Pressures on terrestrial ecosystems: # 3
Rapid population growth, and expansion of human activities increases the amount of waste and pollution generated. This puts pressure on the terrestrial systems in three ways. Firstly, the environment is the ultimate receptacle for waste products, and the large volumes that are currently generated challenge this capacity. This problem is acute in urban areas, where waste is concentrated into localised areas, and the surrounding area is under pressure to provide suitable disposal sites. Secondly, the environment is often used as a "detoxifier" of waste. Many harmful substances (such as oil) are broken down by naturally occurring bacteria or other micro-organisms, leaving less-toxic or non-toxic substances such as water and carbon dioxide. High urbanization and industrialization rates pressure the environment's capacity to do this, due to the large volumes of pollution, and the highly toxic nature of some of the substances produced. Many local authorities are encountering soil and groundwater pollution problems, particularly with solid waste dumping, and where pollutants are contained in sewage sludge (President's Council, 1991). Thirdly, the release of waste and pollutants into the environment put pressure on the natural functioning of ecosystems. For example, large volumes of sulphur dioxide and nitric oxide which are released into the air by industrial and agricultural activities are deposited on the soil, vegetation, and water bodies, where they form acids. Organisms are specifically adapted to function within a certain range of acidity, and if the addition of more acid changes the conditions, the organisms cannot function as well, or even die. Large areas of the Eastern Highveld and Mpumalanga are currently showing reduced productivity due to acidification.
Many local authorities are encountering soil and groundwater pollution problems, particularly with solid waste dumping (President's Council, 1991). By far the biggest contributor to the solid waste stream is mining waste (72,3%), followed by pulverized fuel ash (6,7%), agricultural waste (6,1%), urban waste (4,5%) and sewage sludge (3,6%) (van der Merwe & Vosloo, 1992). The production of waste is aggravated by increasing urbanization, resulting in localised concentrations of waste, and pressure around cities for waste disposal sites. Urbanization and industrialization can also cause pollution, especially where pollutants are contained in sewage sludge.
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| Acacia saligna |
Invasion of exotic trees and shrubs, especially in the wetter regions of South Africa such as the Fynbos Biome and in riparian habitats specifically, pose a severe threat to plant and animal diversity (Macdonald 1989). The invasion process has many ecological impacts which include alteration of soil nutrient cycling, reduction of runoff and subsequent streamflow, increased river bank erosion, altered fire intensity (Macdonald & Richardson 1986) and reduction of light to the forest floor or near to the ground (Midgley et al., 1992). The Catalogue of Problem Plants in Southern Africa [Wells et al. 1986] lists 789 species, some of which, for example Acacia saligna and Hakea sericea, have dominated areas to the extent that natural vegetation has been almost lost. Others, for example pine and eucalyptus trees, present a threat to ecosystem functioning, because they use greater amounts of water than the natural vegetation, and therefore reduce the amount of runoff that reaches the streams and rivers. These impacts reduce the diversity and cover of indigenous plant species, but may be reversible even after up to 20 years of dense infestation (Macdonald & Richardson 1986).
Introduced animals have also reduced South Africa's biodiversity, a few examples being the Argentinian ant, the Himalayan thar, the European starling, the house sparrow and the black rat. On South Africa's islands, house mice, rabbits, and feral domestic cats threaten the native species. Some of the most drastic impacts of invasive animal species have been recorded in South African rivers, where alien fish, and to a lesser extent invertebrate and reptile species, have altered habitats and successfully out-competed native fauna. Up to 60% of the threatened endemic freshwater fish of South Africa may be threatened by introduced fish species such as trout, carp and bass (DEA&T 1997). The mite Varroa has caused the collapse of up to 30% of bee colonies in KwaZulu-Natal, Western Cape, and other parts of South Africa. Invasion by alien birds and mammals seems to be of lesser importance (Macdonald et al. 1986).
Climate change alters the physical environment in ways that directly affect living organisms. Changing temperature and water availability conditions are likely to induce stresses in vegetation and component plant species, and may encourage mobile organisms to alter their distributions in the medium to long term. Climate change will possibly cause a gradually increasing pressure on the tolerance limits of native species, (i.e. if the average temperature increases by 2°C, the plant or animal species may not be able to withstand a fluctuation of a further 1°C). By causing these stresses in native species, climate change could also favour the success and spread of alien plant species (Richardson et al. 1999).
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There is also information about Terrestrial Ecosystems in the following reports:
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Metropolitan reports:
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Last update: October 1999
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