Perhaps the earliest impact on the resources of the basin was the general elimination of most of the large mammals, first through hunting and later as a consequence of farming and fencing. Overgrazing by livestock has damaged rangelands and wetlands causing erosion and changes in species composition. Agriculture along river courses has introduced nutrients and pesticides into wetlands, increased salinity, and altered riparian species composition. Pollution and salinity have increased as a result of industry, mining and urbanisation. Alien plants and animals have also changed the ecosystems. Aquatic invertebrates are good indicators of the status of an ecosystem and change as the ecosystem changes. In the Orang-Senqu River there is an abundance of filter-feeders, in particular the Blackfly Simulium chutteri and downstream of Vanderkloof dam a drop in the presence of a predaceous caddisfly. Outbreaks of blackfly are attributed to stable flow conditions, particularly high winter flow, deterioration in water quality, and encroachment of in-stream vegetation. A number of aquatic invertebrates have also declined and possibly disappeared from the Orange River system, including mayflies, snails, a large Elmid beetle, and a leech species which was known to be parasitic on hippopotami, the latter becoming extinct. By contrast, an invasive snail Physa acuta has spread dramatically. Fish communities in the Upper and Lower Orange River are considered extensively modified, and their status is deteriorating, mainly because of deviation from the natural flow and deterioration in water quality. The poor ecological status and negative trends of the Orange River are a result of both in-stream and peripheral changes. The Types of ImpactThe impacts of human activities on biodiversity can be broadly characterised as:
The main human activities driving these impacts are:
These two aspects of human influence are discussed below, along with the preliminary assessment of ecological status of the Limpopo River basin. Human Impacts on BiodiversityWater qualityWater quality is known to play a prominent role in determining the distribution of aquatic organisms. Impacts on water quality, as described in detail in the water quality section, can eliminate aquatic and other species, influence their adaptation to the environment and their community structures. Water pollution also affects microbial and ecological processes such as metabolic rates and nutrientcycling processes. Water QuantityReduction in streamflow for irrigation and water supply results in an overall reduction in the availability of water in the downstream ecosystems. As water is a fundamental building block of aquatic life, reduction in streamflow has a direct impact on the biodiversity resources, limiting growth in all organisms. Furthermore, flow regulation of storage dams can have a negative effect on aquatic and riparian ecosystems. Seasonal fluctuations in water availability are often a fundamental part of the growth cycle of plants, particularly those adapted to living in the riparian zone of a river the size of the Orange-Senqu. Alien invasive speciesAlien invasive species often do not have natural enemies in the invaded region. They can however compete with indigenous species for space, nutrients and sunlight. Dense invasions of aquatic plants can alter the flow of rivers and streams, disrupting the aquatic ecosystem. Reduction of light penetration reaching the deep portions of the water systems, and changes in bank vegetation resulting in erosion, alter and affect the aquatic environment. Invasion by alien species has important socio-economic consequences. Box: Socio-economic Consequences of Increased Invasion of Alien Species.
Source: UNDP-GEF 2008
Human Activities Driving Biodiversity LossAgricultureAgriculture has a series of known impacts on biodiversity:
MiningMining has a series of known impacts on biodiversity:
IndustryIndustry has a series of known impacts on biodiversity:
Infrastructure developmentInfrastructure developments have a series of known impacts on biodiversity:
UrbanisationUrbanisation has a series of known impacts on biodiversity:
Mapping the Human Footprint IndexOne measure of the impact of human activities on the landscape is reflected in the Human Footprint Index, undertaken by SEDAC at Columbia University in the United States (Last of the Wild Data Version 2, 2005b), shown in the map below. This analysis shows the direct impact of human activities onthe natural environment, clearly illustrating the wide scale transformation in parts of the Orange-Senqu River basin through urbanisation and agriculture. The Human Footprint Index expresses the relative human influence in a region as a percentage. Its values range from zero to 100, with a value of zero representing the least influence, and a value of 100 representing the strongest influence.
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