Sand dunes can form anywhere that the essential requirements of a supply of dry sand and the wind to move it exist together. In Britain, these conditions are confined to the coast, but in other parts of the world they occur inland as well. The basic process that forms coastal sand dunes and desert sand dunes is similar, although the resulting dunes are different in shape, because of the way that coastal vegetation modifies the effect of the wind.
Once waves have built up a beach of sand on the coastline, of a height that is only reached by exceptional tides or storms, sand dunes may form. Formation is triggered by obstacles to the wind, such as tidal litter, shingle or vegetation, which slow the wind and cause deposition of sand. Of these, vegetation is the most important, as it is able to keep growing upwards through the accumulating sand. Other obstacles, once buried, cease to be of importance unless they are re-exposed. As will be detailed later, this pioneer vegetation is perfectly adapted to its role in sand trapping. It forms the crucial first stage which gives rise to the succession communities that make up a sand dune system.
The youngest stages of this system, the foredunes, are not only the most susceptible to natural erosion from storms and high tides, but they are also the most attractive for recreation. Unfortunately, the foredune vegetation, which is so well adapted to coping with conditions of sand, drought and maritime exposure, is the least suited to withstand the effects of trampling. Most conservation work is thus concentrated on these vulnerable foredunes. Work may include protecting the dunes that still remain, and recreating denuded dunes by trapping wind-blown sand using fencing, brushwood and transplanted vegetation. As visitor pressure is usually the most important agent in the destruction of sand dunes, this factor must be tackled first by providing suitable access routes to the beach.
Other important sand dune work involves management of the fixed or back-dune vegetation, both to maintain particular communities of plants and animals, and to conserve individual species of importance.
The plant life of British dunes in general is extremely rich. Altogether more than 1,000 species of flowering plants and ferns may be found (Coastal Ecology Research Station, 1973). Many of these are introduced species which have been spread by man, often in the course of afforestation within the last century, or by birds. [n addition to the vascular plants, several hundred species of lichens, bryophytes, fungi and algae occur in the dune systems.
The reason for this immense richness, in striking contrast to the relative simplicity of salt marsh flora, is the highly variable conditions within the dune environment. This creates a complex mosaic of habitats according to climate, substrate, water table, soil chemistry, exposure and other factors which are discussed further in the next section. This mosaic is further diversified by the plants themselves, particularly as they modify wind scour, protect the soil surface, increase its organic content and water-retaining capacity and form a sheltered platform for the colonisation of other species. Near the landward edge of the dunes, grazing may further modify the flora towards increasing diversity.
The invertebrate life of dunes and slacks is as diverse as the plant life. Common and conspicuous insects include grasshoppers, earwigs, and many species of beetles, butterflies and moths. Sand-burrowing hunting wasps and bees are abundant on open dunes, while several species of bumble-bee inhabit older dunes. Crane-flies hover in mating swarms in late spring; later in the year their leather-jacket grubs destroy many young shoots of marram grass. Large flies include the robber and horse flies. Spiders are notable, especially wolf and jumping spiders. In the pools and marshes of wet slacks, dragonflies, mayflies and caddis flies live out most of their lives as aquatic larvae before developing as free-flying adults, while pond skaters, water boatmen and whirligig beetles remain always in or on the water. The common banded snail (Cepaea nemoralis) and garden snail (Helix aspersa) are also frequently found.
Among the vertebrates, certain dune systems are notable for their colonies of natterjack toads and sand lizards, two species which are rare and declining in this country mainly due to loss of habitat. Shore-nesting birds include terns on beaches and embryo dunes and shelduck in old rabbit burrows on fixed dunes. Hawks, owls and other birds of prey hunt the dunes and slacks while birds of passage such as fieldfares and redwings sometimes winter in vast flocks among extensive sea buckthorn thickets where they feed on the plentiful berries. On fixed dunes and dune heaths and grasslands, species such as skylark and meadow pipit are typical. The mammals include several species of voles and mice, and, more importantly for their effect on dune vegetation and therefore on stability, rabbits. Where man allows it, sheep and less frequently cattle and horses graze the dunes and dune grasslands, with effects which are discussed here. Foxes are major predators of small mammals and shore-nesting birds, while, in Wales, polecats destroy many rabbits. Man himself, although seldom an inhabitant of dunes, exercises more influence than any other species through his use or misuse of this environment.
Sand dunes are not only fascinating places of study for the botanist or zoologist, but also for those interested in ecology, geomorphology, hydrology or the development of soils. This interest stems not only from the form of the dunes and the wildlife contained at any one time, but in the way the dune system changes and develops over time, changes which are more rapid than in any other type of landform. These changes may be induced by man or by natural causes, and vary from the slow development of a plant community, to the transformation of a foredune by an overnight storm. Development over the years can be seen in the successive dune ridges and slacks, and in historical changes including alteration of the coastline, and the covering of farmland and dwellings by the movement of sand. As will be seen in Chapter 2 – Management Planning, management decisions are not easy to make in a system which is inherently unstable.