Boundaries

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 [569].

Freshwater habitats

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).

Terrestrial habitats

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.

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

Poor