The major and interacting environmental factors of climate and soil together determine the fundamental and original diversity of ecosystems which this country contains and which has to be provided for in our national series of sites requiring conservation. The present pattern of variation in ecosystems has, however, to be seen as the result of gross climatic changes during the period since the last advance of the Quaternary ice, modified increasingly during the last 2000 years by the impact of man. The general picture was of a slowly warming climate allowing the gradual spread of major plant formations and their associated fauna in a wave-like sequence to the north and west across Britain, as types with increasing warmth requirements and luxuriance successively replaced those adapted to cooler conditions but possessing less competitive power. The open fjaeldmark and tundra which first colonised ground freed from the ice were invaded by birch, juniper and willow scrub of the taiga type. Pine forest spread and took over from birch, but was itself later replaced by mixed deciduous forest, mainly of oak, ash, elm and hazel, but locally dominated by beech, alder, lime, hornbeam and yew. By migrating northwards and upwards, the earlier plant formations each survived where their optimum conditions remained, so that the time sequence of Late-glacial and Post-glacial vegetation types became zoned latitudinally and altitudinally within Britain.

The fjaeldmark and tundra complex became restricted to high levels of the mountains of northern and western Britain, though with lower-lying outposts in situations where closed woodland could not develop, as on exposed coasts and areas of bare rock and unstable, shallow soils; its limits are taken to define montane conditions. Below this on mountains were zoned submontane scrub and woodland, and the lowlands of the south and east were evidently covered with great tracts of forest. During the whole of the Post-glacial Period, woodland has thus been the climatic climax formation over most of Britain, and limited in its extent mainly by high altitude, severe wind exposure and waterlogging of the ground. Primitive man probably kept limited areas clear or thinly covered with trees, but populations were for a long time too sparse to have much effect in reducing forest cover. Woodland floristic composition varied according to geographical position and soil type but in general there was probably a greater diversity in age and size of the trees, and a more constant development of a shrub layer, than is usual in British woodlands of the present.

Around 5500 B.C. the climate became markedly wetter, and the widespread occurrence of buried tree stumps at the base of peat deposits suggests that, in the higher rainfall districts of western and northern Britain, the extent of forest became substantially reduced by the spread of mire vegetation over ground where drainage was poor. Yet, during this Atlantic Period, thermophilous trees and other plants probably reached their maximum extension and abundance in Britain. About this time, the rising sea-level cut the land bridge formerly connecting Britain with the rest of Europe, so that the immigration of new species was much curtailed for many groups of organisms.

Later still, around 500 B.C., the climate became cooler again, and this was followed by a slight downward and southward shift in the main climax vegetation zones. During this Sub-Atlantic Period, which has continued, with minor climatic fluctuation, up to the present day, human activity probably first began significantly to alter the general pattern of vegetation cover in Britain. Forests were destroyed on an increasing scale, to provide land for cultivation and grazing of domestic animals, and for fuel, including charcoal for smelting of iron. Woodland clearance, particularly rapid in Norman times, has only been compensated by extensive re-afforestation during the last 50 years. Animals dependent on the forest were either incidentally or deliberately eradicated or reduced, though new ecosystems, especially grasslands, were created, and some reached a fair measure of stability under the existing management practices. New species of plant and animal were introduced by man, either casually or purposely, and some of these arrived here so long ago that the concept of a native species is of doubtful validity.