Impacts on terrestrial ecosystems: # 2

Impaired Functionality of Terrestrial Systems:  

Three main indicators of impaired functionality are addressed, namely water availability and quality, loss of productivity, and adaptability or resilience to change.

Water availability and quality: 

Water flowing from catchments with forest plantations growing there, is usually of high quality, which assists water resource protection. However, best estimates indicate that plantation forests consume about 1.5 billion cubic metres of water per year more than would be consumed in the natural condition; this is about 3% of SA's available surface water. In addition, locally, plantations have encroached riparian zones, and because surface runoff from plantations is reduced, there is less clean water to dilute waste water released downstream, affecting pollution management programmes.

Prosopsis sp.

Alien invasive plants also impact on water resources and runoff from catchments. It has been estimated that the additional volume of water used by alien vegetation across the country, in comparison with the natural vegetation, is some 3 300 million cubic metres, roughly 7% of mean annual runoff (Versveld et al. 1998). For example, alien plants in the Northern Cape and Western Cape Provinces use about 15% to 16% of mean annual runoff. Invasions by Prosopis species in the arid interior of the country could be using as much as 190 million cubic metres of water per year, thus depleting groundwater resources vital for farmers in these regions (Versveld et al. 1998). Unfortunately, many farmers use Prosopis as livestock fodder, creating tension between costs and benefits of alien plant control (Hoffman et al. 1999). It has been estimated that controlling alien invasive plants in the next 20 years could cost as much R600 million per year for the next 20 years, equivalent to R5.4 billion in present day value. (Versveld et al. 1998). Furthermore, at a conservative expansion rate of 5% per year, the impacts of alien vegetation could double in 15 years.

Pollution of water courses by industrial and domestic effluents, agricultural run-off, and acid mine drainage, compounds this problem by making what little water is available, less able to support plant and animal life. Reduced water quality can also be harmful to human health if supplied as drinking water, and frequently cannot be used in industrial and agricultural processes. Even water used for cooling in many manufacturing processes, has to be of a certain standard, otherwise the equipment becomes clogged up with lime scale.

Loss in Productivity:

The principle effects of soil degradation are soil impoverishment and greater susceptibility of vegetation to drought. Nutrient loss resulting from soil erosion has been shown to reduce plant growth and community development (Thornes 1985). Available per capita agricultural land in 1970 was 0.86 hectares. Through rapid population growth, this decreased to 0.5 hectares per capita in 1980 and it is estimated that this will decrease further to 0.2 hectares per capita by 2020 (Fuggle & Rabie, 1983). Significant degradation of soil and vegetation will mean even less area available per person and may impact on food security and lead to increased dependence on imports. The costs of replace nitrogen, phosphorus and potassium have been estimated to run to R1.5 billion annually. Possibly more important is that the loss of soil organic matter and clay minerals during erosion reduces the water holding capacity of soil, exacerbating drought conditions substantially (Laker 1993). Downstream effects of erosion such as reservoir siltation may reduce the capacity for water storage, making the country more susceptible to water shortages in times of drought. Eutrophication of surface waters by nutrient rich silt also occurs, adversely affecting water quality. These impacts on dams are estimated to cost in the order of R200 million annually (Braune & Looser 1989). The following table shows expected and actual siltation rates of selected dams.

Soils also lose their ability to support plant growth when they become polluted. Acidification of soils has occurred in parts of the country due to deposition of acidic substances produced by industrial activities. Seepage from landfill sites also contributes to pollution of soils, as does illegal dumping of hazardous wastes, and accidental spillages of oil, chemical products, and heavy metals. Many of these are highly toxic, not only to plants, but also to the micro-organisms living in the soil. Pollution with toxic wastes can render the soil barren for many years. In some cases the pollutants are flammable, and increase the risk of fires, which may cause further damage to the ecosystem.

Adaptability/ resilience to change:

Figure 2.14 Probable reduction in grassland biome due to climate change.

Predictions of the effects of climate change on natural ecosystems in South Africa, over the medium to long term, have included reduced spatial extent of the Grassland biome (Ellery et al. 1991), an increase in the extent of the Desert biome (Macdonald & Midgley 1996), and the occurrence of novel plant communities (Rutherford et al. 1995). The reduction of Grassland biome area may occur (see Figure 2.14), as present limits to tree distributions in grassland areas (minimum temperature and fire effects) are relaxed, and trees invade and thicken (Ellery et al. 1991, Midgley et al. 1998). A recent analysis (Rutherford et al. 1999) based on climatic factors which limit biome distribution, has shown how increasing aridity in the Northern Cape may expose vegetation to stresses not yet experienced by the country's biomes (see Figure 2.15), thus creating a novel climatic environment. A recent analysis casts doubt on the potential ability of the current reserve network to retain current plant species richness levels (Rutherford et al. 1999), and has shown that the Augrabies Falls National Park in the Northern Cape stands to lose over a third of its plant species, with little chance of significant species immigration.

Figure 2.15a-b Potential increase in aridity in the Northern Cape may lead to changes in geographic distribution of biomes.

>     Terrestrial Ecosystems: Responses

There is also information about Terrestrial Ecosystems in the following reports:
Metropolitan reports:
	Cape Metropolitan Council (1998 edition)		Durban Pilot Study
	Greater Johannesburg Metropolitan Council (1999 edition)		Greater Pretoria Metropolitan Council (1999 edition)