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 km to 200 km inland, divides the ecoregion into two parts: one between 550 m to 900 m in elevation, the other between 900 and 1,300 m (Palmer and Hoffman 1997).
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 (Venter et al. 1986). Mean maximum temperatures in mid-summer (January) exceed 30°C, whereas mean minimum mid-winter (July) temperatures are below freezing (Palmer and Hoffman 1997). Rainfall is highly seasonal, peaking between December and March (Palmer and Hoffman 1997). Annual rainfall ranges between 100 mm to 500 mm, decreasing from east to west and from north to south (Palmer and Hoffman 1997, Desmet and Cowling 1999). Variability in inter-annual rainfall tends to increase with increasing aridity (Schulze 1997).
Shallow, weakly developed lime-rich soils cover much of the region (Watkeys 1999). The soils are principally underlain by sediments of the Dwyka Formation, which are covered by the Ecca and Beaufort groups respectively (Lloyd 1999). The Karoo dolerite dykes and sills were formed when molten rock intruded into the pre-existing rocks of the Ecca and Beaufort shales (Lloyd 1999). 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 (Watkeys 1999). The fossil record contained in the rocks of the Nama Karoo goes back more than 3 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 (Engel et al. 1968, Meadows and Watkeys 1999). 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 (Lloyd 1999). The rocks and fossils of the area also indicate the diverse environments that have been found here over hundreds of millions of years (Dean and Milton 1999a).
Dwarf shrubs (chaemaphytes) and grasses (hemicryptophytes) dominate the current vegetation, their relative abundances being dictated mainly by rainfall and soil (Palmer and Hoffman 1997). As a rule, shrubs increase and grasses decrease with increasing aridity (Palmer and Hoffman 1997). Heavy grazing by domestic livestock can obscure this pattern, however, by suppressing the grass component (Lovegrove 1993). 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, Rhus lancea, and Tamarix usneoides (Palmer and Hoffman 1997).
The Orange River Basin is the region’s main drainage system, although many of the watercourses that feed it are seasonal (Lloyd 1999). 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 access to their breeding grounds in the upper reaches of the Sak River (Lloyd 1999). When 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 (Lloyd 1999).
Towards the Orange River, the Fish River flows through a canyon that is second in size only to the Grand Canyon of America. The canyon is 161 km long, up to 27 km wide, and in places almost 550 m deep with sheer, precipitous sides. The Fish River normally flows only during the rainy season, but pools of varying size remain throughout the year (Barnard 1998).
There is little published data regarding species richness or endemism for the Nama Karoo flora. Gibbs Russel (1987) calculated 2,147 species occurred in a central area of 198,000 km2, of which 377 (16 percent) are endemic. 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 (Desmet 2000). 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 (Desmet 2000).
The fauna of the Nama Karoo is relatively species-poor (Vernon 1999). There are few strict endemics, as 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 (Hilton-Taylor 2000). The most vulnerable of the Nama Karoo’s vertebrates is the riverine rabbit (Bunolagus monticularis), classified as "Endangered" in the South African Red Data Book because of habitat destruction by agriculture (Smithers 1986). The quagga, (Equus quagga) a Nama Karoo near-endemic, was hunted to extinction in the 19th Century (Skinner and Smithers 1990).
Among birds, the ferruginous lark (Certhilauda burra, VU) (Dean et al. 1991) and Sclater's lark (Spizocorys sclateri, LR) are strictly endemic to this ecoregion, while another five species are near-endemic: Karoo chat (Cercomela schlegelii), tractrac chat (Cercomela tractrac), red lark (Certhilauda burra), 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 (Ardeotis kori) and Ludwig’s (Neotis ludwigii) bustards, and the red lark (Dean et al. 1991, McCann 2000, Barnes 2000).
The reptile fauna contains at least 10 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 (Werger 1978). This arid 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 (Vernon 1999). 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) (Vernon 1999).
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 (Lovegrove 1993). 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 (Lovegrove 1993). 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 (Lovegrove 1993). 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 (Kingdon 1997). Luckily, fences do not limit birds, and many species, particularly granivores, still travel hundreds of kilometers to find rainfall (and hence, food) patches (Dean and Milton 1999b).
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 (Skead 1982). Sedentary domestic livestock graze selectively compared to the catholic tastes of their native nomadic counterparts (Roux and Theron 1986). This change in the grazing regime is thought to be responsible for alterations in both plant species composition and cover (Roux and Theron 1986), which ultimately influence ecosystem functioning. On a smaller scale, disturbances associated with heuweltjies (ancient termitaria) (Moore and Picker 1991) maintain habitat heterogeneity and patchiness within the landscape. Termite activity makes the soils of heuweltjies finer, moister and more alkaline than their surrounds (Midgley and Musil 1990). The plant communities that grow on these mounds are thus very different than the surrounding matrix (Lovegrove 1993). 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 (Armstrong and Siegfried 1990, Milton and Dean 1990).
>Very little – less than 1 percent – of the Nama Karoo is protected (Cowling 1986, Barnard et al. 1998). The only large park present in this ecoregion is the Fish River Canyon Park. This park is situated at the south of the Fish River where it flows through its 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 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 (Barnard et al. 1998).
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 lions and plains zebras (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 area holds the national Namibian record for the most regional extinctions (Griffin 1998).
Types and Severity of Threats
Most of the ecoregion is now rangeland for livestock grazing (Hoffman et al. 1999), and therefore still intact, although heavy grazing has left parts seriously degraded (Lloyd 1999). The issue of degradation and grazing practices is complex, however, and requires further investigation (Hoffman and Cowling 1990, Dean and Macdonald 1994, Hoffman et al. 1999). The use of poisoned carcasses by livestock farmers to kill "problem" animals such as black-backed jackal (Canis mesomelas) and caracal (Felis caracal) often results in poisoning of nontarget raptors (Lloyd 1999, Anderson 2000). Some species, like the martial and black (Aquila verreauxii) eagles, perceived to prey on domestic livestock and poultry, may be intentionally targeted (Anderson 2000). Drownings in farm reservoirs are also responsible for a significant number of raptor mortalities in the ecoregion (Anderson 2000). Simple and effective solutions to this problem are currently being promoted in farmer extension programs (Anderson 2000).
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 (Lovegrove 1993, Lloyd 1999). 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 (Milton et al. 1999). These exotics disperse efficiently, lack natural predators and can outcompete indigenous plants for water, nutrients and light (Lovegrove 1993). 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 (Milton et al. 1999). Some progress has been made in addressing the problem, particularly in the area of biological control. Mining is important in the region and also threatens the ecology, although in some cases, attempts are being made to rehabilitate the land as far as possible (Lovegrove 1993). At present, open-cast mining for zinc looks likely to proceed 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 (Lovegrove 1993).
Justification of Ecoregion Delineation
This ecoregion, along with the Succulent Karoo, roughly falls within the ‘Karoo’ biogeographic province of Udvardy (1975). The boundaries of the ecoregion were taken from the Nama Karoo biome of Low and Rebelo (1996), and extended north to Keetmanshoop roughly around the 900 m contour (WWF 1998). 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 m elevation, and has more extreme temperatures and more variable rainfall compared to the adjacent Succulent Karoo ecoregion.
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Prepared by: Colleen Seymour
Reviewed by: In progress