The Nyl River forms a 70-km-long grassy floodplain that is one of the largest in South Africa, running from 10 km south of Mookgopong north to Mokopane. The area, known as Nylsvley, is located in extensive flat to undulating terrain between 1 050 and 1 080 m a.s.l. The floodplain starts to widen at the western edge of Nylsvley Nature Reserve and in some places attains a width of 5–6 kilometres. At its northern end it narrows and assumes the character of a normal river. The only prominent hills are Maroelakop (1 140 m a.s.l.) and Stemmerskop (1 090 m a.s.l.), which are close together in Nylsvley Nature Reserve. The system derives its floodwaters from rain that falls in the nearby foothills of the Waterberg range (Waterberg System IBA). About 12 tributaries drain this area and discharge their water into the floodplain at various points along its course. In general, the climate is semi-arid, with three fairly distinct seasons: a hot, wet season from November to April, during which the Nyl River occasionally floods; a cool, dry season from April to August; and a hot, dry season from August to October. The annual average minimum and maximum temperatures are 12 °C and 26.4 °C respectively, and the mean rainfall is about 600 mm a year.
This is the largest wetland of its kind in South Africa and it is basically a grass-dominated, seasonally inundated floodplain, which in years of poor rainfall may not be flooded at all. It requires rainfall of at least 10% above the annual mean to produce significant inundation. The ephemeral nature of the wetland and the kind of vegetation and microhabitats it provides account for the floodplain being attractive to a wide variety of wetland bird species, many of which breed in large numbers in years of suitable rainfall. The dominant grass in flooded areas is Oryza longistaminata, which may grow up to 2 m tall and provides cover, food and nesting material for many wetland birds.
Bushes and trees, including many Vachellia (formerly Acacia) species, are scattered singly and in clumps throughout the floodplain and provide nesting sites for herons, bitterns and egrets. The floodplain has very little permanent reedbed vegetation. A particularly interesting aspect of the vegetation is the occurrence of the tropical grass Paspalidium germinatum, which is recorded from nowhere else in South Africa. In some of the high-lying areas the vegetation comprises broad-leaved savanna and grassland, with Eragrostis pallens and Digitaria eriantha dominating in the latter.
The area has a list of 426 bird species. The floodplain occasionally erupts with activity, holding up to 80 000 birds in years of high rainfall. Nylsvley is particularly important as it attracts large numbers of rare and locally threatened waterbirds. In particularly wet seasons, Dwarf Bittern Ixobrychus sturmii, Little Bittern I. minutus, Allen's Gallinule Porphyrio alleni and Lesser Moorhen Gallinula angulata are common. Other rare species, such as Slaty Egret Egretta vinaceigula, Streaky-breasted Flufftail Sarothrura boehmi and Baillon's Crake Porzana pusilla, breed erratically, whenever conditions are suitable. Corn Crake Crex crex occurs here in the austral summer. The wetland also occasionally supports extremely large numbers of Great Egret Egretta alba, Little Egret E. garzetta, Yellow-billed Egret E. intermedia, Squacco Heron Ardeola ralloides, Black-crowned Night Heron Nycticorax nycticorax, African Spoonbill Platalea alba and Southern Pochard Netta erythrophthalma. Yellow-billed Stork Mycteria ibis and African Grass Owl Tyto capensis occur in the flooded grasslands and Black-winged Pratincole Glareola nordmanni is occasionally seen in large numbers in the drier grassland surrounding the floodplain. Almost every species of South African duck is found here at one time or another, some in very large numbers.
The surrounding woodland holds Secretarybird Sagittarius serpentarius and small numbers of White-backed Vulture Gyps africanus. Cape Vulture Gyps coprotheres is a scarce visitor. Other woodland specials include Red-crested Korhaan Lophotis ruficrista, Southern Pied Babbler Turdoides bicolor, White-throated Robin-Chat Cossypha humeralis, Kalahari Scrub Robin Erythropygia paena, Burnt-necked Eremomela Eremomela usticollis, Barred Wren-Warbler Calamonastes fasciolatus, Marico Flycatcher Bradornis mariquensis, Crimson-breasted Shrike Laniarius atrococcineus, Southern White-crowned Shrike Eurocephalus anguitimens, Burchell's Starling Lamprotornis australis, Scaly-feathered Finch Sporopipes squamifrons, Violet-eared Waxbill Uraeginthus granatinus, Black-faced Waxbill Estrilda erythronotos and Shaft-tailed Whydah Vidua regia.
Globally threatened species are Black-winged Pratincole and Secretarybird, while regionally threatened species are African Grass Owl, Greater Painted-snipe Rostratula benghalensis and Black Stork Ciconia nigra. Common biome-restricted species are Kurrichane Thrush Turdus libonyanus, Barred Wren-Warbler, Burchell's Starling and White-bellied Sunbird Cinnyris talatala. White-throated Robin-Chat is fairly common and Kalahari Scrub Robin is uncommon. Great Egret, Squacco Heron, Black-crowned Night Heron, Dwarf Bittern, Southern Pochard, African Snipe Gallinago nigripennis, White-backed Duck Thalassornis leuconotus, African Spoonbill, Spur-winged Goose Plectropterus gambensis and White-face Whistling Duck Dendrocygna viduata are congregatory species.
Threatened species that may be found in Nylsvley Nature Reserve are African rock python Python sebae, Jalla's sand snake Psammophis jallae, greater dwarf shrew Suncus linex, Kuhl's bat Pipistrellus kuhliisubtilis, honey badger Mellivora capensis, Selous mongoose Paracynictis selousi, aardwolf Proteles cristatus, brown hyaena Hyaena brunnea, leopard Panthera pardus, African wildcat Felis lybica and aardvark Orycteropus afer. Roan antelope Hippotragus equinus and tsessebe Damaliscus lunatus have been re-introduced to the reserve.
The South African endemic Lowveld flat gecko Afroedura langi, Natal purple-glossed snake Amblyodipsas concolor and northern crag lizard Pseudocordylus transvaalensis may occur within the area, as may the southern African endemic Kalahari tent tortoise Psammobates oculiferus in the arid savanna and Van Dam's round-headed worm lizard Zygaspis vandami on alluvial sands in mesic savanna.
The system is reliant on rain falling in the Waterberg (IBA SA007) and inundating the plain. Any impoundment or disturbance to the flow of the handful of rivers that contribute to the floodplain could seriously impact Nylsvley. The river is subjected to small-scale damming, dykes and the extraction of sand, all of which may alter the delicate flooding regime that drives this dynamic system. Another threat to the IBA is the large amount of water extracted from the floodplain by centre-pivot irrigation schemes. Uncontrolled fires are a problem, as demonstrated by the fire that devastated large parts of the floodplain in 2013. On the other hand, regular block burns are not conducted sufficiently on the floodplain, which is leading to the encroachment of bush and the loss of grassland, as well as some grassland patches becoming moribund. Part of the reason for this is that the wetland remains wet in winter, making it difficult to conduct burns in it. In 2013–2014 a number of applications were received to mine on the floodplain. If approved, mines could have a detrimental impact on the trigger species in the IBA.
The Nylsvley Nature Reserve is located within the IBA and was established in 1967. It is also a Ramsar site. The reserve is well managed and has a management plan, although there is not enough funding or staff to implement it. The management plan also does not address issues relating to bird conservation in the reserve, for example the management of the grasses in the wetland. It is hoped that in 2015 an area east of Mokopane will be declared the Grootvaley Protected Environment. A concerted effort should be made to obtain Protected Environment status for the area outside the nature reserve.
The Friends of Nylsvley provide valuable support to the reserve’s staff.
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