Read the text “Human Impact on the Natural Environment” and complete the table after it.

HUMAN IMPACT ON THE NATURAL ENVIRONMENT

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.

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.

Modification of Landforms. 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.

Direct Alteration of Landforms. Man has a direct effect on the shape of landforms by excavating and pilling 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 machine 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 are among the forms of scarification. Strip-mining is one of the most devastating examples of landform 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 are all visible manifestations of recent changes in the surface form of the ground.

Equally obvious as man-created landforms are coal tips and other waste heaps from mining and quarrying. Many of these features are geomorphologically unstable, allowing various forms of mass movement to generate. When saturated by heavy rain, spoil tips are 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 many of its children. Similar problems may arise on other constructed slopes: the large number of earth flows 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, Keele 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.

Indirect Effects: Slopes and Rivers. By far the most important of all man’s effects on landforms are 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.

Multiple shoe-string rills and gullies on hillsides are often a typical manifestation of man’s indirect effect on slopes. They are presently found in many parts of the world, notably in semi-arid 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 and other 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, hillslopes became eroded and valley bottoms were heavily silted. This may have been the result of a tendency towards greater aridity, but many experts believe that human overpopulation and overgrazing by goats were important contributory factors.

● Name man-created direct and indirect alteration of landforms. Complete the table.

(Bryant, Richard H.. Physical Geography Made Simple, Oxford, 1993)

Date: 2014-12-22; view: 1205