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# of Endemic Species
Major Habitat Type:
xeric freshwaters and endorheic (closed) basins
Liz Day, The Freshwater Consulting Group, Cape Town, South Africa
Paul Skelton, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa
Botswana; Namibia; Zimbabwe
The semi-arid Kalahari ecoregion has no perennial natural surface water and includes some of the largest salt pans in the world (Hughes & Hughes 1992). The endorheic Ntwetwe and Sua Pans together make up the Makgadikgadi system — a salt pan complex occupying a total of about 12,000 km2 (Allan et al. 1995; Tyler & Bishop 1998). This system is itself one of four major pan systems in southern Africa; the others are Etosha, Hakskeenpan, and Grootvloer-Verneukpan (Lloyd & Le Roux 1985). This ecoregion includes the northern portion of the Kalarahi desert and its endorheic systems that sometimes flow into the numerous small pans in the south, the Makgadikgadi Depression, or the Okavango .
Main rivers or other water bodies:
The Makgadikgadi is fed by the seasonal Boteti River, which drains from the Okavango Delta, and the episodic Nata River, which rises in the Hwange National Park near the southwestern border of Zimbabwe (Hughes & Hughes 1992). North of Makgadikgadi, two other large pans, Nxai and Kgama Kgama, together cover about 230 km2 (Hughes & Hughes 1992), and immediately south of Makgadikgadi a pan named Lake Xau occurs. In addition to these large pans in the north, the Kalahari also includes numerous smaller endorheic pans, including several within the Central Kalahari Game Reserve in central Botswana. Pans in this reserve are associated with the fossil river valleys of Deception and Okwa (Tyler & Bishop 1998).
Rainfall in the Kalahari ecoregion is infrequent, unreliable, and patchy (Van Rooyen 1984), with progressively less rain falling from north to south; mean annual rainfall in northern Kalahari is 400-550 mm, and in central Kalahari is 250-450 mm (Harrison et al. 1997). Most of this rain falls during summer (October to March), often in the form of short-lived thunderstorms (Hughes and Hughes 1992). Summers are hot (reaching 45°C) and winters are cold (as low as –6°C). Potential evapotranspiration is high, exceeding precipitation rates in every month of the year (Ellery & McCarthy 1998).
Following heavy rainfall, ephemeral rivers with small catchments flow briefly, and the numerous pans, normally bare or covered with sparse grass and herbs, may retain water for a short time (Tyler & Bishop 1998). Many of these pans have calcrete floors, which aid in water retention (Harrison et al. 1997). In the north of the ecoregion, occasional limestone outcrops give rise to small freshwater pans and springs (Hughes & Hughes 1992).
The two main pans comprising the Makgadikgadi system retain water (at about depths of 15-25 cm) for somewhat longer periods than other pans in the ecoregion. Direct precipitation contributes substantially to the water level of these pans. In addition, water enters Sua Pan from the Nata River during the summer rainy season, while much later in the year, residual water from the Okavango system enters Ntwetwe via the Boteti River – the end-point of the Okavango/Boteti system (Hughes & Hughes 1992). The pans are alkaline, and salinity is usually extremely high during periods of inundation. For most of the year, however, the pans are completely dry and salt-encrusted.
Nxai and Kgama Kgama Pans rely almost exclusively on rainfall for inundation. Further south, Lake Xau formerly received water from the Boteti River during conditions of high flow, but the Mopipi Dam now intercepts this water (Tyler & Bishop 1998).
The Makgadikgadi pans are comprised of salt-encrusted sands, with an aquatic biota dominated during the rainy season by blue-green algae. Along the margins are halophytic plant communities; Sporobolus spicatus and Odyssea paucinervis grasses dominate the saline fringes, and species such as Portulaca oleracea, Sporobolus tenellus, and Suaeda fruticosa occur in salt marshes along the slightly wetter fringe areas (Hughes and Hughes 1992). The surrounding vegetation community includes grasslands, low tree and bush Acacia savanna, and stunted mopane Colophospermum mopane woodland (Tyler and Bishop 1998). The few islands isolated in the large pans are densely vegetated with woodland species. A grassy peninsula dividing Sua and Ntwetwe Pans contains a granite outcrop – Kubu Island – dotted with stunted baobab trees (Adansonia digitata) (Comley 1994).
Like the greater Makgadikgadi pans, Nxai and Kgama Kgama Pans are both set in savanna woodland or open forest. Nxai Pan, which occupies a fossil lakebed, is usually covered by short grass. The pan contains numerous small treed islands with Acacia erubescens, A. nigrescens, C. mopane, Syzygium cordata,and Adansonia digitata (Hughes and Hughes 1992). The channel of the Boteti River supports well-developed riparian woodland. In the river valley Hyphaene palms extend north to Nxai Pan. Their fruit, vegetable ivory, is used by local people for making necklaces and walking stick heads.
Extreme conditions and a scarcity of freshwater result in a depauperate fish fauna; fish enter the pans only with the floodwaters (Hughes & Hughes 1992; Skelton 1993). Most of these immigrants die upon the desiccation of the pans. The nutrient-enriched water created in the pans is therefore most important for the tens of thousands of migrant wading birds attracted to the pans’ rich source of food.
Other noteworthy aquatic biotic elements:
Barren, highly saline, subject to extremes of temperature and dramatic, unpredictable fluctuations in inundation, the Makgadikgadi pans are inhabited only by plants and animals with specializations that allow them to withstand these conditions. Invertebrates, mainly crustaceans (anostracans, conchostracans, and notostracans), are the major aquatic inhabitants of these pans (Lovegrove 1993). These organisms are opportunistic, many of them passing the dry months or years as heat- and desiccation-resistant eggs (Kok 1987). When standing water becomes available, the eggs quickly hatch and the organisms mature and begin reproducing, sometimes asexually, until conditions begin to deteriorate again, whereupon they mate sexually, produce fertile eggs, and die. The egg stages of these animals protect them from abrasion by windblown particles, from very intense sunlight, and from tiny invertebrate predators. Within the eggs, the embryos withstand long periods of diapause by becoming ametabolic (Lovegrove 1993).
During the wet season, these crustaceans provide food for greater flamingoes (Phoenicopterus ruber), while lesser flamingoes (P. minor) feed on blue-green algae. The Makgadikgadi pan system provides globally important breeding sites for both species (Harrison et al. 1997). Along with Etosha in Namibia, Makgadikgadi is the only place in southern Africa where these species breed in large numbers on a fairly regular basis, with the largest known African breeding colony of P. ruber occurring at Sua Pan. When the waters dry out in this pan, the young chicks migrate across the pan to the Nata Delta. Many thousands of chicks die en route(Penry 1994). Among other birds that breed or feed in the pans during periods of inundation is the vulnerable wattled crane (Grus carunculatus), which in wet years flocks to the pans from its stronghold in the Okavango Delta (Harrison et al. 1997).
Across the ecoregion, the scarcity of surface water is a limiting factor to the distributions of many species, and few birds use the pans when they are dry. Amphibian species are restricted to hardy opportunistic species, able to aestivate for long periods and emerge to lay rapidly-maturing eggs as soon as patches of standing water occur (Harrison et al. 2001).
The salt-encrusted sandy edges of the Makgadikgadi pans also provide habitat for an endemic lizard, the spiny agama (Agama hispida makgadikgadiensis) (Finlayson & Moser 1991). Historically, large mammalian herbivores moved between areas of standing water or good grazing associated with the mineral-enriched pans. These herbivores often traveled vast distances at a time, followed by their predators (Hughes & Hughes 1992). Today, veterinary cordon fences restrict the movement of these herds, whose numbers have also been decimated by past hunting activities (Tyler & Bishop 1998). Large herds of blue wildebeest (Connochaetes taurinus), for example, once occurred in the Makgadikgadi area, but their numbers have been drastically reduced, possibly by as much as 90% (Hughes & Hughes 1992).
Justification for delineation:
This xeric ecoregion is defined by the northern portion of the Kalarahi desert and its endorheic systems. The Makgadikgadi region is believed to be the relic of an ancient inland-draining sea, formed some 140 million years ago with the uplifting of the edge of southern Africa (Ellery & McCarthy 1998). During this time, the Cuando, Kafue, and upper Zambezi Rivers drained into the Makgadikgadi depression. Subsequent capture of these rivers by the lower Zambezi diverted flow from these systems into the Indian Ocean, with the result that the Makgadikgadi lost most of its catchment (Thomas & Shaw 1991).
Level of taxonomic exploration:
Allan, D. G., Seaman, M. T., et al. (1995)"The endorheic pans of South Africa" In Cowan, G.I. (Ed.). Wetlands of South Africa. (pp. 75-101) Pretoria, South Africa: Department of Environmental Affairs and Tourism.
Comley, P. Meyer S. (1994). "Traveller’s guide to Botswana" London, UK: New Holland Publishers.
Ellery, W. N.,McCarthy, T. S. (1998). "Environmental change over the decades since dredging and excavation of the lower Boro River, Okavango Delta, Botswana" Journal of Biogeography 25(2) 361-378.
Finlayson, M.,Moser, M. (1991). "Wetlands" Oxford, UK: Facts on File.
Harrison, J. A., Allan, D. G., et al. (1997). "The atlas of southern African birds, including Botswana, Lesotho, Namibia, South Africa, Swaziland and Zimbabwe, Volume 1. Non-passerines" Johannesburg, South Africa: Birdlife South Africa.
Harrison, J., Burger, M., et al. (2001) "Conservation assessment and management plan for southern African frogs. First Draft. January 2001. Southern African Frog Atlas Project". Cape Town, South Africa. Avian Demographic Unit, University of Cape Town.
Hughes, R. H.,Hughes, J. S. (1992). "A directory of African wetlands" Gland, Switzerland, Nairobi, Kenya, and Cambridge, UK: IUCN, UNEP, and WCMC.
Kok, D. J. (1987). "Invertebrate inhabitants of temporary pans" African Wildlife 41(5) 239.
Lloyd, J. W.,Le Roux, A. (1985) "A conservation assessment of the Verneukpan-Copperton area". Cape Town, South Africa. Department of Nature and Environmental conservation.
Lovegrove, B. (1993). "The living deserts of Southern Africa" Vlaeberg, South Africa: Fernwood Press.
Penry, H. (1994). "Bird atlas of Botswana" Pietermaritzburg, South Africa: University of Natal Press.
Skelton, P. H. (1993) A complete guide to the freshwater fishes of Southern Africa. South Africa: Southern Book Publishers, Halfway House.
Thomas, D. S. G.,Shaw, P. A. (1991). "The Kalahari environment" Cambridge, UK: Cambridge University Press.
Tyler, S. J.,Bishop, D. R. (1998)"Important Bird Areas of Botswana" In Barnes, K.N. (Ed.). The important bird areas of southern Africa. (pp. 103-122) Johannesburg, South Africa: BirdLife South Africa.
Van Rooyen, T. (1984). "The soils of the Kalahari Gemsbok National Park" Supplement to Koedoe 1984 45-61.