Pressures on terrestrial ecosystems: # 2
The transformation of ecosystems is considered to be the most important human impact in the southern African subcontinent (Macdonald 1989). The conversion of natural ecosystems for intensive agricultural or forestry production, or for grazing purposes, have been identified as major pressures on plant diversity (Cowling & Hilton-Taylor 1994). Land pressures arise from forestry because of the large tracts of land required (about 7-10 000 hectares for a large modern sawmill, and 50-100 000 for a pulp mill of the necessary scale). Pressures on water arise because plantation forests and many crops require large amounts of water compared to natural vegetation, and are therefore located in the areas with higher rainfall in South Africa. Pressures on habitats and biological diversity arise because cultivation and afforestation often involve replacement of natural or near-natural habitats rather than of already transformed habitats. In the process, the surface area of natural habitat has been reduced and fragmented, and impacted by pollution, alteration of hydrological cycles , and by management actions which are aimed at increasing production (e.g. intensive veld burning). Fragmentation of habitats is a serious problem because it can also lead to species extinctions. Populations of plants and animals need large areas of land in order to find sufficient food and mates, during changes in seasonal and annual climate. When habitats are fragmented, (unless they are joined by corridors), animals and plants cannot migrate from an area where food and water are scarce, to an area where food and water are more plentiful, as is the case in unbroken natural habitats. Fragmentation also effectively reduces the size of the habitat, as the perimeter-to-area ratio is increased, and many animals and plants are not suited to life at the edge of the habitat (where it may be more exposed to light and wind, and predation, or where there are fewer suitable nesting sites) and therefore these species cannot survive. This reduces the population size, and hence local extinctions are more common.
 |
| Applying agricultural pesticides |
 |
| Eutrophication |
Pressure to increase productivity of agricultural systems to meet domestic and international demands has encouraged widespread use of fertilisers, and other agrochemicals. Application of fertilisers is important to achieve optimum yields. However, it is equally important not to over-fertilize. Excessive use of reduced (energy rich) sources of nitrogen is the biggest cause of soil acidification, following its biological oxidation to nitric acid (Barnard, 1977; Beukes, 1997). Over-fertilisation also increases the concentration of nitrates in underground and surface waters, and of phosphates in surface waters. This causes pollution and eutrophication (see Freshwater Systems and Resources). Figure 2.3 shows fertilizer usage in South Africa over the last 40 years. Fertilisation peaked in the late 70's and early 80's, but has remained fairly constant for the last ten years. The decline was due in part to drought and consequent lack of finances. The decline in nitrogen application could well be beneficial to the environment, limiting soil acidification and nitrate pollution (van der Merwe, 1995), as long as fertilization does not become limiting to plant growth and yield.
 |
| Figure 2.3 Fertilizer usage |
Production pressures have also encouraged prolific monoculture which reduces biodiversity, and depletes soil nutrients faster than planting a variety of crops. In 1997 the area planted to maize, for example, was 4.4 million hectares (nearly 4% of the country), although not all of this was under monoculture. This, combined with the high economic costs associated with crop failure, encourage increased use of agrochemicals.
In some parts of the country, demand for agricultural productivity has led to marginalisation of farmers, forcing them to cultivate unsuitable soils, in risky climatic areas, in order to produce food.
These pressures, especially when combined with poor cultivation techniques (such as ploughing down the slope, removal of plant material after harvesting, and not managing irrigated soils correctly) leads to increased runoff and soil erosion. Chemical degradation of soils (acidification, salinity or pollution) can also occur, resulting in reduced ability to support plant life.
|
There is also information about Terrestrial Ecosystems in the following reports:
|
|
Metropolitan reports:
|
 |
|
|
|
|
|
|
|
Copyright © 1999 Department of Environmental
Affairs and Tourism. All Rights Reserved.
Site maintained by the Directorate Environmental Information and Reporting
Last update: October 1999
|
.gif){kind=small}