4.4 Wetlands
4.4.1 Awareness
4.4.2 Definition
Wetlands are transitional environments. In a spatial context, they lie between dry land and open water - at the coast, around inland lakes and rivers or as mires draped across the landscape. In an ecological context, wetlands are intermediate between terrestrial and aquatic ecosystems. In a temporal context, most are destined either to evolve into dry land as a result of lowered water tables, sedimentation and plant succession, or to be submerged by rising water-tables associated with relative sea- level rise or climatic change. Geologically, individual wetlands are ephemeral features of the earth's surface, subject to change, although the stratigraphic record is replete with examples of past wetlands such as the Carboniferous coal swamps of Laurasia. Scale is important, however. Large wetlands change relatively slowly and may be perceived as permanent by the human eye, whereas small marshes may be erased by flooding or sedimentation during a single storm. Of particular significance in human terms, especially in this technological age, are the rates of change and the extent to which wetland change may be accentuated by human activity.
Wetlands are denned as biophysical flatlands or slopes with perennial water tables at or near the surface. Their foundations may lie upon poorly consolidated debris or on rocky substrate but, because their usually low energy environment favours accumulation, they soon acquire a cover of sand, mud or organic matter. In the following discussion, a broad distinction is made between coastal wetlands found in estuaries, lagoons, deltas and some open coasts, and interior wetlands found in riverine, lacustrine and palustrine environments whose only common denominator is a high water-table. In most respects this is also a distinction between salt-water and freshwater wetlands, except that in arid lands interior marshes are commonly saline, whereas estuarine wetlands respond to fresh, brackish and saltwater influences.
The few areas of marsh and fenland which remain undrained bear their own characteristic vegetation of reeds, rushes, sedges, thickets of alder, birch and willow, and many other plants which grow only in waterlogged soil. This vegetation is essentially natural, though its detailed distribution is often determined by human activity. Thus regular cutting of the reeds and sedges for thatching and leaf litter prevents the trees and bushes colonising the marsh or fen by destroying their seedlings and saplings, while the reeds and sedges spring again from their underground parts.
A very few 'raised bogs' of the kind quite numerous in the central Irish Plain still survive in the west and north of Britain, and they possess an extremely distinct and interesting plant population. Most of them have long since been destroyed by draining and peat cutting.
The aquatic vegetation inhabiting rivers, lakes, pools, canals and ponds is again quite distinct and essentially natural, though many of its individual habitats, such as canals and ponds, have actually been provided by man.
Finally there is the vegetation of the sea coast, of the salt marshes, sand dunes, shingle beaches and sea cliffs. They bear quite special, characteristic vegetation, determined by the different kinds of maritime habitat, wherever it has not been destroyed and replaced by artificial constructions such as sea walls.
4.4.1 Awareness
There are a number of distinctive major landscapes in the world that have caught the attention of many disciplines as being a focus for their studies. Arid lands, mountain lands, tundra lands, polar lands, Mediterranean lands, mid-latitude grasslands, tropical rainforests and savannas, for example, have all at some time been considered by botanists, biologists, ecologists, soil scientists, historians, archaeologists, geographers, economists and land managers, to name but a few, from their varying points of view. Wetlands are another such major landscape, but it is only since the late 1960s that they have engaged the attention of a range of scholars in an effort to understand their variety and complexity, yet essential unity.
The relative recency of this interest has much to do with the character of the wetlands themselves, and with the way that they have been perceived in the past. Although wetlands occupy about 6 per cent of the earth's surface, unlike other landscapes of comparable size they are not climatically based or induced and therefore do not occupy large contiguous stretches of land. Giving them their simplest definition, wetlands are lands with soils that are periodically flooded. Therefore they are ubiquitous and found in nearly every climatic zone from the tundra mires of the poles to the tropical mangroves of the equator, and in every continent except Antarctica.
With very few exceptions, such as the Everglades of Florida or the Fens of eastern England (with original areas of about 10,000 and 4000 km2respectively) they rarely cover large areas. More often than not they are found in scattered locations, and are intermittent and local in their occurrence. Consequently, the growth of knowledge about wetlands, which has revolved around the problem of defining, classifying functions, characteristics and values, has been delayed until research workers began to see the commonalities which bound them together.
In addition to the real problem of coming to grips with wetlands as a physical entity there has been the problem of their perception. Hitherto, wetlands have been considered wastelands and therefore worthless. Their transformation through draining, dredging and infilling seemed a fitting fate for them. In recent decades, however, wetlands have assumed a new attraction and value for a great variety of reasons, not least of which is the fact that they are being eliminated at an alarming rate and are rapidly assuming the status of an endangered species. At the same time there is a growing appreciation of their natural functions, and the values that humans attach to them. They are one of the most fruitful areas of archaeological research, and they are the ideal setting in which to study the interactions between physical processes and human actions that encapsulate and exemplify many of the themes of man's impact on his environment. Also, the new-found beneficial functions of wetlands seem in danger of being lost with draining and infilling.
Before the early 1960s wetlands were largely neglected and unappreciated, and were probably the most poorly understood of landscapes and ecosystems, being neither sound land nor good water. In Europe, academic research on and knowledge about them broadly revolved around three foci of interest. First and earliest were botanists who explored the processes of bog and mire formation and geomorphologists, hydrologists, and sedimentologists who looked at the dynamics of coastal marsh formation particularly since Late Quaternary times. A second, and later, focus of research was the palaeobotanical analysis of peat bog structures, with its associated implications for understanding past climatic regimes and sea- level changes, the advent of related vegetation change, 14C dating and palynology was crucial to this endeavour which was closely associated, in Britain at least, with the work of Godwin. A third focus of research had its origin in seventeenth-century antiquarian interests, but came to serious academic fruition in more recent decades with the work of historians, and more particularly of historical geographers. These workers were concerned with unravelling the long-drawn-out process of human adaptation and change to a hazardous environment, and the consequent modification and evolution of the cultural landscape.
In the United States there was the rapid adoption after about 1950 of integrated ecological studies by workers that simulated intellectual curiosity about wetlands. In addition, the setting up of research centres devoted to the study of wetlands gave an added focus to enquiry.
Second, there was the active and practical interest in wetlands stimulated by the recreational, hunting and wildlife enthusiasts. Their interest in the wild and the primitive, perhaps best expressed as a concern or love of 'nature' and 'wilderness', was part of a deeply-held ethos that was, and still is, central to much of American history and culture. During the immediate post-war period, the Fish and Wildlife Service - the guardian of the duck-breeding grounds - became concerned about the impact of continuing wetland losses on fish and wildlife populations. The loss was approaching 121 000 ha/yr. This report was one of many strands that fed the increasing awareness of the general public and Federal agencies to environmental issues during the early 1960s and which gave wetland preservation a popular basis of support. The general public became aware of wetland losses by dredge and fill operations in estuarine and coastal locations, and, in addition, their newly discovered positive functions for flood protection and water quality maintenance were also stressed. By 1972, many coastal states had enacted legislation to protect their shores.
In England and Wales events did not proceed in the same way. There was no wildlife/wilderness lobby comparable with that of the United States, although the activities of the Royal Society for the Protection of Birds (RSPB), the Nature Conservancy Council (NCC) and the privately initiated venture of the Wildfowl and Wetlands Trust, founded by the late Sir Peter Scott at Slimbridge on the Severn Estuary, kept alive the idea of preserving breeding grounds, particularly for migratory birds. However, such concerns were peripheral to the aim of maximizing food production. The spectre of low agricultural production since the 1930s, food shortages and the need for greater self-sufficiency during the Second World War and the immediate post-war period put the emphasis on agricultural expansion and intensification. Government support came under the Land Drainage Acts of 1930 and 1976 which organized catchment boards to oversee drainage activity. Liberal grants were made available by the government to these boards and to individuals to drain and upgrade wetland. So overwhelming was this emphasis that it was not until the 1970s that the first murmurs were heard about the deleterious effects of land draining on wildlife and flora. A very similar ethos and sequence of events prevailed in much of the rest of Europe, even more beset with problems of post-war reconstruction). The Common Agricultural Policy (CAP) of the European Economic Community (EEC) has encouraged even greater wetland conversion.
During the 1970s, events gathered momentum in the United States. The recognition of the importance of wetlands for water quality control and a growing appreciation of their recreational and aesthetic qualities led to the Federal government's taking a greater responsibility for their preservation.
4.4.2 Definition
In England, wetlands have been large enough to acquire regional names, such as the Fens of eastern England, the Broads of Norfolk, the Carrs of Humberside, the Levels of Somerset and the Mosses of Lancashire. However, these names say little about their composition or origin: the Somerset Levels and Fens consist of organic peat soils, marine clays and river alluviums, Romney Marsh is almost entirely composed of marine sands and shingle, and the Carrs are composed of peat. Further confusion arises with the use of yet other names, such as mires, bogs, sloughs, swamps and marshes. For example, in Somerset the individual low-lying peat and alluvial levels are called 'moors', a term common in the Netherlands and Germany for high peat lands.
There are about 90 English terms to describe peat lands, peats and peat-like qualities, and undoubtedly the list has grown since then. In the United States, few terms occur uniformly throughout the country in either scientific or vernacular usage. Unlike Europe, little distinction is made between peat and non-peaty wetlands, perhaps because peatlands comprise a smaller proportion of wetlands in the 48 lower states. The term 'marsh' is generally reserved for wetland dominated by graminoids, grasses or herbs, and 'swamp' for wetland dominated by woody plants, as in the cypress swamps and river bottom hardwood swamps of the South. 'Bog' is used to denote ombrogenous mires, but the distinctions between all these three are not always clear. Fen is rarely used, but there are some distinctive additional terms, such as wet meadow, wet prairie, glade and pothole in the northern Plains, and pocosin in the Carolinas.
The distinguishing feature about all these types of wetland is the interplay between the land and the water, and consequently they partake of the characteristics of both. The invasion of water can be caused by a variety of factors such as the periodic overflowing of rivers in valley bottoms or flood plains, the rise and fall of tides along the coast, impeded surface flow due to tilting, uplift or landslip, occasional tidal inundation caused by land subsidence or by unusual climatic events, deposition of sediments in estuaries or deltas, the impediment of subsoil draining by impervious lower strata or horizons, and particularly by permafrost, or the rising of the water- table above the surface level. All these contribute to standing water, or to saturated or waterlogged soils. The causes are infinite and often work in combination; for example, storm surges can block river outfalls at high tide causing the overflow of already bank-full river discharges many kilometres inland.
If hydrology is the key to the formation of wetlands it is not necessarily the total explanation of their distinctiveness. We all know wetlands when we see them because of their characteristic vegetation of, for example, swamp grasses, sedge or cypress trees and intermittent patches of water, but it is difficult to describe the common features of that vegetation which makes the wetlands stand apart from other landscapes. Part of tecological composition and character which arises from the fact that they are situated at the junction between dry-land terrestrial ecosystems and permanently wet aquatic ecosystems. They differ from both, but at the same time partake of the characteristics of either. This is one of the reasons why wetlands have been so neglected and their distinctive features and commonalities have not been appreciated until recently.
At the simplest level we can say that their soils are formed and conditioned by standing water or waterlogging and are adapted to anoxic biochemical processes. Their vegetation is adapted to wet conditions (hydrophytes) because it is water- covered for at least a part of the growing season and is thus deficient in oxygen. It also decomposes slowly and contributes to the process of wetland formation by either trapping silt or forming, in time, actual soil (peat). Much of the fauna is adapted to dwelling in either deep water (fish and shellfish) or dry land (waterfowl) but moves seasonally into the wetland. To these three obvious and crude distinguishing ecological features of soils, vegetation and fauna (but note some qualifications about these in desert interior wetlands, we must add a number of other ecological functions that are less obvious and more subtle. Wetland soils are physically volatile and are in constant flux with the decomposition and erosion of sediments with river flow, flood and tidal shift. Additionally, different vegetation flourishes or dies with changes in predominant water sources, e.g. rushes die if water becomes too rich in nutrients. The interaction between water level, sedimentation and decomposition is finely balanaced, and within the soil there are biochemical processes at work as energy flows through the ecosystem leading to the transformation and trapping of nutrients. All these factors lead to the creation of a highly diverse ecosystem, partaking of aquatic and terrestrial sources, which means that wetland ecosystems are amongst the most productive in the world so that they and their products have been a constant lure to humankind.  With regard to wetland biology, suffice it to say, the physiological and behavioural similarity of plants and animals and the physical conditions found in the wetlands of the world have much in common, although they vary enormously in detail.
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