Location and General Description
The Namib Desert ecoregion extends along the coastal plain of western Namibia, from the Uniab River in the north to the town of Luderitz in the south. It extends inland from the Atlantic Coastline to the foot of the Namib Escarpment, a distance of between 80 and 200 km. The ecoregion can be divided into two areas: the Central Namib (from the Uniab to the Kuiseb Rivers) and the Southern Namib (from the Kuiseb River to the town of Luderitz). In the north, the Central Namib merges with the Northern Namib or Kaokoveld Desert ecoregion, and in the south it merges with the Succulent Karoo ecoregion, which extends up the west coast of South Africa.
A climatic transition belt crosses the coastline north of Walvis Bay in Namibia, and divides the coastal area into a northern area receiving summer rainfall, considered the Kaokoveld Desert ecoregion and a southern area receiving winter rain (the Namib Desert ecoregion). The narrow strip of land within this transition belt (about 50 km) is the most arid area in southern Africa and receives sporadic, unpredictable rainfall without a clear seasonal pattern (Williamson 1997).
The most important climatic feature of the Namib Desert is its sparse and highly unpredictable annual rainfall. The annual mean ranges from 5 mm in the west to about 85 mm along its eastern limits (Lovegrove 1993). There is extraordinary variation in rainfall between years, with the driest areas of the desert having the highest variation. The low rainfall of the Namib has two causes. Firstly, the easterly trade winds emerging over the Indian Ocean lose most of their humidity when rising over the eastern escarpment of the Drakensberg Mountains in South Africa. On their way over the continent, they lose further moisture and reach the western escarpment of Namibia warm and dry. These air masses move down onto the low-lying Namib and produce extremely hot and dry winds. Secondly, the prevailing local southwesterly winds, cooled down by the Benguela current along the west coast of South Africa, produce an inland movement of cold air, which is overlain by the warmer, lighter air. This inversion prevents the convectional rise of the cool, humid air and thus no clouds are formed (von Willert et al. 1992). The cool air does, however, regularly form a stable layer of fog, which is blown inland as far as 50 km. This coastal fog is the life-blood in the Namib and is an important factor contributing to the remarkably high diversity of animal life in this extremely arid environment (Lovegrove 1993).
Based on coastal fog, the Namib can be divided into three areas running from west to east: coastal, central, and inland. The coastal area has a mean annual rainfall of 2 to 20 mm and has thick fog on more than 180 days of the year. Air temperatures are low as a result of the cool air coming off the Benguela Current, and daily and seasonal temperature changes are minimal, with daily highs and lows differing by as little as 2° to 5 °C (Barnard 1998). Up to about 50 km inland the mean annual rainfall increases from 20 mm to 50 mm. Fog, while still important to desert organisms, occurs on only about 40 days in the year. Still further inland, fog is rare, and the mean annual rainfall increases from 50 mm to a maximum of 85 mm in places. Daily and seasonal temperatures increase sharply and become highly variable, with temperatures of below 0° and above 50°C recorded at one location (von Willert et al.1992).
One of the more characteristic features of the Namib Desert is its severe paucity of surface water. The Southern and Central Namib are bisected by ephemeral rivers that are normally dry and flow at irregular intervals, the most important of which is the Kuiseb River (Jacobson et al. 1995). On the rare occasions that this river does flow, its passage to the sea is blocked by sand dunes. The only temporarily flowing rivers to occasionally reach the ocean are the Swakop River and the Omaruru River (north of Swakopmund).
Between Luderitz and the Kuiseb Rivers, the Southern Namib is made up of a belt of sand dunes 320 km long and about 120 km wide. These spectacular linear and crescent-shaped dunes reach elevations of 300 m. A strip of low sand hummocks of about 2 m high runs between this sea of dunes and the ocean. The dunes march northwards, driven by prevailing southerly winds, and are then brought to an abrupt halt by vegetation of the Kuiseb riverbed, which forms a wind barrier. To the north of the Kuiseb River, the dunes give way to the gravel plains of the Central Namib that are dotted with inselbergs of granite and limestone (Lovegrove 1993, Barnard 1998).
The formation of true soils with well-defined profiles is absent because of the low moisture status of the region (von Willert et al. 1992). The soils are made of raw minerals and are sandy and sometimes calcareous or with calcareous crusts, composed of particles in a wide range of sizes. Salt crusts are common on the soil close to the ocean, and the soils are brackish as far as the inland limit of coastal fog. Characteristic soils of the ecoregion include arenosols and weakly developed shallow Halomorphic soils (Barnard 1998). Gypsum accumulations are also characteristic of the ecoregion, particularly in the Central and Southern Namib areas (Pallett 1995).
The Southern Namib is an expansive area of large, shifting dunes. This area is botanically unexplored but is believed to be largely devoid of vegetation (Giess 1971, White 1983, Lovegrove 1993). The few species known to sparsely vegetate the dunes are the perennial grasses Stipagrostis sabulicola and S. gonostachys as well as Monsonia ignorata and the succulent Trianthema hereroensis. These perennial plants are found dotted on the lower slopes of the dunes and have adapted to the constantly shifting sands. Hummocks are formed between these sand dunes and the coast. These are formed by the nara plant, the pencil plant (Arthraerua leubnitziae) and the dollar plant (Zygophyllum stapfii) (Giess 1971, Werger 1978, White 1983).
To the north of this is the Central Namib. A narrow strip of vegetation (up to 200 m wide) follows the Central Namib coastline north of the Swakop River. A common feature of this strip are dwarf shrubs filled with sand between their branches, such as Psilocaulon salicornioides, Zygophyllum clavatum, Salsola aphylla, and S. nollothensis. At rocky sites a variety of dwarf shrubs grow, including Drosanthemum luderitzii, Ruschia sedoides, and Tetragonia arbusculoides as well as the annual grasses Stipagrostis hermannii, S. namibiensis, and S. subacaulis. Inland from this strip of vegetation are vast gravel plains that are largely devoid of vegetation except for fields of colorful lichens, including the species Teloschistes flavicans, Parmelia convoluta, and Usnea spp. Some of these lichens are not attached to any substrate and are known as vagrant lichens or Wanderflechten. Examples of these are Xanthomaculina convoluta and Parmelia hueana (Wessels 1989). Towards the extreme east, the gravel plains become more vegetated with annuals (mostly Stipagrostis species) as the predominant life form. These annuals, which lie dormant through extended drought periods as seeds, grow rapidly after a rainfall of anything more than about 20 mm and transform the landscape into a sea of grass. Widely scattered shrubs, mainly the African moringo (Moringo ovalifolia), Euphorbia virosa and species of Commiphora, are also present on rocky outcrops on these plains (Giess 1971;Werger 1978; White 1983).
A dense growth of Sporobolus robustus or more open communities of Eragrostis spinosa are found along dry riverbeds within the ecoregion. Trees of Acacia erioloba are also scattered along the riverbeds. Along the Swartkop and Kuiseb Rivers the A. erioloba forms dense stands with Acacia albida, wild tamarisk (Tamarix usneoides), the mustard tree (Salvadora persica), and the exotic Nicotiana glauca, native to South America (White 1983).
The Namib Desert is believed to be the world’s oldest desert and it has been arid for at least 55 million years (Barnard 1998). The convergence of the Benguela upwelling and the hot interior have maintained, and perhaps increased this aridity in recent times, but they did not generate the aridity. The region, isolated between the ocean and the escarpment, is considered to be a constant island of aridity surrounded by a sea of climatic change (Armstrong 1990). The arid conditions probably started with the continental split of West Gondwana 130 million to 145 million years ago when this area shifted to its present position along the Tropic of Capricorn. This lengthy dry period has had a profound influence on the region’s biodiversity. The region has remained a relatively stable center for the evolution of desert species. This has resulted in a unique array of biodiversity with high levels of endemism and numerous advanced adaptations to arid conditions.
The monotypic Welwitschia mirabilis, one of the most remarkable plants in the world, is endemic to the Namib Desert and to the Kaokoveld Desert ecoregion to the north. The distribution of this relict gymnosperm extends from the Kuiseb River in Namibia to Namibe in southern Angola (White 1983). These plants are usually found more than 20 m apart, in broad flat channels on gravel plains. The channels are so shallow as to be barely discernable, but receive floodsheet waters from higher areas during the infrequent rains. The soil becomes moist to about 1.5 m and this subsurface moisture can be retained for years. Welwitschia plants are up to 1.5 m tall and have two fibrous, straplike leaves, which persist throughout the entire life of the plant and curl into fantastic shapes on the ground. They are the longest-lived leaves of any member of the plant kingdom. It is estimated that the largest Welwitschia plants are about 2,500 years old (White 1983, Armstrong 1990, Lovegrove 1993).
In the animals the high species richness and endemism is made up largely of reptiles. They have evolved adaptations to survive in this harsh environment when most birds and large mammals have not. There are almost 70 reptile species in the ecoregion, of which more than 25 are considered endemic to the ecoregion. Five of these are strictly-endemic to the dry Namib Desert, and at least 20 species are regarded as nearly endemic to the ecoregion (WWF database). Several endemic reptiles, including two desert lizards, the wedge-snouted sand lizard (Meroles cuneirostris) and the small-scaled sand lizard (M. micropholidotus), the barking gecko (Ptenopus kochi) and the day gecko (Rhoptropus bradfieldi) are unusual in that they all dive beneath the sand to escape danger (Branch 1998).
The Namib Desert is home to a large number of small rodent species that occur among the rocky habitats in the western deserts, in the sand dunes and in the vegetation of the gravel plains. The gerbil, Gerbillurus tytonis is restricted to the southern portion of the ecoregion (WWF database). Grant’s golden mole (Eremitalpa granti VU) is near-endemic in the Namib Desert, its range extending down into South Africa. This eyeless mole is well-adapted to the desert, able to swim through the loose, dry sands of the Namib dunes. The Namaqua dune molerat (Bathyergus janetta LR) is also near-endemic in the Namib Desert, as are two bat species; the Namib long-eared bat (Laephotis namibensis EN) and the Angola wing-gland bat (Myotis seabrai VU) (Hilton-Taylor 2000).
Larger ungulates are scarce in the Namib Desert, with only gemsbok (Oryx gazella LR) and springbok (Antidorcas marsupialis LR) present (Griffin 1998). Hartmann’s zebra (Equus zebra hartmannae EN) is found in the extreme east of the desert, in the transition belt between the desert and the escarpment. However, they do move further into the desert along vegetated riverbeds (Joubert and Mostert 1975). Gemsbok are the most widespread ungulates in the desert habitat and can survive for weeks without drinking by ceasing to sweat when deprived of water. At such times, their body temperature may reach as high as 45°C, and can be sustained at this level through the heat of the day by a network of fine blood vessels at the base of the brain that exchanges heat to prevent brain damage (Armstrong 1990). The predators of the Namib Desert are cheetahs (Acinonyx jubatus, VU), brown hyenas (Hyaena brunnea, LR) and spotted hyenas (Crocuta crocuta), Cape foxes (Vulpes chama) and bat-eared foxes (Otocyon megalotis). Brown hyenas are common around Luderitz, and this area supports the highest density of this species in Namibia (Pallett 1995). Klipspringers (Oreotragus oreotragus), steenboks (Raphicerus campestris), baboons (Papio ursinus) and leopards (Panthera pardus) occur along the courses of the Kuiseb and Swakop Rivers (Lovegrove 1993). Many species have become locally extinct in the southern areas of the Namib Desert. Lions (Panthera leo), elephants (Loxodonta africana), black rhinos (Diceros bicornis), white rhinos (Ceratotherium simum), giraffes (Giraffa camelopardalis) and hippos (Hippopotamus amphibius) were all shot out by colonial settlers who established themselves along the Orange River. The lower Orange River was once famous for its large numbers of hippos, which came onshore to graze on the floodplains at night (Pallett 1995).
The desert does not have a very high level of avian richness, with only 180 species recorded to date. This is due to the extremely arid terrain and a lack of rivers, with even ephemeral rivers absent in the southern part. The most prominent bird found in the desert is the ostrich (Struthio camelus). Most of the bird life is concentrated along the coastline. The isolated Sandvis area (previously known as Sandwich Harbor) situated on the coast at about 23°S is an area of high species richness. To date, the number of species recorded at Sandvis is 113 and represents a wide taxonomic variety (Berry and Berry 1975). Six birds are considered endemic to the Namib Desert: the dune lark (Certhilauda erythrochalamys), Benguela long-billed lark (C. benguelensis) (Ryan et al. 1999), Gray’s lark (Ammomanes grayi), bank cormorant (Phalacrocorax neglectus), tractrac chat (Cercomela tractrac), and Rüppell’s korhaan (Eupodotis rueppellii). The dune lark is strictly endemic to this ecoregion while the gray’s lark, Rüppell’s korhaan, and C. benguelensis are found only in this ecoregion and the Kaokoveld Desert ecoregion.
The Namib Desert is best known for its high species richness of beetles, particularly those belonging to the family Tenebrionidae (Lovegrove 1993). Many of these have evolved methods of condensing fog as a source of water. The head-standing beetle (Onymacris unguicularis) for example, creeps to the crest of a dune when fog is present. It then faces into the wind and stretches its back legs so that its body tilts forward, head down. As fog precipitates onto its body and runs down into its mouth the beetle drinks (Armstrong 1990).
A curious feature of the Namib Desert is circles on the ground that are accentuated by abrupt changes in the vegetation pattern. These circles, known as "fairy rings" are most noticeable from the air. They occur on the eastern fringes of the desert from southern Angola all the way through this ecoregion, characteristically within the Stipagrostis grassland community. The origin of these circles has generated interest and speculation for centuries (Lovegrove 1993). Various hypotheses have been advanced for their formation, from geomorphological phenomena, to plants having allelopathic exclusion effects, to being animal derived features. A similar phenomenon occurs to the south of the Orange River in the form of heuweltjies (originally termitaria) that are about 30 m in diameter and 1 m high. Recent evidence has shown that the harvester termite Microhodotermes viator most likely creates these heuweltjies. It is now thought that the fairy rings of the Namib Desert are created through a similar process by one of three possible termite species: Hodotermes mossambicus, Psammotermes allocerus, or Baucaliotermes hainsei (Moll 1994).
The present conservation status of the Namib Desert is good as most of the ecoregion is intact and is protected in extensive conservation blocks (Maggs et al. 1998). The Namib-Naukluft National Park (49,768 km2) is the largest conservation area in southern Africa and protects the central area of this ecoregion. The park runs from Swakopmund in the north to Luderitz in the south. The southern part of the park is taken up by the large expanse known as Diamond Area No. 2, to which public entry is strictly prohibited. The park covers gravel plains, the dune sea, the eastern semi-desert and the Kuiseb River and is therefore a good representation of the Central and Southern Namib vegetations (du Plessis 1992). The well-known Soussusvlei is located within this park and is surrounded by the highest sand dunes in the world. The vlei itself is a large shallow hollow that fills with water during the infrequent rains (Stuart and Stuart 1992).
To the north of the Namib-Naukluft National Park lies the National West Coast Tourist Recreation Area. This area extends for 180 km up the coast and is under less stringent protection than the national parks. The Cape Cross Seal Reserve is located within this area and protects one of the largest colonies of Cape fur seals (Arctocephalus pusillus) in southern Africa. Between 80,000 and 100,000 seals live on the rocks here.
Types and Severity of Threats
A major threat to the Namib Desert is the impact of off-road driving. The impact is the greatest on the gravel plains where depressions left by vehicles remain for more than 40 years because the rainfall is too episodic and sparse to erase them. These tracks are unsightly and cause long-lasting damage to the lichen fields. Lichens are particularly sensitive to mechanical damage as they grow extremely slowly and cannot quickly repair damaged thalli. Most of the damage is done by mining company vehicles when on prospecting expeditions (Lovegrove 1993).
The major threat to the Namib-Naukluft National Park is the drop in the water table along the Kuiseb River. This is caused primarily by the extraction of ground water by the Department of Water Affairs at two sites near Walvis Bay (Lovegrove 1993). The extracted water supplies the domestic consumption of Walvis Bay and Swakopmund and the enormous demands made by the Rossing Uranium Mine near Swakopmund. At present, the Department of Water Affairs is trying to meet the requirements for water by prospecting for more underground water sources. If water were to be found, roads, pipelines and powerlines would have to be constructed through the most pristine dune desert in the world (Lovegrove 1993). The Kuiseb River and the vegetation within it acts as a windbreak to the southerly winds, retarding the northwards movement of the dune sea onto the gravel plains. The destruction of this natural barrier would have serious ecological consequences in this part of the Namib Desert (Lovegrove 1993). Another threat to the Namib-Naukluft Park is the Topnaar pastoralists who graze large herds of goats and small groups of donkeys over the Kuiseb Riverbed and along the edge of the dunes. The livestock have overgrazed the understory plant growth and fallen Acacia seedpods of the riverbed and are competing for food with wild animals, such as gemsboks.
Collectors of succulent plant species are also having an impact on the flora to the south of the ecoregion. Illegal trade in species such as the halfmens Pachypodium namaquanum is thought to be considerable (Maggs et al. 1998).
Justification of Ecoregion Delineation
The Namib Desert is nested within the ‘Namib’ biogeographical province of Udvardy (1975) and the ‘Namib Desert’ unit of White (1983), with the eastern boundary taken from the 100 mm rainfall isohyet. The northern limit is at the Huab River, which a geographic boundary (Giess 1971) between the summer rainfall (northern) and winter rainfall (southern) deserts along this coastline. The southern portion of White’s ‘Namib Desert’ unit, although referred to as the southern Namib, shares affinities with the Succulent Karoo. Thus, the portion south of Luderitz, Namibia was appended to the Succulent Karoo ecoregion.
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Prepared by: Amy Spriggs
Reviewed by: In progress