Table 24.1, which relates sediment yield to various categories of land use within a small area. The contrast between open cultivated land and forested areas is readily apparent.Man's relation with his natural environment is a complex one. While he is subject to certain natural controls and events, he also acts as the dominant force in many of the Earth's physical and biological systems. The relationship has changed with time. For thousands of years, the direction and extent of his progress were to a considerable measure dictated by his physical environment, which sometimes presented him with very difficult obstacles. Increasingly, man has become capable of altering his physical environment to suit himself. Although the object of these alterations was to improve his living conditions, in some cases they have created major long-term problems, and in still others they have been catastrophic, both for the natural environment and himself.
This book has attempted to explain the operation of natural systems and processes, but equally it has been stressed at several points that man is an important influence on many of them. In some parts of the world, the environment has been so transformed that few elements of its original nature are detectable. Even extreme habitats such as the tundra or hot deserts only sparsely populated by man have not escaped untouched, since they are often the most sensitive to the slightest interference. Many apparently natural systems are in fact control systems in which man acts as a regulator either consciously or inadvertently. At best, except for large-scale weather phenomena, natural systems are mostly modified systems. In this chapter we shall consider some of the ways in which climate, landforms, soils and ecosystems have been inadvertently altered by man.
Mining and quarrying, deforestation, the introduction of exotic plants and animals, the use of agricultural machinery, the building and use of tracks and roads, and the overgrazing of pastures, have all, singly and in combination, profoundly altered landforms and caused accelerated erosion and deposition to occur. Where man excavates or piles up material himself, he can be regarded as a direct agent of change; where he causes natural landform processes, such as wind and water action, to accelerate or diminish, he is acting in an indirect manner. Indirect effects are by far the most widespread. Much of this influence occurs accidentally or secondarily to some other purpose; conscious attempts to influence landform processes—for example, by building coastal groynes or by reafforestation—are inevitably expensive and limited in extent.
Man has a direct effect on the shape of landforms by excavating and piling up earth, reclaiming land from the sea and causing subsidence through mining. These activities have greatly increased since the Industrial Revolution with the development of enormous machinc power and explosives for moving material. Railway and motorway construction provides many familiar examples of man-created slopes, embankments and cuttings. Land scarification is sometimes used as a general term for disturbances created by the extraction of mineral resources; open-pit mines, quarries, sand and gravel pits arc among thoforms of scarification. Strip-mining is one of the most devastating examples of landfomr alteration of this kind. Although common in the United States, it does not occur on a widespread scale in Britain, except as a method of mining Jurassic ironstone in Northamptonshire. The effects of subsidence are common in most of the older coal-mining areas of Britain. Switchback roads, perched canals, fractured buildings and flooded depressions or flashes arc all visible manifestations of recent changes in the surface form of the ground.
Equally obvious as man-created landforms arc coal tips and other waste heaps from mining and quarrying. Many of these features are geomorpho-logically unstable, allowing various forms of mass movement to generate. When saturated by heavy rain, spoil tips arc frequently subject to sliding and flowage, supplying sediment that clogs stream channels. In 1966 at Aberfan in Wales, a major disaster occurred on a spring-saturated coal waste heap which moved as an carthflow, destroying part of the village below, including a school and many of its children. Similar problems may arise on other constructed slopes: the large number of carthtlows triggered during the building of the Panama Canal is a well-known example. More recently, the building of new trunk roads and motorways in Britain has encountered slope failure in several instances: at Port Talbot, Kecle and Sevenoaks, excavation reactivated slope shear planes which were last active under periglacial conditions during the Devensian glaciation. These sites required extensive engineering works to stabilise or avoid the slopes.
By far the most important of all man's effects on landforms arc those connected with his interference with the natural vegetation, in particular with the clearing of forest for agricultural purposes. There is a close relationship between the amount of vegetation cover and erosion rates on hillslopes, and hence with the amount of sediment in streams. A stable vegetation cover acts as an effective regulator of natural erosion, protecting the ground from direct raindrop impact, absorbing some of the run-off, and making the slope more cohesive. With the removal of the vegetation, the surface loses its plant litter, causing a loss of soil structure, cohesion and porosity. Overgrazing has similar effects, and the introduction of animal pests such as the rabbit into Australia has also had a detrimental effect on slope stability. Some idea of the effect of vegetation on erosion can be gleaned from Table 24.1, which relates sediment yield to various categories of land use within a small area. The contrast between open cultivated land and forested areas is readily apparent.
Multiple shot-string rills and gullies on hillsides are often a typical manifestation of man's indirect effect on slopes. They arc presently found in many parts of the world, notably in semiarid regions susceptible to tropical downpours. In an area such as South Australia, the recent date of a great deal of gully and sheet erosion on slopes is testified by the burial of fence-posts andothcr man-made debris. There is evidence that in some long-settled areas of the world, like western Europe, where gullies are not now a prevalent feature of the landscape, they were more widespread in past times when the natural vegetation cover was first removed. We may note that it is not always easy to distinguish between the effects of man and a changing climate on hillslope erosion. For example, in the Mediterranean area during the latter part of the Roman period, there was an increasing loss of soil fertility, hillslopcs became eroded and valley bottoms were heavily silted. This may have been die result of a tendency towards greater aridity, but many experts believe that human overpopulation and overgrazing by goats were important contributory factors.
High rates of hillslope erosion by overland flow are a natural state in some localities, creating badlands as in South Dakota where much of the area is underlain by almost impervious clay formations. But badlands can be artificially produced where accelerated erosion proceeds unchecked. Instances of this extreme form of slope degradation became common in the poorly farmed areas of the southern Appalachians in the United States in the 1920s and did much to bring the whole problem of soil erosion and soil conservation to public attention.
The alteration of infiltration and run-off on slopes by modifying the vegetation inevitably has a profound effect on adjacent rivers in at least two respects: by increasing both the discharge and also the sediment supply. There seems little doubt that many of the floods in mid-latitude rivers would not occur it' the vegetation in the drainage basin were in its natural state. Evidence has been put forward that the increase in the frequency of floods in recent decades in the river Severn at Shrewsbury is related to improvements in land drainage in the catchment area. The effect of this is to prevent the fields from retaining a large proportion of the normal precipitation and from releasing it slowly. At times of flood, discharge levels become higher and achieve their maximum more quickly. Another way in which discharge levels may be affected in similar fashion is through urbanisation; the ground surface is rendered impervious by buildings, paths and roads, and precipitation is channelled directly to rivers through drains and sewers. 
Fig. 24.1 illustrates the effect of urbanisation on flood peaks, with attendant damage to river banks, properties and farmland.
It is difficult to assess quantitatively the importance of man-induced slope erosion on the sediment load of major rivers since we have few pre-interference data. Most investigators are generally agreed that cultivation has greatly increased the sediment load of rivers of' south-cast Asia, Europe and North America, perhaps by a factor of two or three above the geological norm for the world. The effect of this increase is of considerable importance in the construction of dams and canals; in severe cases, large amounts of sediment supply may also cause valley aggradation, destroying productive land capacity. Specific operations of man which lead to local cases of river silting and aggradation include mining operations, urbanisation and highway construction: all these arc sources of excessive sediment. However, much can be done to reduce the effects of building operations—for example, by keeping to a minimum the periods during which bare areas arc exposed, and by using sediment basins to trap coarser sediment.
The phenomenon of the dustbowl in the Great Plains region of America in the 1930s is a well-known example of man-induced land erosion. The area was former grassland underlain by rich brown and chestnut soils, but both overgrazing and ploughing contributed to the catastrophe which caused the widespread abandonment of farms. A great expansion in wheat cultivation in the early years of the decade was followed by a series of droughts; the soil, largely exhausted of its natural fertility, was subject to deflation and particle drifting of disastrous proportions.
The dustbowl situation is by no means unique. In the marginal areas around today's hot deserts, such as the Thar desert of Pakistan and India, and the Egyptian desert, a great deal of deflation is initiated by grazing animals. In other deserts, as in the central Sahara and the south-west United States,desert pavements (Chapter Eight) normally contribute little coarse dust, but this protective layer is easily destroyed by wheeled vehicles, exposing finer-textured materials.
In Britain, coastal dunes are highly susceptible to deflation when interfered with by man. Constant trampling or vehicular traffic quickly destroys the protective grass vegetation, initiating blowouts or landward migration of the dunes. On the Dutch coast, protection of the dune systems from degradation by man is vital as these give protection to large inland areas lying below sea-level.
Man can have relatively little impact on the forces that govern waves, tides and currents, but he has had some effect on coastal erosion and deposition at the shoreline by building various structures and by removing beach material for ballast or construction. Before the nineteenth century, the erection of small piers or breakwaters to protect harbours was one of the few ways in which coastal processes could be locally modified. In the last two centuries, the urbanisation of many coastal areas in Britain has often paid little attention to local erosion factors, and one can find many examples where even a few yards of erosion by the sea may spell disaster for a heavily built-up area. Hence various engineering structures such as groynes, breakwaters and seawalls have had to be built to check marine erosion. However, these are not only extremely expensive to build and maintain, but often defeat the object of the exercise, since by checking erosion in one place they may lead to its increase elsewhere.
This may be illustrated by considering the effect of a single groyne or jetty in checking the movement of beach material 
(Fig. 24.2). Groynes have been widely used on the shingle beaches of the south coast of England. By preventing the longshore movement of the shingle, this may starve the shore on the downdrift side of the groyne of its supply of beach material, leading to a narrowing of the beach and an increase in direct wave attack on the cliff behind. Ideally, when groynes have trapped their maximum quantity of sediment, beach drift will be restored to its original rate for the shoreline as a whole. Where, however, large harbour breakwaters arc involved, as at Newhaven and Folkestone harbours, the effects may be more permanent, both in creating large depositional structures on the updrift side of the breakwaters, and in accelerating erosion rates in chalk cliffs further along the coast.
Atmospheric circulation systems operate on such a large scale that one is perhaps inclined to doubt that man's activities would have any appreciable effect on them. However, it is known that the global heat balance has changed over the last few decades, and we might ask ourselves how much of this- is a result of man polluting the atmosphere. It is certainly evident that pollution has marked local effects on the atmosphere. The problem is not so much to establish that man has an impact on the atmosphere but to evaluate it in comparison with the natural forces of change. Atmospheric changes induced by man may be grouped into three categories: the introduction of solids and gases not normally found in the atmosphere (pollutants); changes in proportions of the natural component gases of the atmosphere; and alterations of the Earth's surface in such a way as to affect the atmosphere. A fourth type of impact, planned weather modification, is considered in the next chapter.
To city-dwellers the most obvious way in which man has affected the atmosphere is through pollution. Pollutants include particulate matter, both solid and liquid particles, and gaseous substances such as sulphur dioxide (SO2), oxides of nitrogen (NO, NO2, NO3), carbon monoxide (CO) and hydrocarbon compounds. But not all man-made pollution comes from cities. Isolated industrial activities frequently create a footprint of atmospheric pollution in areas of countryside downwind from the industrial site: particularly infamous examples in Britain include smelters and brickworks. Mining and quarrying activities also send large amounts of mineral dust into the air. Even man-induced forest and grass fires as well as bonfires, can greatly add to particulate pollution at certain times of year.
Atmospheric pollutants are conducted upward from the emission sources by rising air currents as part of the normal convective processes. Larger particles settle under gravity and return to the ground as fallout. Smaller suspended particles are brought to the Earth by precipitation as washout. By a combination of the two processes the atmosphere tends to be cleaned of pollutants, and in the long run a balance is achieved between the input and output of pollutants, although there are large fluctuations in the quantities stored in the air at a given time. Pollutants arc also eliminated from the air over their source areas by winds which disperse the particles into large volumes of clean air in the downwind direction. Smoke stacks are intended to take as much advantage of this as possible. The passage of a cold front accompanied by strong winds is usually very effective in sweeping away pollutants from an urban area, but during stagnant anticyclonic conditions concentrations may rise to high values, sometimes producing a smog (Chapter Sixteen).
Once in the atmosphere, the primary pollutants undergo a number ofchemical reactions, generating a secondary group of pollutants. For example, sulphur dioxide (SO1) combines with oxygen and suspended water droplets to produce sulphuric acid. This acid is harmful to organic tissues and is also very corrosive. Photochemical reactions are brought about by the action of sunlight: for example, sunlight acting on nitrogen oxides and organic compounds produces ozone (03). Another toxic chemical produced by photochemical action is ethylene.
The harmful effects of atmospheric pollution on plant and animal life are manifold. For humans, many pollutants are irritant to the eyes and dangerous to the respiratory system. During the London smog of December 1962, more than 4,000 additional deaths occurred. Ozone in urban smog has a severe effect on plant tissues; atmospheric sulphuric acid has wiped out lichen growth in many urban areas of Britain. Lead and other toxic metal particles are a particular cause of concern for human health. In addition, pollution also causes many millions of pounds worth of damage to materials: limestone structures suffer greatly in certain British cities unless treated with preservatives.
The global effects of foreign particles in the atmosphere in altering radiation and heat balances is difficult to assess. There have always been major natural sources of particles in the atmosphere, including forest fires and large volcanic explosions. Careful monitoring by U.S. scientists of temperature trends and dust amounts at various heights in the atmosphere has led to the tentative conclusion that man's contribution to atmospheric particles may have far-reaching effects on tropospheric processes—for example, on the rainmaking mechanism—but perhaps little effect on processes in the stratosphere.
Of the main natural constituent gases in the atmosphere, carbon dioxide (CO2) and oxygen (Or) arc the most critical from an environmental viewpoint, for both arc inextricably involved in the biochemical cycles between atmosphere and the surface of the Earth. Although nitrogen comprises four fifths of the atmosphere, its inert chemical nature relegates it to a minor role in this respect. Oxygen and carbon dioxide arc naturally added to the atmosphere by `outgassing' from the Earth's interior. The work of plants has been essential in removing carbon dioxide from the atmosphere and storing it as coal and other fossil organic substances. Before the Industrial Revolution, carbon dioxide levels appear to have been about 290 parts per million (p.p.m.) in the atmosphere. But in the last hundred years or so, this amount has increased by about ten per cent, largely because of man's use of fossil fuels 
(Fig. 24.3). It has been suggested that, in contrast to the effect of solid particles, an increased level in carbon dioxide content will increase the temperature of the atmosphere, since the gas is an absorber of long-wave radiation. However, although the use of fossil fuels continues to accelerate, we know that temperatures have fallen in the last two decades (Chapter Seventeen) and the link remains unproven.
It has been pointed out also that man's large-scale combustion of hydrocarbon fuels requires a large quantity of oxygen to be withdrawn from the atmosphere and converted into carbon dioxide and water vapour. There is therefore the possibility of a lowering of the oxygen content of the atmosphere to levels which might have a detrimental effect on animal life.
Changes in water vapour levels brought about by man through combustion and alterations to the vegetation cover could in theory markedly affect global radiation and heat balances in the same manner as changes in carbon dioxide levels. But water vapour content varies greatly from place to place and it is difficult to measure global changes. It seems unlikely that there would be a general build-up of excess atmosphere water vapour through combustion, as it would rapidly return to the oceans as precipitation. A special case, however, is the emission of water vapour and various other substances by jet aircraft. These emissions occur in the stratosphere, where the water vapour content is normally very small. The condensation trails (contrails) of aircraft can often be observed to spread laterally and develop into cirrus clouds. These clouds are highly reflective and can have an effect on the Earth's albedo.
Meteorological processes close to the ground arc extremely sensitive to the character of the Earth's surface, and man's alteration of this through deforestation, agricultural practice and urbanisation has had several important effects. One result of these activities is to alter the rate of evapotranspiration. The complete removal of a forest cover will sharply reduce transpiration and thus the amount of water returning to the atmosphere in vapour form. The draining of a swamp will have a similar effect. Just what impact, if any, this has on air masses of large vertical extent, is uncertain.
Another important consequence of surface change is to alter the temperature characteristics of the atmosphere nearest the ground. We noted one example of this in Chapter Sixteen: closely built urban areas develop their own heat island on calm nights in summer. Equally impressive arc the changes in the heat budget brought about when an irrigated area is created in an arid region. The albedo of a light-coloured desert area is about 25-30 per cent; there is very little water for evaporation so that all the incoming radiation is available for the direct heating of the air. With irrigation, the albedo drops to10-15 per cent, and the incoming energy is used up almost entirely in evaporating water. Thus direct heating of the air above the irrigated area is very slight, and day-time temperatures become markedly lower than in the surrounding desert.
A third climatic clement that may be modified when man alters the ground surface is the wind. Trees and hedges effectively brake the wind. causing a simultaneous diminution in evaporation and in the carbon dioxide exchange close to the ground. Garden walls or thick tree belts may so effectively still the air immediately to leeward as to cause frost pockets on cold nights.
In the latter part of Chapter Twenty-Two we observed that primitive man managed to live as part of the natural ecosystem without altering its main characteristics. Large-scale burning of grasslands may perhaps be regarded as an exception to this. With the beginnings of agriculture, far-reaching effects, both obvious and subtle, were introduced into ecosystems. Man gradually became more sophisticated in knowing just how much to modify an ecosystem in order to harvest the crop he wanted. In achieving this end, he has inevitably simplified ecosystems, disrupted nutrient cycling, introduced alien species and eliminated others, and caused pollution. Only in recent years has there been an awareness of some of the consequences of ecosystem modification. Some of the efforts to redress the balance arc considered in the next chapter; this section summarises the more serious consequences of man's impact.
The most general effect of man on ecosystems is that he tends to simplify them. This comes about because man's prime concern is to direct energy and material cycling in the system towards himself so that he can easily crop them. Species other than the ones he wants to crop are regarded as weeds or pests, and he attempts to eliminate them. Hence, reduction in species diversity, often to a single species population, is a notable characteristic of man's impact on ecosystems. Food webs are also made much simpler in this process. In arable farming, man removes all other consumer organisms and crops the primary producers. In pastoral farming, he retains a single herbivore species, sheep or cattle, and himself occupies the position of sole carnivore. Again, this brings man into a state of active competition with all plants and animals apart from his favoured ones.
The degree of simplification varies enormously. in remote areas still only inhabited by hunters and gatherers, man may in fact add another trophic level to the rest of the food web. Primitive shifting agriculture in tropical rainforests represents only a temporary simplification and cropping of the natural system as the plot is only cultivated for a few years and then abandoned. On the other hand, grazing economics exhibit a much greater degree of ecosystem simplification. Unless the pastoralism is sufficiently wide-ranging to allow pastures to recover, selective grazing by the domesticated species leads to the eradication of the most palatable plant species, allowing tougher grasses or xerophytic plants to predominate, thus simplifying and downgrading the pasture. Modern arable farming represents perhaps the most extreme form of simplification, producing a highly artificial type of ecosystem.
Ecosystem simplification of this type often results in disastrous side-effects. A single species population, such as a field of wheat or a herd of cows, offers great opportunity for the development and spread of disease, pests and parasites. The potential for survival in ecosystems is much enhanced in multi-species populations: the greater the species diversity in any assemblage, the better the chance will be of a balanced interrelationship between organisms. Man-created monocultures are thus ecologically unstable and can only be sustained at the price of high inputs of energy (e.g. tpachinery, weeding) or matter in the form of chemical fertilisers. Without these, loss of soil fertility quickly sets in, with all the dangers of soil erosion. Intensively managed monocultures can be very high-yielding, but only at the expense in the long run of drawing on energy supplies from non-renewable sources, the fossil fuels.
Ecosystems cannot operate without efficient nutrient cycling. A major consequence of man's simplification of ecosystems is that he inevitably destroys major nutrient reservoirs, notably the natural vegetation and the soil system. To maintain yields he attempts to replace the loss by injecting fertilisers into the system.
When chemical fertilisers are applied to the land, many of the elements contained in them are retained by the soil, adding to the clay-humus complex. However, certain ions arc not retained, and among them is nitrate, an important constituent of most fertilisers. Nitrate is being added to the soil from fertilisers and nitrogen-fixing plants at a much faster rate than it can be broken down by denitrifying agents in the soil. Being soluble, it is rapidly leached out into rivers and lakes. Here, the increased nitrogen input permits the accelerated growth of plants, algae and other phytoplankton: this chemical enrichment resulting in increased productivity is called eutrophication. Unfortunately, in extreme form the outcome is ultimately harmful, since the plants and organisms die and decompose at such a rapid rate that oxygen levels fall until aquatic life becomes impossible. A severe example of eutrophication has occurred in recent years in Lake Eric, North America, where deep layers of decaying organic matter have covered large stretches of the shoreline.
This example of man's impact on nutrient cycling in ecosystems is by no means unique: similar problems of eutrophication arise with the phosphates contained in detergents, fertilisers, and normal sewage effluent.
The extinction or reduction in numbers of plant and animal populations is a well-known consequence of man's impact on the environment. Often the species become endangered not so much by hunting or conscious elimination, but by the disruption and fragmentation of habitats. Some species, particularly large predators, require an extensive area of specialised habitat in which to breed and hunt. and fragmentation of this by man's interference has frequently had disastrous effects. The marsh harrier (Circus aeruginosus), a large raptorial bird of recdbeds and fens, is a prime example of this. The systematic draining and reclamation of fens and wetlands in Europe and North America has had a devastating impact on the population levels of the bird. Very few pairs have succeeded in raising fledgelings in Britain in recentyears, and it seems likely that it will soon be lost as breeding species from the British Isles.
A contrary but equally far-reaching effect has been the accidental or purposeful introduction of alien species into ecosystems. Some animals and plants, because of their greater genetic adaptability and high reproductive rates have often made places for themselves at the expense of native species. Others have taken advantage of the new artificial ecosystems that man has created. For example, the starling (Sturntus cake-Iris) and the rock dove (Colundm !ivies) were once cliff-dwellers, but they have now established themselves in cities throughout the world, roosting in the `cliffs' of city centres. The gradual extension in the distribution of the starling in North America 
(Fig. 24.4), following its introduction in New York in 1891, represents an interesting example of an artificially introduced species that has successfully competed with native birds for living space in a man-made environment.
We have seen that the evolution of all ecosystems is primarily determined by the amounts of energy and matter which flow through them in chains or webs. They are further maintained by intricate patterns of chemical cycling. Under natural conditions, ecosystems have been in a state of ecological equilibrium. With the increasing impact of man. their essential characteristics arc altered, so that now signs of severe imbalance or a declining efficiency arc beginning to be observed in many of them. This is shown, for example, by the progressive devastation of formerly good fertile agricultural or grazing land through over-intensive use; in the reduction of species when secondary forest replaces primary forest; in a general loss of biological productivity; and in an increasing amount of pollution.
>Billings, W. D., Plants and the Ecosystem (Chapter S). Macmillan, London, 1964.
>Boughey, A. S., Man and the Environnent, Macmillan, New York,1971.
>Jones, D. K. C., 'Man-made landforms', in The Unquiet Landscape, edited by Brunsden, D., and Doornkamp, J. C., IPC Magazines, London, 1974.
>Smith, R. L., The Ecology of Man, Harper and Row, London, 1972.
>Strahler, A. N., and Strahler, A. H., Environmental GeoScience, Hanitton, California, 1973.
>Turk, A., et at, Environmental Science, Saunders, London, 1970.
>Wagner, R. H., Environment and Man (2nd cdn.), Norton, Ncw York,1974.
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