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Impact Of Land Degradation

Aside from the direct effects of reduced vegetation cover, especially from overgrazing, wood collection and deforestation, human-induced land degradation does not appear to be a serious issue over the greater part of the global desert area. Salinization can be an important problem in some oases, but in true, open deserts, natural vegetation cover is nearly always sparse, there is no soil structure, and little or no soil organic matter to degrade. As will be seen in more detail in Chapter 6, most of the human-induced pressures on the desert biome tend to concentrate on the deserts' edges, that is, the transitional ecotones between deserts and non-desert regions where some productivity can be derived from the land, and in the more humid environments inside the desert biome, such as oases and desert mountains, or sky-islands. The true deserts, however, which cover the vast majority of the biome, are normally too arid and too inhospitable to be the direct target of large-scale development.

Some impact, however, does occur, and at a global scale its cumulative effect can be significant. The deterioration of vegetative cover mostly increases the amount of dust particles suspended in the atmosphere. Populations with more direct exposure to this phenomenon are more likely to develop allergies and other respiratory ailments. Sources of dust are largely restricted to severely disturbed areas, as well as some natural sources such as lake beds and ephemeral stream channels. Newly remobilized dunes become an issue for humans when they advance over settlements and infrastructure.

Military activities and off-road vehicles do cause extensive, lasting damage to the fragile desert cover. The Mesopotamian shrub desert is fascinating from the ecological and cultural perspectives. Located in the Tigris and Euphrates River valleys, it is also an important winter stopover for migrating Eurasian birds and a refuge for the endangered and sparse populations of wolves, hyenas, leopards, oryxes, gazelles and wild boars. This desert, considered a cradle of civilization, has been greatly impacted by the recent Iraq wars.

Grazing pressure on the desert, and especially on the desert margin, is the most extensive agent of land degradation. For example, the Chihuahuan Desert in Mexico is in a vulnerable conservation status because cattle grazing and browsing have damaged sensitive desert scrubs and riparian habitats. The degradation of these riparian habitats, coupled with the loss of springs as a result of aquifer depletion and the diversion of streams for irrigation, have had a great impact on wildlife that depends on water sources.

Although mining activities affect small areas directly, they have significant impacts on surrounding areas. When the mine reaches the end of its life, the site is normally abandoned and remains a mixture of deteriorated materials, mining by-products, and unproductive rubble, usually coarse, and often of extremely toxic chemical composition, which is unfavourable to colonization by plants and animals. These sites have left a legacy of polluted land and groundwater, with wide impacts through the redistribution of toxic elements by wind and flash floods, in many deserts.

Chile and Argentina have abandoned mines of copper, lead, and nitrate in the Puna that are potential sources of contamination due to inadequate rehabilitation after their closure and the risk of chemical spillage (Romero and others 2003). The mining centres located at high altitude in the Dry Andean Puna, a mountain desert, near the source of rivers that feed irrigation systems or provide populated areas with drinking water, are particularly dangerous and require special consideration. The Rio Grande in the Quebrada of Humahuaca receives water from the Yacoraite River that drains from the eastern face of the Aguilar Mountain, which is currently being mined. The water of the Yacoraite has high levels of lead, iron, manganese, and molybdenum unsuitable for both llama grazing near the mines and the inhabitants of the Quebrada downstream (Figure 4.9). At the same time, this area has been declared by UNESCO as a World Heritage Site and is also an important tourist destination. The freshwater of the Rio Grande is used for irrigation of orchards (early season vegetables) that are commercialized in large urban centres whose population is put at risk by the metal discharges (Box 4.2).

Mining and salt extraction of sulphates, borates and others may also contribute to desert wetland pollution and consequently affect principal sources of water. Modern mining methods are very water-intensive. In addition, mining companies often excavate beneath the water-table and must pump and remove the groundwater, in the so-called practice of "dewatering." Dewatering can cause failures of springs and wells, land subsidence, and also threatens oases, wetlands and irrigation.

Oil spills on land and in freshwater bodies are frequent in some Palearctic deserts, and very damaging to the environment. Spilled oil affects surface resources and a wide range of subsurface organisms that are linked in a complex food chain that includes human food sources. Additionally, they can harm the environment by direct physical damage, that is, through the lethal coating of animals and plants, and by the toxicity of oil itself.

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