trees. These facts are the starting point for the conservation of downland because it indicates that grassland of today is stable only because of the grazing of animals— mainly domestic sheep, and wild rabbits. It has been inferred, from the concentration of artifacts of late Neolithic culture on the Wessex downs, and, to a less extent, on the South Downs, that the original forest covering these chalk hills must have been at least partly removed by 2500 B.C.; and it may well be that in these areas a good deal of the chalk grassland turf is about 4,000 years old. But in other areas, e.g. the North Downs, there was no such widespread early culture, and the origin of the open downland must be a good deal more recent. Woodland of today many not be all that ancient; for in several places lynchets,' or old Celtic ploughing terraces, can be detected in present day mature beech woods.
The term limestone is commonly used to describe calcareous strata that are harder and older than chalk. As a guide to their distribution it may be said that in the British Isles the chalk areas occur in east and south-east England where the young rocks are exposed; next, to the north and west, are situated the older Oolitic limestones—rather soft yellowish stone so beautifully used for building in the Cotswolds, for example; and farther north and west, in a larger arc, are to be found the Carboniferous or Mountain limestones, e.g. the Mendips, Gower, the Great Orme, the Craven district of Yorkshire, and the Derbyshire Dales. Elsewhere in north and west Britain the limestones are small in extent and most of the rocks are ancient siliceous strata. In Ireland, however, there is a great deal of Carboniferous limestone; but little is free from peat covering which is maintained as an organic ground cover in that very wet climate. In general, much of the geographical variability of lowland calcareous grassland over the country as a whole may be accounted for by differences in climate.
Regional differences in biodiversity
Regional botanical differences can be generalised in terms of distinctive groupings of plants. For example in Matthew's oceanic southern biodiversity element, Helianthemum apenninum is restricted to the outcrop of Devonian limestone and to certain areas in the Mendips; Koeleria vallesiana is restricted entirely to the Mendips. Polygala calcarea an important species of the Chalk and Oolitic limestone within the oceanic southern element is more widespread but reaches its highest abundance in the western chalk grasslands and its northern limit on the Oolitic limestone in Rutland.
Twenty-six European species, most of them rare or local plants of calcareous grasslands, are included in Matthew's continental southern element, which in Britain has a markedly southern distribution with the highest concentration of species in south-east England, south of a line joining the Bristol Channel and the Wash. Ophrys fuciflora, 0. purpurea and 0. simia are restricted to the chalk in Kent while 0. sphegodes is found in Kent, Sussex, on the Isle of Wight and on the Jurassic formation in Dorset. More widespread species in this group includes Hippocrepis comosa which extends northwards to the carboniferous limestone in northern England but is only common in the chalk grasslands of southern England. Other species in this group, each with their own special distribution pattern but included in this major south-eastern floristic element are:
- Aceras anthropophorum, Ajuga chamaepitys. Anacamptis pyramidalis, Asperula cynanchica, Blackstonia perfoliata, Buxus sempervirens, Carex humilis, Cephalanthera damasonium, Daphne laureola, Himantoglossum hircinum, Spiranthes spiralis, Trifolium scabrum and Trinia glauca.
Helianthemum canum, Hornungia petraea and Linosyris vulgaris are, however, western species in Britain and found mainly on Carboniferous limestone.
The continental element includes many of the rare species characteristic of the chalk and Oolitic limestone in the south and south-east which are mostly absent from the western Carboniferous limestone. This group includes Pulsatilla vulgaris, Bunium bulbocastanum, Dianthus gratianopolitanus, Hypochaeris maculate, Ophrys insectifera, Orchis militaris, 0. ustulata, Orobanche elatior, Phleum phleoides, Phyteuma tenerum, Carex ericetorum, Senecio integrifolius and Seseli libanotis.
Cirsium acaulon, which is included in this group also occurs on the carboniferous limestone, but it is significant that at the northern limit of its range, in Derbyshire and Yorkshire, it occurs only on south to south-west facing slopes. The northern and sub-montane calcicole element in the flora is well represented below 1,000 ft. on the carboniferous limestone of Northern England, but only one species (Antennaria diolca) in this floristic element reaches the chalk, although the moss Rhytidium rugosum is abundant on the chalky boulder clay in the Breckland. Examples of northern species relatively abundant on the lowland carboniferous limestone of the Pennines, the Peak District and Morecambe Bay are: Viola lutea, Crepis mollis, Trollius europaeus, Prunus padus, Rosa villosa, Draba incana, Cirsium heterophyllum, Geranium sylvaticum, Epipactis atrorubens, Melica nutans, Saxifraga hypnoides and Asplenium viride.
Much the same factors which appear to determine the distribution of plant species are relevant to the distribution of animals. The proximity of the Continent of Europe to the eastern chalk in Kent and Sussex is undoubtedly related to the particular richness of the fauna in these counties. Relatively few species seem to be more numerous or only present in western Britain, although the insects of the Cornish peninsula, particularly associated with vegetation of the calcareous maritime serpentine rocks at the Lizard, probably include many species which only occur in this region in Britain. Species such as the Small Blue butterfly Cupido minimus are undoubtedly commoner in Western Britain than in the East.
Many animal species have distributions in Britain which are markedly southern. Some occur only along the south coast of England, others are not found north of the Thames while others again occur only up to the Midlands. It is often difficult, especially in the case of predacious species, to determine whether these are restricted to limestone formations or not. Limestone in southern England warms up quickly and shares with arenaceous and cindery substrates an abundance of thermophilous animal species which is not found so frequently on the colder clay soils.
The richness of phytophagous animals on limestone in southern Britain is frequently easily related to the occurrence of particular foodplants and the abundance of these. It seems probable that the physical characteristics of limestone rocks and soils, the occurrence of these rocks and soil abundantly in southern England, and the growth and diverse vegetation on limestones are the most important factors in determining the distribution and abundance of the animals. Chemical factors are of over-riding importance in some cases, for example the occurrence of many terrestrial Mollusca. The proximity of the sea seems to be a powerful modifying influence, with many maritime species being particularly associated with limestone cliff tops and cliff faces.
The history of grasslands in Britain whether in the long term (i.e. throughout the Pleistocene), medium term (Prehistoric) or recent (Historic) has been important in determining the composition of the fauna. Palaeoecological studies have shown how different the fauna of Britain has been in recent geological time. The creation of grasslands from the primaeval forest is reflected in the fossil faunas collected from different sites. In historic times the introduction of the Rabbit (Oryctolagus cuniculus), and the extinction of the Great Bustard (Otis tarda) and near extinction of the Large Blue_ butterfly (Maculinea anion) are examples of the greatly modifying influence directed by human activity on the fauna of calcareous grasslands.
Targets and sites
In the Biodiversity Challenge, and agenda for conservation in the UK (1994), produced by a group of voluntary conservation organizations the targets for lowland dry calcareous grassland were stated as:-
- Develop and/or maintain appropriate management of the existing resource, and protect sites from damaging activities.
- Restore adjacent areas where possible, to create a buffer zone for the main site ar a mosaic of bare soil habitats, short-grazed and tall-sward pastures and scrub of various ages.
- Establish the true extent of the remaining resource as an essential first step to achieving these targets.
The total extent of unimproved grassland habitats in the UK is estimated to be less than 0.3 million ha (UK Action Plan) with only a proportion of this figure 'supporting its characteristic biodiversity'. In 1972, Dorset had only 2,268 ha of unimproved cha grassland left compared with 28,000 ha in 1811. Best estimates suggest that the total resource of lowland unimproved calcareous grassland is now less than 20,000 ha in the UK. Priority areas for action include:
- Salisbury Plain,
- Chiltern Scarp,
- the North and South Downs,
- Cotswold Scarp,
- the Mendips,
- the Lincolnshire and Yorkshire Wolds,
- Anglesey and the Vale of Clwyd in Wales,
- Breckland (CG7b),
- the magnesian limestone grasslands in Durham (CG8),
- the Dorset Downs,
- the Isle of Wight,
- the Carboniferous limestones of Derbyshire,
- the Pembroke coast (CG1b)
The grasslands are under threat from changes in farming practice, particularly:
- abandonment of grazing on steep slopes which leads to encroachment by coarse grasses and scrub
RDB vascular plants of lowland calcareous grasslands, such as Gentianella anglica, tend to be more characteristic of the short, grazed pastures provided by low-intensity sheep grazing, mixed livestock grazing or unmanaged rabbit grazing.
Examples of threatened species
Examples of key species whose conservation is closely linked to the conservation of unimproved lowland calcareous grasslands are listed below. A suggested conservation target is given for each species.
- Buellia asterella (starry Breck lichen)
- Reduce scrub encroachment by appropriate management and reinstate rabbit-grazing. Prosecute botanists illegally collecting material. Known from only three areas in the Suffolk Breckland. Characteristic of rabbit-grazed, calcareous, sandy, lichen-dominated turf.
- Barbula glauca (glaucous beard-moss).
- Known from only one site in Wiltshire, in a dry chalk crevice. Threatened by shading, rubbish dumping and collecting, the site should be managed appropriately and collecting discouraged by prosecution.
- Flowering plants
- Althaea hirsuta (hairy mallow)
- Continue to restore appropriate management at threatened Somerset site and only Kent site. Monitor recovery of these populations and monitor status of third native population. Known from only three native sites. A plant of disturbance patches on south-facing calcareous slopes.
Gentianella anglica (early gentian) Maintain in 25 sites on chalk in southern England. A UK endemic species.
- Orchis militaris (military orchid)
- Maintain conservation management to enhance populations at only two known sites in the UK, both owned by the FC. Establish the extent of genetic variation between British and continental plants using gene or enzyme analysis. Provide suitable management at other historic sites to allow natural recolonisation.
- Decticus verrucivorus (wart-biter grasshopper)
- Survey Wiltshire chalk grasslands for further populations. Re-establish in the only former known site in the county.
- Gomphocerripus rufus (a grasshopper)
- Maintain range on limestone grassland. Hesperia comma (silver-spotted skipper) Halt decline and restore range.
- Siona lineata (black-veined moth)
- Maintain remaining populations at two chalk grassland sites in Kent. Restore to other sites in previous range.
- Burhinus oedicnemus (stone-curlew)
- Target from the RSPB /JNCC species action plan. Increase the population breeding in England to 200 pairs by the year 2000, within the present range which also includes arable and calcifugous grassland. Encourage recolonisation of the past breeding range within England. Increase the proportion of the population nesting in semi-natural grassland habitats. An important part of the population nests on chalk grassland in southern England.