Kalahari xeric savanna
The Kalahari xeric savanna is an ecoregion in southern Africa characterized by a harsh climate, where temperatures may increase by 45°C from night to day, and rainfall is infrequent. Rain falls only during the summer on the reddish-brown Kalahari sands , pelting the savanna with violent, localized storms. Although this area is semi-arid, there is an impressive diversity of migratory birds and large mammals, both herbivorous and carnivorous. A considerable amount (approximately 18 percent) of this ecoregion is protected. Where it is not protected, heavy grazing has degraded the habitat. Fences are a serious problem because they obstruct the migratory routes of ungulates, and they pose significant threats to biodiversity in unprotected areas.
Location and general description
The Kalahari xeric savanna ecoregion stretches across northwestern South Africa, southern Botswana and southeastern Namibia. Most of it lies on the level plains of the Kalahari Basin, interrupted by long parallel sand dunes in the south. The Kalahari sands themselves extend from the northern Cape in South Africa to the Democratic Republic of Congo, and there is yet to be consensus regarding their origin or age. The sands of this ecoregion vary in depth, and are underlain mainly by calcrete. In places, the rock below the sands harbors substantial mineral wealth, in the form of diamonds, copper, and coal. The Kalahari sands are generally nutrient poor. A thin layer of iron oxide is responsible for their reddish-brown color, although water leaching in areas of higher rainfall or near pans commonly causes the color to fade.
There has been a long term hydrological change in this region, which began with the diversion of a major river flow through the Botswanan portion of the region initiated by a geological rift zone anomaly hundreds of thousands of years ago. This flow alteration eliminated one of the largest permanent lakes in all of Africa, and replaced that depression with a seasonal wetland, the Makgadikgadi Pans. This change affected not only the water table, but also the amount of evaporation and total humidity and precipitation of the region.
Temperature fluctuations in this ecoregion are extreme. In the southern Kalahari, the temperature on winter nights can plummet to 1°C, while soaring to 30°C during the day. Similarly, a cold summer night may drop to 5°C, whereas daytime temperatures may exceed 45°C. Nothing is gentle in this hostile environment. Even the rain, when it falls, arrives most frequently in violent but brief thunderstorms. Rainfall is also remarkably patchy, with great differences occurring between sites only a few kilometers (km) apart. Average annual rainfall is highest in the northeast and lowest in the southwest, ranging between 150 and 500 millimeters (mm). Inter-annual variability in rainfall increases from 25 percent in the east to 40 percent in the west. Latitude, high atmospheric pressure, and the barrier created by the Drakensberg Mountains between the Kalahari and the Indian Ocean are largely responsible for this climate. Topography and elevation are also influential, and although the Kalahari xeric savanna occurs at elevations between 600 meters (m) and 1600 m, most of it lies above 1000 m.
In less arid areas, the vegetation is open savanna with grasses (Schmidtia spp., Stipagrostis spp., Aristida spp., and Eragrostis spp.) interrupted by trees (e.g. camelthorn (Acacia erioloba), grey camelthorn (A. haematoxylon), shepherd's tree (Boscia albitrunca), false umbrella thorn (A. luederitzii), blackthorn (A. mellifera), and silver cluster-leaf (Terminalia sericea). Shrubs include Grewia flava, Ziziphus mucronata, Tarchonanthus camphoratus, Rhigozum trichotomum, Acacia hebeclada, and Lycium spp. In drier areas, most large trees occur in ancient riverbeds, and the rolling red dunes are sparsely populated by smaller A. erioloba, A. haematoxylon, and B. albitrunca trees, as well as broom bushes (Crotalaria spartioides) and dune reeds (Stipagrostis amabilis). The creeping tsamma melons (Citrullus lanatus), gemsbok cucumbers (Acanthosicyos naudinianus), and wild cucumbers (Cucumis africanus) are important sources of water and food for both humans and animals.
Plant species richness per unit area in the Xeric kalahari savanna is among the lowest of all the southern African ecoregions, and it is estimated that less than three percent of the plants are endemic. Animal endemism is also low; there are no strictly endemic birds and only three near-endemics, only one near-endemic amphibian, one strictly endemic reptile (Typhlosaurus gariepensis) with nine near-endemic reptiles, and a single near-endemic small mammal, Brants's whistling rat (Parotomys brantsii).
However, despite the low rates of endemism, the diversity of large mammals at all levels of the food chain is remarkable for such an oligotrophic and arid system. Some flora and fauna are almost synonymous with the region. These include the camelthorn tree, gemsbok (Oryx gazella), sociable weaver (Philetairus socius), and Kalahari lion (Panthera leo).
Although not a separate species, the Kalahari lion exhibits behavioral adaptations to surviving in this harsh environment. It lives in relatively small groups, has larger home ranges and hunts smaller prey more often than lions of more mesic areas, although larger prey, particularly gemsbok, still represents the greatest mass of food eaten. Its appearance is also different, being taller at the shoulder, lighter, and with many of the males possessing black manes. The camelthorn is the only common large tree in the region. It provides nesting and foraging sites, shade, food, and refuge for a variety of plants and animals. The animals of the Kalahari Xeric Savanna show a range of adaptations to the extremes of this arid environment. While gemsbok are marvelously physiologically adapted, the impressive communal nests of the sociable weaver (up to six metres long and two metres high, weighing as much as 1,000 kilograms (kg) and housing up to 300 birds) are so well insulated that they substantially buffer the temperature extremes of the outside air.
Apart from the Kalahari lion, the ecoregion boasts an impressive array of other large predators, mainly in protected areas. These include the cheetah (Acinonyx jubatus VU), leopard (Panthera pardus), spotted (Crocuta crocuta LR) and brown (Hyaena brunnea LR) hyena, and wild dog (Lycaon pictus EN). The representation of smaller vertebrate predators is also remarkable. Among the mammals are the aardwolf (Proteles cristata), caracal (Felis caracal), black-backed jackal (Canis mesomelas), honey badger (Mellivora capensis), African wild cat (Felis lybica), black-footed cat (Felis nigripes), striped polecat (Ictonyx striatus), common genet (Genetta genetta), bat-eared and Cape fox (Otocyon megalotis, Vulpes chama), as well as meerkat (Suricata suricatta) and three species of mongoose, banded (Mungos mungo), slender (Herpestes sanguinea), and yellow (Cynictis penicillata).
Raptors include the secretary bird (Sagittarius serpentarius), a number of eagles including the martial eagle (Polemaetus bellicosus), a variety of owls, including the giant eagle owl (Bubo lacteus) and a collection of falcons, goshawks, kestrels, and kites. Among the reptilian predators are the boomslang (Dispholidus typus typus), Cape cobra (Naja nivea), puff adder (Bitis arietans), and rock monitor (Varanus exanthematicus albigularis), as well as geckos, lizards, and skinks. As in many arid areas, the amphibian fauna is not particularly species-rich, but does include the giant bull frog (Pyxicephalus adspersus) that has a diverse diet, preying on small birds, rodents, reptiles, and insects. A number of scorpion species also inhabit the ecoregion, of which the two most important families are the Scorpionidae and Buthidae. Both prey predominantly on insects, and species of the latter often prey on the less venomous members of the former.
Although the cheetah is the fastest predator on land (capable of reaching speeds of 100 km per hour), its slight build and timid disposition make it subordinate to other carnivores, and it is often robbed of its prey. To avoid this, the cheetah usually hunts during the day, when most of its competitors are inactive. Its main prey are springbok (Antidorcas marsupialis, LR), although when game is scarce, it will also hunt young or weak hartebeest (Alcelaphus buselaphus, LR), gemsbok, and wildebeest (Connochaetes taurinus, LR), as well as duiker (Sylvicapra grimmia) and springhare (Pedetes capensis), among others.
When antelope numbers are low, porcupines (Hystrix africaeaustralis) feature prominently in the diet of the Kalahari lion. Numerically, they can represent as much as 32 percent of the annual kill. The porcupine's sharp quills make hunting them hazardous. If the quills cannot be removed from the skin, they can cause severe, and even fatal, infections. In times of scarcity, however, porcupines offer a high fat meal on a limited menu.
The Kalahari's ungulates have evolved reproductive strategies that minimize the risk of predation on the young. Females of wildebeest and springbok herds give birth within a week or two each other. Because the young are very vulnerable for only about the first month of their lives, and most predators are territorial (thus limited in number, eating only until satiated), the chances of survival for each calf or lamb are much greater if births are synchronous than if they were staggered.
Approximately 18 percent of the ecoregion falls within protected areas, the largest being the Central Kalahari (although not all of this reserve is within the ecoregion) and adjoining Khutse Game Reserves in Botswana. Together they cover 77,800 square kilometers (km2). The Kgalagadi Transfrontier Park (KTP), Africa's first Peace Park, was recently proclaimed and is also large, at 34,390 km2. The KTP is the official union of the former Kalahari Gemsbok National Park (South Africa) and the Gembsok National Park (Botswana). Fortunately, game have always been able to move freely between the two countries, but the establishment of the Peace Park opens the possibility of tourists having the same freedom of movement. Common strategies for both conservation management and tourism development will also be shared across the border.
Types and severity of threats
Fences (farm, veterinary and border) are primarily responsible for the precipitous decline in wild animal populations in the Kalahari since the 1960s. Construction of veterinary fences began in 1954, but continues today. Animals like the blue wildebeest and hartebeest, not truly adapted to aridity, must migrate to obtain food and water. In the urge to migrate, some animals become caught in the fences, while others suffer slow and painful deaths through thirst and starvation. After mining, the cattle industry is Botswana's second largest revenue earner. Botswana's agreement with the European Union in terms of Lomé Convention concessions means it is paid up to 60 percent more for beef than its competitors in the open market, and receives 92 percent of imposed import tariffs back as subsidies. These incentives have encouraged the increase in Botswana's national herd to more than three million animals, twice the number of the human population. The EU's strict import regulations are behind the construction of veterinary fences, through the requirement that cattle be isolated from game, in spite of the fact that conclusive proof that foot-and-mouth disease originates in buffalo or Blue wildebeest is still lacking. Moreover, it is open to question whether the benefit distribution is equitable, because along with cattle smallholders, large-scale farmers and middlemen also profit from the protocol. In some ways, however, the fences may be beneficial to wildlife, in that they also serve to exclude cattle from wildlife areas and reduce conflict with humans. The Botswana government has also shown itself willing to make some concessions. In 1998, it pledged to realign and roll back some fences in the north of the country.
As in other areas where migratory patterns have been disrupted in favor of fences and livestock farming, the change in grazing practices has lead to habitat degradation through alteration of the natural grazing patterns. Thus, although the habitat of the Kalahari Xeric Savanna is not highly fragmented, much has been degraded by heavy livestock grazing. Heavy grazing results in an increase in woody plants and a decrease in grass cover, a phenomenon known as bush encroachment. Not only does this reduce rangeland carrying capacity, but the change in vegetation structure and plant communities also has repercussions for the native fauna. For example, a number of bird species, some of which are listed in the South African Red Data Book, are seriously affected by habitat alteration associated with heavy grazing (e.g. raptors and the Kori bustard). Interestingly, wetland and migratory birds appear indifferent to habitat change brought about by heavy grazing. Attempts to deal with bush encroachment through the application of arboricides leads to further habitat degradation.
Livestock farmers often use poisoned carcasses to kill "problem" animals such as black-backed jackals (Canis mesomelas) and caracals (Felis caracal), resulting in the poisoning of nontarget raptors. Some species, like the martial and black (Aquila verreauxii) eagles, perceived to prey on domestic livestock and poultry, may be intentionally targeted. Drownings in farm reservoirs are also responsible for a significant number of raptor mortalities in the ecoregion. In South Africa, simple and effective solutions to the problem of reservoir drownings are currently being promoted. Livestock farmers shoot other predators that wander onto their land, as various cases of lion, Painted hunting dog, and hyena shootings have illustrated.
The Painted hunting dog is probably the species of most concern in the Xeric Kalahari Savanna, listed in the global IUCN and South African Red Data Book as endangered, due to eradication of their prey and shooting by farmers. Efforts to conserve this species are hindered by their large home ranges and tendency to roam vast areas. Other species of special concern in the Southern African context are the African wildcat, honey badger (Mellivora capensis), pangolin (Manis temminckii), aardvark (Orycteropus afer), kori bustard (Ardeotis kori), bateleur (Terathopius ecaudatus), martial eagle, whiteheaded (Trigonoceps occipitalis), lappetfaced (Torgos tracheliotus), and Cape (Gyps coprotheres) vultures.
Justification of ecoregion delineation
The boundaries for the Kalahari Xeric Savanna are based on White's (1983) Kalahari deciduous Acacia wooded grassland and bushland and Kalahari/Karoo-Namib transition, with a small transition area of undifferentiated woodland to Acacia deciduous bushland and wooded grassland. A majority of the ecoregion encompasses the plains of the Kalahari Basin, and extends beyond the Kalahari biogeographical province of Udvardy (1975). Despite low levels of endemism, the ecoregion hosts a number of migratory species.
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