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).
Credit : World Wildlife Fund
Picture Credit : Google
