Ecoregions

Zambezian halophytics

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Magkadigkadi Pans Central District, Botswana. Source: R.D. Taylor

The Zambezian halophylics ecoregion includes two spatially disjunctive units in Southern Africa: The Makgadikgadi Pan complex in Botswana and a smaller hypersaline unit in southern inland Mozambique. One of the largest saltpans in the world, the Makgadikgadi Pan complex in Botswana stretches out over 12,000 square kilometers. Surrounded by the semi-arid Kalahari savannas, the pans experience a harsh climate, hot with little rain, and are normally a vast, glaring expanse of salt-saturated clay. These pans are sustained by freshwater from the Nata River, and more infrequently, from input from the Okavango Delta by way of the Boteti River. Saline- and drought-tolerant plant species line the pans, with grasslands further out. A wide variety of wildlife can be found on the grasslands, including ungulates and large predators. The pans are also the only viable breeding site for flamingos in Southern Africa, and harbour a variety of other bird species. However, they are increasingly threatened by exploitation for soda mining, and wildlife are constrained by veterinary fences. This ecoregion also includes the scattered saline grasslands around the Changane Valley in southern Mozambique. This latter grassland area has endured a lengthy civil war, and more recently, devastating floods.

Location and general description

The Zambezian halophytics ecoregion covers the Makgadikgadi Pan complex in Botswana. The Makgadikgadi Pan is an ephemeral hypersaline lake complex situated in the northeastern part of Botswana, southeast of the Okavango Alluvial Fan. Covering over 12,000 square kilometres, it is one of the largest saltpan complexes in the world. The complex consists of two major pans, Ntwetwe Pan (106 x 96 kilometres) and Sua Pan (112 x 72 kilometres), surrounded by a number of smaller pans. The largest of these are Nxai Pan, Kgama-Kgama Pan and Kudiakam Pan, all situated to the north of Ntwetwe Pan. To the south is Lake Xau, which is no longer a lake, but rather a saline grassland. Halophytic communities are widespread in the Changane Valley in southern Mozambique, which runs in a north-south direction generally tracing the 34°E line of longitude, from about 22°S to about 24°S latitude. The saline affinity vegetative communities scattered throughout this valley range from wetlands to seasonally flooded grasslands.

caption Source: WWF

The Makgadikgadi Pans complex is situated within the Kalahari Desert - a flat, semi-desert plain extending across much of central Southern Africa. The climate of the Kalahari is semi-arid, with droughts occurring in an approximately seven-year cycle. The rainfall, which is highly variable, falls mostly in the summer months, between October and March, with November to February generally being the wettest months. Most of the rain falls as thunderstorms, depositing between 15 millimeters (mm) and 90 mm of rain within a few hours. During the austral winter months (May to August) there is virtually no rain, and no surface water to sustain vegetation. The mean annual rainfall at the Makgadikgadi Pans is between 450 and 500 mm, with between 40 and 50 percent variability. The pans have a harsh climate, with high diurnal and seasonal temperature ranges. Midday summer temperatures average around 35°C and reach a maximum of 44°C. Austral winter is a long, dry period of sunshine and cloudless days with midday temperatures up to 17°C, dropping to a low of – 8°C at night. In late austral winter, high velocity winds blow across the pans, restricting visibility by stirring up the considerable loose saline surface soil. According to White, the Changane Valley of southern Mozambique is also semi-arid, with an annual rainfall of between 400 and 600 millimetres, mostly falling between October and April. In the past few years however, rainfall in this area has increased dramatically causing major flooding and widespread devastation. In March 2000, 500 mm fell in merely three days.

The Makgadikgadi Pans are in a depression (the Kalahari Basin) that once held an enormous prehistoric lake that covered most of northern Botswana. The Zambezi, Okavango and Kwando/Chobe Rivers fed this ancient Lake. The formation of various faults on the southern extremity of the East African Rift Valley diverted the flow of these rivers away from the lake, causing it to slowly dry up. This drying process concentrated salts in the lake bed, eventually leaving flat, salt-saturated clay pans (the Makgadikgadi Pans complex). The lake was its deepest in the area of the Ntwetwe and Sua Pans, which were the last areas to dry up, and are today the most saline. Steep escarpments of white calcrete, marking the perimeter of the ancient lake, run along the shores of the pans, rising between 70 and 100 meters (m) above their surface. The only body of water to remain from the once massive lake is the Okavango Alluvial Fan in the northeast of Botswana.

There are two rivers that are important to the Makgadikgadi Pans complex. The first of these is the Boteti River, which flows sporadically out of the Okavango Delta and empties into the southern portion of Ntwetwe Pan. Owing to massive evaporation rates and a sandy substrate, the outflow of the Okavango Alluvial Fan is only five percent of its inflow. It is these dregs that flow irregularly into the Boteti River (or into the Nhabe River to the west of the ecoregion). Low rainfall in the Angolan Highlands has meant that in recent years little water has flowed from the Okavango River into the Boteti River, and the Ntwetwe Pan has not received water from the Boteti in over 10 years. The Boteti River also once fed Lake Xau, to the south of the ecoregion, but the Mopipi Dam now intercepts this water and Lake Xau has been dry since 1983. The Nata River, which originates in Zimbabwe, is the most important river in the complex. This river flows more reliably and empties into the northeast of Sua Pan at the Nata Delta, an area of the pan that rarely dries out completely. The Nata delta is essential in sustaining wildlife through the dry austral winter months. During the rainy season Sua Pan is flooded (usually to about 30 centimeters (cm) deep) by inflow from the Nata River. In some years, however, flooding from the river does not occur and the pans are filled only by direct rainfall. In years of poor rainfall the pans may not be flooded at all.

The pans of the Makgadikgadi complex are saline deserts that are almost devoid of macrophytic vegetation. The dominant plant form on the pans is a thin layer of blue-green algae, which covers their surfaces during the rainy season. On the saline fringes two macrophytes predominate; Sporobolus spicatus and the spiny grass Odyssea paucinervis. Salt marshes are found scattered around the wetter fringes of the pans. These marshes support species such as Portulaca oleracea and Sporobolus tenellus. Surrounding the pans at a larger scale (and on less brackish soil) are grasslands dominated by Odyssea paucinervis and Cynodon dactylon with Cenchrus ciliaris and Eriochloa meyeriana dominating the crests of calcrete escarpments. These grasslands have few trees, except to the west where Hyphaene palms fringe the drainage lines, extending north to Nxai Pan. The fruit of these palms is known as vegetable ivory and is used by the local people to make necklaces and heads for walking sticks. To the north and northwest of Ntwetwe Pan individual Baobab (Adansonia digitata), Acacia kirkii, and Acacia nigrescens trees are found scattered throughout the grassland. A grassy peninsula divides Sua and Ntwetwe Pans. A 20 metre high granite outcrop, known as Kubu Island, is located on this peninsula and is dotted with giant baobab trees. Further north, around Nxai Pan, the vegetation is typical of the Kalahari and grades into the Kalahari Acacia-Baikiaea Woodland ecoregion. Nxai Pan itself is a grassland scattered with Acacia trees. Kgama-Kgama Pan, to the north of Nxai Pan is similarly vegetated. Kudiakam Pan, to the south of Nxai Pan, is devoid of macrophytic vegetation, and the notable "Baines Baobabs" are situated on its shores. This impressive group of baobabs are named after the artist Thomas Baines, who painted them in 1862. The vegetation to the south of the Makgadikgadi Pans complex grades into mopane tree savanna dominated by Colophospermum mopane with varying proportions of Acacia nigrescens, Terminalia prunioides and Combretum imberbe. Near the southeastern corner of the Makgadikgadi National Park are a few cactiform (cactus-like stem succulent) plants such as Hoodia lugardii. They have one-meter high thorny stems and masses of star-shaped brown flowers that appear in spring.

In the moderately saline areas of the Changane valley grasslands predominate, scattered with islands of Acacia nilotica kraussiana. The more frequently flooded areas have a higher salinity and are dominated by grasslands interspersed with extensive bare patches. The dominant grasses in these saline flooded grasslands are Eriochloa meyeriana, Sporobolus nitens, and Aristida adscensionis. Near the Changane River, salinity is even higher and species of Arthrocnemum, Salicornia, Atriplex, and Saueda predominate.

Biodiversity features

caption Makgadikgadi Plain, Botswana. Source: Judy Oglethorpe The harsh climate of the Makgadikgadi Pans is unsuitable for most animals. Temperatures are extreme, winds are fierce, vegetation is scarce and water, when present, is salty. The only fauna to permanently inhabit the pans are highly specialized invertebrates. These invertebrates, mainly crustacea, are adapted to withstand the long dry periods and to respond rapidly to rainfall. They have short life cycles and desiccation-tolerant eggs and many can hatch, grow and produce eggs within 24 hours. Ostriches (Struthio camelus) are found nesting on the pans as it affords them protection from scavengers, such as the black-backed jackal (Canis mesomelas).

caption Meerkats in the morning sun, Makgadikgadi. @ C. Michael Hogan Most other animals inhabit the grasslands surrounding the pans. These include red hartebeest (Alcelaphus buselaphus), gemsbok (Oryx gazella), springbok (Antidorcas marsupialis), steenbok (Raphicerus campestris), kudu (Tragelaphus strepsiceros), giraffe (Giraffa camelopardus), Burchells zebra (Equus burchelli), blue wildebeest (Connocheatus taurinus), black-backed jackal, brown hyaena (Hyaena brunnea), spotted hyaena (Crocuta crocuta), lion (Panthera leo), cheetah (Acinonyx jubatus), wild dog (Lycaon pictus) and even elephant (Loxodonta africana) along the Boteti River. The Nxai Pan has a large springbok population and is one of the few places where springbok and impala cohabit. These two antelope are normally separated by habitat preference, but the Acacia savanna surrounding Nxai Pan provides the impala with a suitable habitat while the grass covered pan mimics the desert conditions preferred by springbok. During the dry austral winter season, when surface water throughout the Makgadikgadi Pans dries up, the large mammals congregate around the permanent pools along the Boteti River. After good rains they move away from the river in search of better grazing, usually north to the Nxai Pan National Park. They return to the river when the rains cease, usually in early May. The area around the Nata Delta is stocked with cattle and competition for grazing has driven wildlife away except for the occasional springbok or black-backed jackal.

There are many reptiles such as tortoises, rock monitors, snakes and lizards inhabiting the grasslands surrounding the pans. The Makgadikgadi spiny agama (Agama makarikarica) is endemic to the Makgadikgadi Pans complex. This agama inhabits the edges of the pans but it is difficult to see as it buries itself in the sand during the heat of the day.

For most of the year the pans are devoid of birds, except for ostriches and species such as the chestnut-banded and Kittlitz’s plover (Charadrius pallidus, C. pecuarius). The only hospitable area during these times is the Nata Delta, which has a permanent water source and a small resident population of waterbirds including grebes (Podiceps spp.), cormorants (Phalacrocorax spp.), ducks and plovers (Charadrius spp.) with a few flamingos (Phoenicopterus ruber, Phoeniconaias minor) and pelicans (Pelecanus spp.). The grasslands surrounding the pans support a moderate bird fauna with species such as ostriches, secretary birds (Sagittarius serpentarius), kori bustards (Ardeotis kori), korhaans (Eupodotis spp.), sandgrouse (Pterocles spp.) and francolin (Francolinus spp.) being common. The Hyphaene palms to the west of the pans are nesting sites for, among others, the greater kestrel (Falco rupicoloides) and the palm-nut vulture (Gypohierax angolensis). After good rains the pans are transformed into a vibrant paradise, attracting thousands of waterbirds, most of which come to breed on the pans. Wattled and southern crowned cranes (Grus carunculatus, Balearica regulorum), saddle-billed, marabou and open-billed storks (Ephippiorhynchus senegalensis, Leptoptilos crumeniferus, Anastomus lamelligerus), African fish eagles (Haliaeeetus vocifer), black-necked grebes (Podiceps nigricollis), Caspian terns (Hydroprogne caspia), eastern white and pink-backed pelicans (Pelecanus onocrotalus, P. rufescens), geese and waders such as avocets (Recurvirostra avosetta), black-winged stilts (Himantopus himantopus), plovers, sandpipers and teals (Anas spp.) congregate around the pans. The most spectacular arrival are the greater and lesser flamingos (Phoenicopterus ruber and Phoeniconaias minor) that flock to the pans in their thousands. The Makgadikgadi Pans are designated a "Wetland of International Importance" and an "Important Bird Area in Botswana".

The flamingos migrate to the pans from Namibia and East Africa to feed on the newly hatched shrimps and crustaceans (greater flamingos) or on the flourishing blue-green algae (lesser flamingos) and to breed on the flat pans. Greater and lesser flamingos breed at two sites in southern Africa, Etosha Pan in Namibia and the Makgadikgadi Pans, but only the Makgadikgadi Pans have proved to be viable. Sua Pan itself is the largest breeding area in Africa for the greater flamingo. When the waters once again dry out, the young chicks migrate northwards across the pan towards the Nata delta, with several thousand chicks dying of exhaustion en route.

Prehistory in the ecoregion

Much like the origins of man in the Waterberg and Transvaal regionsto the south, (Hogan, 2008) significant recovery of Early Stone Age tools has derived from Makgadikgadi as vestiges of habitation by Homo sapiens and even Homo habilis. A considerable amount of the extant collection is exhibited at Jack's Camp Museum located in the north end of the pans. Field reconnaissance in the dry lakebeds continues to yield further surficial specimens of early Stone Age tools and late Stone Age projectile points.

Late Stone Age man would have lived here in huts made of sticks and grass in nomadic groups of 15 to 60 individuals. The women would have concentrated on gathering fruits and nuts, while men created snares of twine to catch springhares emerging from their burrows. Springbok and eland were hunted by spear and arrow, as the Later Stone Age tools had become quite sophisticated. Although no rock paintings are found in the pans, somewhat north in the Tsodillo Hills there is a wealth of animal and human subject rock art, some dating as early as 24,000 years before present.

Current conservation status

The Zambezian Halophytics are relatively undisturbed, and large blocks of natural habitat remain intact. The human population in the Makgadikgadi Pans area is very low, and large expanses of this area are uninhabited. Most of the ecoregion is, however, not formally protected.

The extreme west of Ntwetwe Pan is protected under the Makgadikgadi National Park and Nxai Pan, together with Kgama-kgama Pan, is protected in the Nxai Pan National Park. In 1993 these two parks were amalgamated to form the new Makgadikgadi and Nxai Pan National Park, which covers an area of more than 5,500 km2. During the amalgamation, the Nxai Pan National Park was extended southwards to meet the Makgadikgadi National Park (along the Maun to Nata road). This extension included the Kudiakam Pan and Baines Baobabs within the new park. While the new Park is extensive, it does not include much of Ntwetwe Pan and completely excludes Sua Pan, which is the only viable breeding site of the greater and lesser flamingo in southern Africa. The Nata community has established its own informally protected area, the Nata Sanctuary. This reserve includes the northern area of Sua Pan and the Nata Delta. The reserve is managed by the local people for the sustainable benefit of both people and wildlife in the area, and provides protection to an important breeding area for waterbirds, particularly the white pelican, and for Palaearctic migrants. However, the reserve does not incorporate the flamingo breeding area, which remains outside of the protected areas network.

Types and severity of threats

There is a small, low technology salt mine on the shores of Ntwetwe Pan, which is not seen as a major threat to the pan. Of more importance is the soda ash extraction plant on Sua Pan, situated on a spit at its northeastern edge. This plant, constructed in 1989 by Soda Ash Botswana, consists of a network of wells covering an area of 500 square kilometres. A brine solution is pumped from the wells to the main plant where soda ash and salt are extracted. Water is not a major requisite for the process and fears that water will be diverted from the Okavango Alluvial Fan to the plant are groundless. The size of the plant is relatively small compared with the total area of Sua Pan and it is not expected to have much direct impact on the pan. The threat that this plant poses to the area is associated with its infrastructure; roads, a railway, power lines and a township have already been established around the plant. However, the plant has stimulated the local economy and has also shown commitment to the environment. The company has liased with the Kalahari Conservation Society on a number of environmental issues and have even buried their power lines to prevent injury to flying flamingos at night. The plant is located directly between flamingo feeding and nesting grounds.

Uncontrolled tourism, particularly motorbike tours, is a major threat to the fauna of the Makgadikgadi Pans. Sightseeing parties and vehicles disturb breeding waterbirds, particularly flamingos and pelicans, causing them to become apprehensive and even to desert their eggs and young.

Another threat to the Makgadikgadi Pans complex is the potential alteration of water flowing into the pans for irrigation schemes elsewhere in the region. A major diversion of water flow from the Nata River, for example, would have a deleterious impact on the ecology of the pans. It would reduce flooding in Sua Pan, leading to huge flocks of flamingos, pelicans and other waterbirds suffering breeding ground diminution. The flamingos could then lose their only viable large breeding ground in southern Africa. Water diversion could also result in the drying up of the Nata Delta, an essential area for waterbird survival during the dry austral winter months. Diversion of water from the Boteti River would have similar devastating effects, as the permanent pools of this river are an essential over-wintering area for mammals and birds in the area.

A major threat to wildlife throughout Botswana has been the erection of veterinary cordon fences. These fences have an indirect effect on the Lake Xau area of this ecoregion. Wildebeest and hartebeest, which spend the wetter months along the boundary between Ghanzi and Kgalagadi (in the Kalahari Xeric Savanna ecoregion), move northeast towards the Central Kalahari Game Reserve during the dry season. Here they encounter the Kuke and Makalamakedi veterinary fences and are channeled into the Lake Xau and Boteti River areas. This has caused major overgrazing in these areas and the antelope, in competition with cattle for food, die of starvation. In the drought of the 1980’s, 90 percent of the wildebeest population and 83 percent of the hartebeest population died. Fences to the north of the Makgadikgadi Pans have similarly prevented the southwards migration of buffalo and zebra from the Okavango Alluvial Fan during the wet season leading to a massive decline in populations since 1987. The veterinary cordon fences have been erected without environmental impact assessments and are having a large impact on populations of wild ungulates. A veterinary fence protrudes into the eastern area of Sua Pan. This fence caused the entanglement and death of immature flamingos until the Department of Animal Health and Production removed the mesh from the fence in 1997. Unfortunately the fence continues to impact the flamingo population as significant numbers of adults die every year through collisions with the fence wires.

The cattle industry in Botswana is a threat to the areas surrounding the pans and to the Makgadikgadi and Nxai Pans National Park. Overgrazing in these areas has put cattle in direct competition with wildlife. The government has strongly supported and subsidized the cattle industry and in many cases, overstocking, poor range management and illegal occupation of protected land have resulted. Cattle are fast encroaching on the unfenced Makgadikgadi area of the National Park, particularly along the Boteti River.

Illegal hunting is also a threat to the wildlife surrounding the pans. Poaching within the Makgadikgadi and Nxai Pans National Park is rife and there is little enforcement of hunting limits outside of the Park. Legal hunting of large game birds is negatively impacting populations as there is no bag limit or closed season.

Justification of ecoregion delineation

This ecoregion is geographically and biologically distinctive. It encompasses the entirety of the saline Makgadigkadi Pans and associated fringing vegetation in Botswana, as well as the halophytic vegetation assemblies of the Changane Valley.

References

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Disclaimer: This article contains some information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

 

 
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Citation

Fund, W. (2014). Zambezian halophytics. Retrieved from http://www.eoearth.org/view/article/157212

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