Climatically, edaphically and topographically, the southern and eastern lowlands of Britain, i.e. mainly England, is the region most suited to arable farming and human settlement, and it is this part of the country in which the original ecosystems have been most profoundly modified. Virtually all 'natural' habitat, unaffected by man, has gone and relatively little of even a semi- natural character remains. Woodland is still the major semi- natural habitat in the lowlands and its remnants from the main period of clearance were, until very recently, composed largely of native species, managed and cropped for their economic importance. Much forest land was also traditionally kept as sporting preserve, rather than for the value of its timber, and the creation of royal hunting reservations or chases has been instrumental in preserving large areas of forest land. The open park woodland associated with large country houses has also provided a particular type of wooded habitat which is largely confined to Britain. During the last few decades, a waning demand for slow-growing hardwoods, and an increasing need to manage all woodlands for an economic return, have contributed to the accelerating replacement of native tree species by alien conifers. These new woodland dominants profoundly alter the ecosystem in its subsidiary components, notably the field communities and dependent animals, and usually create a much impoverished type.

The sophistication of modern agricultural practice is leading to the steady eradication in arable areas of all habitats and higher forms of life extraneous to the crop itself, i.e. the destruction of hedges and hedgerow trees, elimination of weeds and filling-in of ponds. In some of the lowlands, large areas have been kept under permanent grassland as pasturage for domestic animals, especially cattle and sheep. While these are communities produced by man, some of the grasslands, especially on the Chalk, are long established ecosystems of considerable biological richness and value in their own right. Here, again, the last three decades have seen great inroads into the remaining area of semi-natural habitat. Many of the permanent pastures have been ploughed and converted to arable crops; even where they are retained as grassland there has often been fertilising and re-seeding, which has completely changed a botanical composition developed over centuries. Often, the only pastures to escape destruction or modification have been on ground too steep to plough, though with modern implements even steepness of slope is no longer such a limitation.

In the lowlands, areas of acidic sand and gravel have been sufficiently infertile to discourage attempts at farming, and so have tended to retain semi-natural acidic grassland, scrub and heath, notably with heather, bracken and gorse. Often this semi-  natural complex, probably derived from original woodland, is associated also with common rights or maintenance of game preserves. Major areas of this kind occur in south-east England in a discontinuous belt from East Anglia to the New Forest and Isle of Purbeck. The area of such habitats has, however, contracted greatly since 1945, for the application of modern agricultural techniques can convert these types to farmland of reasonable productivity.

Rivers and streams are well represented all over Britain. Those of the uplands tend to be the eroding type, with swift-flowing turbulent and often rocky courses, whilst those of the lowlands are mainly the 'depositing' type, slower-moving through alluvial lands built up by their sediment over the ages. Upland open waters are also predominantly oligotrophic whereas many of those in the lowlands are eutrophic.

The major lake-forming processes in Britain were those of glacial erosion, and the mountain regions of north Wales, the Lake District, Galloway and the Scottish Highlands have large numbers of lakes and tarns. In the lowlands, glacial deposition was dominant and consequently fewer natural lakes were formed. South of the area covered by the ice at its maximum extent, natural bodies of standing water are scarce indeed. Because of the local scarcity of natural standing waters in the south, artificial lakes which have achieved a degree of naturalness (in certain cases, this can be a relatively rapid process) such as some gravel pits, reservoirs, ponds and especially the mediaeval peat diggings which form the Norfolk Broads are of considerable biological interest. Open waters, particularly rivers, in lowland areas are vulnerable to forms of disturbance not encountered in other habitats. Many lakes in lowland Britain have in recent years been receiving increasing amounts of nutrients from sewage effluents and from agricultural land and these have led to a number of adverse biological changes, especially in depletion of flora. Most lowland rivers are to some extent modified by the construction of artificial barriers and channels, and the abstraction of water, which affect the rates of flow and alter the natural substrate, or by the discharge of effluents including hot water, suspended solids, pesticides and other industrial toxins, domestic sewage and detergents.

Many of these forms of modification originate at points within the catchment remote from the scientifically important tracts of river or lake, and activities such as afforestation (involving extensive ploughing and fertilising), land improvement (also involving fertilising) and mining (with discharge of waste material) in hill areas can affect upland rivers markedly. On the whole modification is less severe in upland than lowland areas, but oligotrophic waters are intrinsically more fragile than eutrophic waters, and many of the finest oligotrophic lakes have been ruined by conversion to water supply and hydro-electric reservoirs. The proposed large-scale movement of water between catchments offers a new threat to the integrity of open waters. There is also a heavy and increasing pressure on open waters for recreational use, which can cause serious disturbance to marginal and submerged plant communities as well as to wildfowl.

Permanently wet ground with a vegetation cover (peat-land) was formerly much more widespread in the lowlands, but has been greatly reduced in extent by drainage and conversion to farmland. The most notable examples are the Fenlands of Huntingdonshire and Cambridgeshire where the original expanse of swampland is now represented only by a few island remnants, maintained with difficulty in a somewhat modified state. Some ground has been difficult to drain, and locally there is deliberate maintenance of high water tables, as in water meadows. Shallow lakes and slow- flowing rivers often have a marginal fringe of swamp vegetation but, in general, there appears to be a tendency for drying out of such habitats, through falling water tables and hydroseral development, and many are progressing to carr woodland or wet meadow. The local areas of raised mire, developed on plains where the peat has been able to grow above the influence of mineral-rich water, are nearly all modified from their original condition and show varying degrees of drying out, with loss of Sphagnum cover. This kind of peat is valued as a source of moss litter and many of the deposits are being worked extensively; afforestation or reclamation for agriculture may then totally eradicate the original ecosystem.

In the north and west of Britain, the cool, cloudy and humid climate is physiologically unsuitable for many arable crop plants. Moreover, there is a prevalence of infertile, acidic soils which can be raised to a reasonable level of productivity only by considerable expense, and much of the ground is too steep, rocky or inaccessible to be cultivated. Arable farming is confined to the rather limited areas of fertile lowland on the plains adjoining the hills or in the upland valleys. The uplands themselves are used mainly as pasturage for herbivores, notably sheep, grouse and (mainly in Scotland) red deer. Cattle were formerly kept in much larger numbers than at present, and many areas once had considerable herds of goats. Pasturage for these animals was obtained by extensive clearance of the forests which formerly covered the uplands to varying altitude, depending on local and regional differences in climate, so that woodlands in the hill country are now fragmented remnants, modified by recent management and frequently by grazing of the field layer. Within the potential tree limit, British uplands are thus covered mainly by mixtures of derived grasslands and dwarf- shrub heaths (especially Calluna heath), subject to heavy grazing and repeated burning. Grassland predominates in the west and on the more basic soils, and heather moor in the east and on the more acidic soils.

While the differences between typical lowlands and typical uplands are fairly obvious, the dividing line between the two is extremely difficult to draw in general. As lowland- upland transition is recognised within most of the main formations, i.e. woodlands, grasslands and heaths, peat-lands and open waters, and in the case of the grasslands and heaths it is made the basis of a separation into two different formations, the upland one representing the distinctive range of mountain communities. There are floral and faunal differences between lowland and upland zones, but they seldom, if ever, give a clear- cut boundary and there is usually a gradual transition from one to the other. In many hill areas, the upper limits of enclosed land give a useful, practical, though man- made boundary for delineating upland sites and habitats. This criterion has the advantage of being ecologically based, for it varies altitudinally according to regional differences in climate as do the natural vegetation zones - a feature which makes it impossible to define any one altitudinal boundary between lowland and upland. Nevertheless, this land-use separation is not always valid for, in the more southerly British hills, vegetation referable in floristics to lowland grassland and heath or peatland often lies above the limits of enclosed land, and grades into more distinctly upland communities only at higher levels.

Woodlands obviously belong mainly to the lowland zone, and the upper edge of a wood is sometimes used to define the boundary of an upland site. However, apart from the fact that woodland is usually patchy on lower hill slopes, with very variable upper limits, and often absent altogether, this formation is itself often separable into distinctive upland types at higher levels and/or in northern localities. In many hill areas, grassland, heath and peatland of upland type extends to well below the actual tree limit. The potential tree limit (which varies from over 610 m to near sea-level, according to geographical position) is regarded as the boundary between 'montane' (higher) and 'submontane' (lower) zones. The lower limits of the submontane zone correspond to the boundary between lowland and upland.

The submontane grassland and heath complex covers great areas of the British uplands, but on wetter ground grades into blanket mire, an acidic peat-forming ecosystem in which the vegetation receives nutrients largely from the atmosphere. Blanket mires cover large expanses of gently contoured land, even down to near sea- level, in districts of high precipitation/ evaporation ratio, and are especially associated with the cool oceanic climate of northern and western Britain. They are variably modified by human influence and show all degrees of drying out and erosion, though some undisturbed areas remain. Though blanket mire was formerly too wet for tree growth, it has in recent years been extensively converted by draining and planting into coniferous forest, as have many areas of deforested grassland and heather moor. Large tracts of treeless upland up to 360- 490 m have thus been given a forest cover but the plantations are largely of non-native softwoods, especially Sitka spruce and lodgepole pine.

The montane zone, restricted to the higher mountains and especially extensive in Scotland, contains a range of dwarf- shrub heaths, grasslands, peatlands, moss and lichen heaths and rock communities, which as a whole probably show a closer approach to natural climax vegetation than any other ecosystem in Britain now. Even these, however, have been modified, mainly by reduction of woody species and herbs, where grazing has been heavy and long established. Recreational developments, notably skiing, hill walking and rock climbing, have in recent years become a factor of some importance locally in mountain districts, and create their own kinds of disturbance to habitat and wildlife.

Coastal land also contains a good deal of habitat in a relatively undisturbed and original state. This is especially true of sea cliffs, which occupy a good deal of the British coast, but applies also to many sand-dune systems and shingle beaches. Stable dunes and machair have been long exploited as grazing or even arable land, and the higher, least sea-swept areas of salt marsh are used as pasturage, and former saltings behind sea walls are now being extensively converted to arable land. Coastlands are mostly unstable habitats in which development of climax woodland vegetation is limited or prevented and they are often subject to erosion and redistribution of water- borne material. The distribution of the different types of coastal habitat is irregular and determined by a combination of geology and geomorpholo-gical processes, but some, such as sand dune and salt marsh, are always limited in extent, and machair is confined to the windy coasts of the western Highlands and islands. With the exception of sea cliffs, much coastland is subject to considerable human pressure, whether for recreation or agricultural, urban and industrial development. If recent proposals for estuarine barrages and reservoirs come to fruition, they will affect salt marshes and offshore sand and mud flats particularly. Estuaries are especially under threat from urban- industrial development, e.g. deep-water ports and installations, and industrial complexes such as those associated with the exploitation of the North Sea oilfields. Coastal lowlands are also favoured sites for nuclear power stations, aluminium smelters and steel mills.

Many of the habitats and communities mentioned have been considerably modified by human influence, and in this sense are no longer natural. Some, such as the permanent grasslands of the lowlands, have been created by man from completely different types, yet they are sufficiently similar to naturally occurring communities in other places to be regarded as semi- natural. The Norfolk Broads were almost completely artificial in origin, yet developed an almost completely natural character, and have become one of the most important wetland complexes in the country. In addition, the diversity of human activities has created numerous habitats and communities which, though often populated by naturally occurring species, have such artificial features that we cannot regard them as even semi- natural. This range of artificial ecosystems includes man-made habitats such as arable farmland, roadside and railway verges, ponds, derelict land, hedges and walls. These are in the aggregate of considerable importance to nature conservation, but are mostly too highly fragmented and dispersed to be dealt with in terms of the key area concept. The range of variation is nevertheless described, as an indication of the nature conservation interest, but specific sites of importance are not mentioned, except where they occur as a bonus interest in key sites chosen for their natural or semi-natural ecosystems.