The Nama Karoo is a vast, open, arid region dominated by low-shrub vegetation, punctuated by rugged relief. Although not remarkably rich in species or endemism, the flora and fauna of the region are impressively adapted to its climatic extremes. The major threats to biodiversity are posed by pastoralism, introduction of alien species of plants, mining and conversion of native habitat to agriculture, all exacerbated by the fact that very little - less than one percent - of the ecoregion is conserved.
Location and general description
Most of the Nama Karoo occurs on the central plateau of the Cape Province in South Africa, although it extends over the Orange River into Namibia in the northwest. The Great Escarpment, which runs parallel to the coast 100 kilometers (km) to 200 km inland, divides the ecoregion into two parts: one between 550 meters (m) to 900 m in elevation, the other between 900 and 1300 m.
The climate is typically harsh. Droughts are common, and both seasonal and daily temperatures fluctuate considerably. Temperature variations of 25°C between day and night are common. Mean maximum temperatures in mid-summer (January) exceed 30°C, whereas mean minimum mid austal winter (July) temperatures are below freezing. Rainfall is highly seasonal, peaking between December and March. Annual rainfall ranges between 100 millimeters (mm) to 500 mm, decreasing from east to west and from north to south. Variability in inter-annual rainfall tends to increase with increasing aridity.
Shallow, weakly developed lime-rich soils cover much of the region. These soils are principally underlain by sediments of the Dwyka Formation, which are covered by the Ecca and Beaufort groups respectively. The Karoo dolerite dykes and sills were formed when molten rock intruded into the pre-existing rocks of the Ecca and Beaufort shales. Dolerite sills, generally more resistant to weathering than the surrounding sandstones and shales, can be seen as the flat-topped hills typical of the Nama Karoo landscape. The fossil record contained in the rocks of the Nama Karoo goes back more than three billion years. The greenstones of the Kaapvaal craton in the northeast have been found to contain unicellular and biogenic filamentous structures, signs of some of the earliest forms of life. The richness of dinosaur and mammal-like reptile fossils of the region is world-renowned, and has provided substantial evidence regarding the origin of mammals. The rocks and fossils of the area also indicate the diverse environments that have been found here over hundreds of millions of years.
The Orange River Basin is the region’s main drainage system, although many of the watercourses that feed it are seasonal. The ecoregion also has a number of pan systems, the largest of which, the Grootvloer-Verneukpan complex, plays an important role during fish migrations, enabling certain species to gain access to their breeding grounds in the upper reaches of the Sak River. When austral summer rainfall is high, the system also provides a link between the Orange and Sak river systems, which may enable an interchange of indigenous fish and other aquatic organisms.
Towards the Orange River, the Fish River flows through a canyon that is second in size only to the Grand Canyon of the USA. The canyon is 161 km long, up to 27 km wide, and in places almost 550 meters deep, with sheer, precipitous sides. The Fish River normally flows only during the rainy season, but lentic pools of varying size remain throughout the year.
There is little published data regarding species richness or endemism for the Nama Karoo flora. It has been calculated that some 2147 species occur in a central area of 198,000 square kilometres, of which 377 (16 percent) are endemic to the ecoregion. Recently, however, an archipelago of mountains within a part of the ecoregion known as Bushmanland have been found to harbor both Nama Karoo and Succulent Karoo type vegetation, as well as a diverse assemblage of succulents endemic to the archipelago itself. A study of the invertebrate fauna of one of these mountains (the Gamsberg) also revealed a collection of succulent Karoo species, well out of their known distribution range.
Dwarf shrubs (chaemaphytes) and grasses (hemicryptophytes) dominate the current vegetation, their relative abundances being dictated mainly by rainfall and soil. As a rule, shrubs increase and grasses decrease with increasing aridity. Heavy grazing by domestic livestock can obscure this pattern, however, by suppressing the grass component. Some of the more abundant shrubs include species of Drosanthemum, Eriocephalus, Galenia, Pentzia, Pteronia, and Ruschia, while the principal perennial grasses are Aristida, Digitaria, Enneapogon, and Stipagrostis spp. Trees and taller woody shrubs are mostly restricted to watercourses, and include Acacia karroo, Diospyros lycioides, Grewia robusta, Searsia lancea and Tamarix usneoides.
The fauna of the Nama Karoo is relatively species-poor, with 481 vertebrate taxa having been recorded. There are few strict faunal endemics, since most animals have extended their ranges into the Karoo from adjacent biomes. One species of small mammal is strictly endemic to the ecoregion: Visagie's Golden Mole (Chrysochloris visagiei, CR). Five other small mammals are near-endemic, Grant's Rock Mouse (Aethomys granti), Shortridge's Rat (Thallomys shortridgei, LR), the Riverine Rabbit (Bunolagus monticularis, EN), Gerbillurus vallinus and Petromyscus monticularis, LR.
The Black Rhinoceros (Diceros bicornis) is a Critically Endangered mammal found in the ecoregion. Another special status Nama Karoo vertebrate is the Riverine Rabbit (Bunolagus monticularis), classified as Endangered in the South African Red Data Book because of habitat destruction by agriculture. The Quagga (Equus quagga), a Nama Karoo near-endemic, was hunted to extinction in the 19th Century. Another charismatic mammal found in the Nama Karoo is the Meerkat (Suricata suricatta).
Among notable birds, the Ferruginous Lark (Certhilauda burra, VU) and Sclater's Lark (Spizocorys sclateri, LR) are strictly endemic to the ecoregion, while another five avafauna are near-endemic: Karoo Chat (Cercomela schlegelii), Tractrac Chat (Cercomela tractrac), Red Lark (Certhilauda burra, VU), Karoo Scrub Robin (Cercotrichas coryphaeus), Red-headed Cisticola (Cisticola subruficapillus), and the Namaqua Prinia (Phragmacia substriata). Other characteristic speces of the Nama Karoo which are regarded as Vulnerable in South Africa are Tawny (Aquila rapax) and martial (Polemaetus bellicosus) Eagles, African Marsh Harrier (Circus ranivorus), Lesser Kestrel (Falco naumanni), Blue Crane (Anthropoides paradiseus), Kori Bustard (Ardeotis kori) and Ludwig’s Bustard (Neotis ludwigii). The Bateleur Eagle (Terathopius ecaudatus) is a Near Threatened avian species of the Nama Karoo.
Non-endemic amphibians found in the ecoregion are African Clawed Frog (Xenopus laevis), Southern Ornate Frog (Hildebrandtia ornata), Angola River Frog (Amietia angolensis), Cape River Frog (Amietia fuscigula), Boettger's Dainty Frog (Cacosternum boettgeri), Red-spotted Namibia Frog (Phrynomantis annectens), Cryptic Sand Frog (Tomopterna cryptotis), Namaqua Dainty Frog (Cacosternum namaquense), Senegal Running Frog (Kassina senegalensis), Tschudi's African Bullfrog (Pyxicephalus adspersus) and Karoo Toad (Vandijkophrynus gariepensis).
The reptile fauna contains at least ten species that are regarded as near-endemic to the ecoregion, but only a few are potentially confined to the Nama Karoo, including Karoo dwarf chameleon (Bradypodion karrooicum) and Boulenger's Padloper (Homopus boulengeri). Many of the endemics, and some of the other species present, are relicts of past drier epochs when desert and savanna biomes expanded to link up with similar biomes in northeast Africa. One of the special status reptiles in the Nama Karoo is the Lower Risk/Near Threatened Speckled Cape Tortoise (Homopus signatus).
This arid biological corridor enabled flora and fauna to move between the two regions. Many discontinuous populations of the same species, genera and families with representatives in each region indicate that the corridor formed many times, most recently about 18,000 years ago. Among the fauna to exhibit this interrupted distribution are the bat-eared fox (Otocyon megalotis), Olive Toad (Bufo garmani), and Fawn-colored and Sabota Larks (Mirafra africanoides, M. sabota).
In the mid- to late-1800s, European travelers and colonists witnessed game migrations numbering millions across the Nama Karoo. One account recalls a herd taking three days to pass through a small town. These migrations are believed to have taken place between the summer rainfall Nama Karoo and southern Kalahari, to the winter rainfall Succulent Karoo. Hunting and fences have now halted this phenomenon forever. Although other game (e.g. wildebeest (Connochaetes taurinus), blesbok (Damaliscus dorcas), quagga (Equus quagga), and eland (Taurotragus oryx) were often involved in these migrations, springbok (Antidorcas marsupialis) were by far the most numerous species. Farmers, who tended to regard them as vermin, competing with their sheep for food, space and water, shot as many springbok as they could, using the carcasses for dried spiced meat. This slaughter, along with habitat loss to fenced livestock farms and a rinderpest outbreak at the end of the 19th Century, reduced springbok numbers dramatically. Springbok are now, for the most part, a form of livestock living on fenced farmland. Luckily, fences do not limit birds, and many species, particularly granivores, still travel hundreds of kilometers to find rainfall (and hence, food) patches.
The major large-scale disturbance to the Nama Karoo ecosystem has been grazing, previously by a variety of indigenous migratory ungulates and now by domestic sheep and goats confined within farm boundaries. Sedentary domestic livestock graze selectively compared to the catholic tastes of their native nomadic counterparts. This change in the grazing regime is thought to be responsible for alterations in both plant species composition and cover, which ultimately influence ecosystem functioning. On a smaller scale, disturbances associated with heuweltjies (ancient termitaria) maintain habitat heterogeneity and patchiness within the landscape. Termite activity makes the soils of heuweltjies finer, moister and more alkaline than their surrounds. The plant communities that grow on these mounds are thus very different than the surrounding matrix. Many animal species may contribute further to the nutrient enrichment of heuweltjies. Aardvark (Orycteropus afer) and steenbok (Raphicerus campestris) often use them as dung middens; Brant’s whistling rats (Parotomys brantsii) frequently colonize them; and sheep prefer to graze (and therefore deposit dung) on the mounds.
Current ecoregion status
Less than one percent of the Nama Karoo is protected. The only sizeable park present in this ecoregion is the Fish River Canyon Park. This site is situated at the south of the Fish River where it flows through a large canyon. The park has recently been enlarged to include adjacent mountains to the west and now extends to the Orange River. The park includes the Ais Ais hot springs, which reach the Earth's surface within the canyon. The establishment of wildlife conservancies on commercial and communal farmlands could improve this situation, with rural communities responsible for the ecological management of large areas in habitats otherwise overlooked for conservation.
The Namibian area of the ecoregion once had high species richness, but low populations of large mammals which were decimated by settlers who entered Namibia at the Orange River and Warmbad areas. Large mammal distributions receded in a northeasterly direction, leaving southern Namaland devoid of vulnerable species such as Lion and Plains Zebra (Equus burchelli). These two species have suffered a 95 percent range reduction over the past 200 years. By the early 1800s, mammal populations in the south of this ecoregion had been decimated, and today this ecoregion holds the national Namibian record for the most regional extinctions.
Types and severity of threats
Most of the ecoregion is now utilised as rangeland for livestock grazing, and while nominally intact, heavy grazing has left parts seriously degraded. The issue of degradation and grazing practices is complex, however, and requires further investigation. The use of poisoned carcasses by livestock farmers to kill wildlife viewed as threats to their livestock, such as Black-backed Jackal (Canis mesomelas) and Caracal (Felis caracal) often results in poisoning of non-target raptors in addition to the mammalian carnivores. Some species, like the Martial and Black (Aquila verreauxii) Eagles, perceived to prey on domestic livestock and poultry, may also be intentionally targeted for persecution by the native farmers. Drownings in farm reservoirs are responsible for a significant number of raptor mortalities in the ecoregion. Simple and effective solutions to this problem are currently being promoted in farmer extension programs.
In addition to pastoralism, alien invasive plants, mining, agriculture, and the collection of succulents and reptiles for the pet trade, also threaten the ecoregion’s biodiversity. A number of introduced ornamental (e.g. some Cactaceae) and forage (e.g. Opuntia, Prosopis, Atriplex, and Bromus spp.) plants, together with a few accidental introductions (e.g. Salsola kali and Argemone ochroleuca) have the potential to seriously alter the region’s ecology and hydrology. These exotics disperse efficiently, lack natural predators and can outcompete indigenous plants for water, nutrients and light. Anthropogenic climate change, increased stocking rates, cultivation of marginal lands and salinization of surface water are all likely to further facilitate the spread of alien invasive plants. Some progress has been made in addressing the problem, particularly in the area of biological control.
Mining is economically important in the region and may threaten the ecology, although in some cases, attempts are being made to rehabilitate the land after disturbance. At present, open-cast mining for zinc is proceeding at the Gamsberg. The possibility of future mining activities on the Gamsberg and other mountains in its archipelago are of great concern. Clearing of natural vegetation for cultivation destroys the natural habitat of many plants and animals. Pesticides used to control Brown Locust (Locustana pardalina) outbreaks also impact wildlife habitat severely, with high concentrations being found at the top of the food chain, particularly in raptors.
Justification of ecoregion delineation
This ecoregion, along with the Succulent Karoo ecoregion, roughly falls within the ‘Karoo’ biogeographic province of Udvardy. The boundaries of the ecoregion were taken from the Nama Karoo biome as defined by Low and Rebelo, and extended north to Keetmanshoop roughly around the 900 metre elevation contour. This ecoregion is distinguished from surrounding ecoregions by a range of environmental parameters including elevation, temperature and rainfall. The Nama Karoo lies between 500 to 1500 metres in elevation, and has more extreme temperatures and more variable rainfall compared to the adjacent Succulent Karoo ecoregion.
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