Major threats to freshwater species in southern Africa

Major threats to freshwater species in southern Africa.

Loss and degradation of habitat is the leading threat followed by water pollution and alien invasive species.

The Olifants and Berg river systems, both in the Western Cape area, support the highest numbers of regionally threatened species.

The Olifants river system: The main threat to fishes is the introduction of invasive alien fishes. The introduction of Smallmouth Bass (Micropterus dolomieu) in the 1930s for angling purposes is regarded as a major threat, with predatory impacts also from Bluegill Sunfishes (Lepomis macrochirus) and Rainbow Trout (Oncorhynchus mykiss). Tilapia sparrmanii has also been introduced and is reported to compete with native species for food. In another case, Sandelia capensis, a species widespread throughout the Western Cape, was introduced to the Olifants River System, where it is not native, via a local farmer in a misguided attempt to use an “indigenous” fish for mosquito control. It is now common and widespread in the Suurvlei River where it competes with Barbus erubescens, a native species, for food and it possibly also predates on juveniles. In another case where a native species has been introduced to an area outside its normal range, Labeobarbus capensis has been introduced above three waterfall barriers into the upper and middle Twee River by the Cape Department of Nature Conservation in an attempt to create a sanctuary for an indigenous fish. It is thought that this has had a negative impact on populations of B. erubescens above the falls. As a final example, Pseudobarbus phlegethon has gone extinct in the Jan Dissels River, a major tributary of the Olifants River, and is no longer found in mainstream areas where it probably occurred before the introduction of alien fishes.

Habitat degradation is another substantial threat to fishes as the Olifants catchment and some tributaries are the focus for intensive citrus, deciduous fruit and vineyard development. The major problem is over-abstraction of water during the dry summer months and the planting of orchards within the 1:1 yr floodline of the river. The absence of a natural riparian zone, and hence buffer area between the river and intensive agriculture, allows fertilisers and copper-based pesticides easy access to the aquatic environment. Two large instream dams (Clanwilliam and Bulshoek) act as barriers to fish migration, and rivers are over-abstracted to fill hundreds of smaller farm dams. Instream dams prevent adults of migrating Labeo seeberi from reaching spawning grounds. The lower reaches of many tributaries have been bulldozed and canalised for flood protection purposes. The excess use of fertilisers and pesticides (many copper based) also poses a substantial threat to indigenous fishes.

The main threats to plants include the drying up of wetland areas, such as small ponds, marshlands and temporary pools, in association with urbanisation. For example, four of the twelve recorded locations of Aponogeton angustifrons have been lost due to urbanisation of Cape Town and Stellenbosch over the past 120 years. These wetland areas are also suffering from increased ploughing for conversion to agriculture, in particular for wheat. This is a major past and future threat to Romulea aquatica which lives in vernal pools completely surrounded by wheat fields. The farmers are largely unaware of the biodiversity within the pools that are being degraded by conversion, by infilling, and through trampling by large numbers of stock. In the case of Moraea stagnalis the main threat is from land conversion for production of Rooibos tea. Oxalis uliginosa is a restricted range plant species threatened by loss of habitat through dam construction where the habitat is completely lost when areas are inundated. As in the case of fish, there is a significant threat to many restricted range species from invasive alien species.

Dragonflies are also suffering due to over-extraction of water leading to loss of habitat. The streams in the Ceres area (where Metacnemis angustata was recorded in 1920) have been radically transformed and some no longer flow at all due to over-extraction of water for the fruit industry. Other threats come from shading of the habitat by alien invasive trees, such as Acacia spp., and loss of habitat through damming of streams. Alien fishes, especially rainbow trout, may also be a threat due to their predation of dragonfly larvae.

The Berg river system: The main threats to the two threatened fish species, Barbus andrewi and Pseudobarbus burgi are once again invasive alien fishes and habitat degradation, with experts agreeing that the predatory impacts of smallmouth bass (Micropterus dolomieu) are the most significant factor driving B. andrewi to local extripation from the Berg. Bass (Micropterus spp.) dominate preferred Barbus andrewi habitat and have effectively halted their recruitment. Over-abstraction of water and pollution have largely reduced the river catchment to a haven for other alien fish species such as Cyprinus carpio and Oreochromis mossambicus both of which are competitors to Barbus andrewi. The main threat to Pseudobarbus burgi, endemic to the Berg River system, is Micropterus dolomieu which has reduced the species to isolated populations in upper reaches of tributary streams. The Rainbow trout (Oncorhynchus mykiss) has a further impact higher up in some of the tributary streams where they are able to survive due to colder temperatures. The relatively recent introductions of another alien species, Clarias gariepinus, will need to be monitored for their potential impact.

As in the Olifants, the main threats to the two threatened odonates, Ecchlorolestes peringueyi and Ceratogomphus triceraticus include alien invasive trees (pines and Acacia longifolia) which shade the habitat and alien invasive rainbow trout which are thought to predate on the larvae. The former threat of habitat removal (mostly for plantation forestry) has largely subsided although C. triceraticus is still threatened by loss of habitat to the wine industry and, to a lesser extent, cattle farming and plantation forestry. Over-extraction of water from streams and possibly pollution from the wine industry are increasing threats.

As in the Olifants system the main threats to plants include the drying up of wetland areas, such as small ponds, marshlands and temporary pools, in association with urbanisation and increased ploughing for conversion to agriculture, in particular for wheat. This is again a major past and future threat to Romulea aquatica which lives in vernal pools completely surrounded by wheat fields. Livestock grazing and trampling, especially by cattle and horses, has likely led to the loss of Cadiscus aquaticus from many of its historic localities. Infilling of wetlands, mechanical damage by heavy machinery, and invasion by alien grasses caused by dumping of cattle feed in dry pools during summer are also significant threats. Eutrophication resulting from run-off of fertilizers used on surrounding ploughed lands is also a problem. One population of Cotula vulgaris is known to be under threat due to groundwater pumping.