Biological diversity in the Succulent Karoo
Stretching along the Atlantic coast of Africa, from southwestern South Africa into southern Namibia, the Succulent Karoo hotspot covers 102,691 square kilometres of desert. Some pockets of this hotspot are scattered within the Cape Floristic Region Hotspot, which borders it to the south. In fact, the Succulent Karoo exhibits a particularly strong floristic affiliation with the Cape Floristic Region, to the point that some have argued convincingly for the region's inclusion as part of a greater Cape Flora.
The Succulent Karoo, which consists primarily of winter rainfall desert, is one of only two hotspots that are entirely arid (the other being the newly recognized Horn of Africa). The region is commonly divided into two zones. The first, Namaqualand, extends along the west coast of South Africa and southern Namibia. It is a winter rainfall desert with a mild climate moderated by cold Atlantic Ocean currents. The mild climate has contributed to the evolution of a rich array of endemic species. The second zone, the Southern Karoo, experiences peaks of rainfall in spring and autumn and has more extreme climate variations than the Namaqualand desert.
Dwarf shrubland dominated by leaf succulents is found throughout the hotspot. These drought-adapted plants have thick, fleshy leaves or stems for water storage. In the Succulent Karoo, there are about 1700 species of leaf succulents, and this dominance is unique among the world's desert s. The recent and explosive diversification of the Mesembryanthemaceae, the largest group, has been described as an event unrivaled among flowering plants. Stem succulents are also found here (around 140 species), as are seasonal bulbs and annuals that display magnificent spring blooms in the open spaces between the shrubs, particularly during the spring in the Namaqualand. Hilly topography within the southern Karoo is dotted with evergreen shrubs and tall aloes.
Unique and Threatened Biodiversity
For an arid region, the Succulent Karoo has extraordinarily high plant endemism, including the richest succulent flora in the world. In total, there are more than 6350 vascular plant species in this hotspot, nearly 2440 of which are endemic (40 percent). Local plant species richness is very high, with an average of 70 species found in 0.1 hectare test plots, and the diversity between sites in the region is also significant. Many plants in the Succulent Karoo, especially succulents, are specialists for a limited range of environmental conditions, producing a phenomenon known as point endemism. Regional endemism is notable at the genus level; 80 genera are found nowhere else in the world.
Notable plant species found in this hotspot include the botterboom (Tylecodon paniculatus), a stem succulent that has glossy leaves in winter and red flowers in summer, and the halfmens ("half human") (Pachypodium namaquanum), a stem succulent endemic to the Richtersveld that can grow up to four metres tall. Clusters of halfmens stems tend to face toward the north, giving the appearance of groups of people gazing northwards. The stems' crowns of leaves, which resemble hairy human heads, enhance the impression. The scientific explanation for this unusual orientation is that the plants, which grow on shaded slopes, lean northwards in order to ensure that their leaves and developing flowerheads, produced during the cool, foggy winter months, are maximally exposed to the sun’s warming rays.
The avifauna of the Succulent Karoo includes more than 225 species, one of which is endemic: Barlow's lark (Certhilauda barlowi). Other species in the region include the black harrier (Circus maurus, VU), which has the most restricted range of the world’s 13 harrier species, Karoo bustard (Eupodotis vigorsii), Ludwig's bustard (Neotis ludwigii), Karoo chat (Cercomela schlegelii), dune lark (Certhilauda erythrochlamys), and dusky sunbird (Nectarinia fusca).
There are roughly 75 mammal species in the Succulent Karoo hotspot, of which two are endemic: De Winton's golden mole (Cryptochloris wintoni, VU), and the Namaqua dune mole rat (Bathyergus janetta). An important flagship species in the region is the riverine rabbit (Bunolagus monticularis, CR), found in the Succulent Karoo, and whose population is thought to have declined by 60 percent over the past 70 years such that no more than perhaps 250 individual remain.
Major concentrations of large mammalian vertebrates, including elephant (Loxodonta Africana, VU), black rhinoceros (Diceros bicornis, CR), and African buffalo (Syncerus caffer), once roamed the gallery forests along the Orange River in the Succulent Karoo. These populations have now disappeared from the hotspot. Today, only smaller herds of gemsbok ( Oryx gazella), mountain zebra (Equus zebra), and springbok (Antidorcas marsupialis) can still be seen there.
Reptile diversity is relatively high in the Succulent Karoo, with more than 90 species, about 15 of which are endemic. All of the endemics are geckos and lizards, representing about a quarter of the nearly 60 gecko and lizard species in the hotspot. These endemics include seven species of girdled lizards of the genus Cordylus, including the armadillo girdled lizard ( Cordylus cataphractus, VU). This lizard has a heavily armored body and spiny tail and is known for rolling into a tight ball when threatened.
As in the rest of South Africa, tortoise diversity is very high in the Succulent Karoo, with seven taxa, two of which are endemic: the Namaqualand tent tortoise (Psammobates tentorius trimeni) and the Namaqualand speckled padloper (Homopus signatus signatus).
Amphibians are poorly represented in the Succulent Karoo with just over 20 species. All of these species are frogs, including one endemic, the desert rain frog (Breviceps macrops, VU), which occurs along the Namaqualand coast of South Africa north to Luderitz in coastal southwestern Namibia.
There are around 26 species of freshwater fish in the Succulent Karoo, none of which are endemic.
Invertebrate diversity is quite high in the Succulent Karoo, and evidence suggests that more than half of the species in some insect groups are endemic. Of the 70 species of scorpions found in the hotspot, nearly 20 are endemic. Monkey beetles, which are almost exclusively found in southern Africa, are concentrated in the area. Along with many types of wasps and bees, these beetles pollinate the hotspot's diverse plant species. Perhaps the most unusual invertebrates found here are the long-tongued flies (Memestrinidae), which can have mouthparts up to 50 millimetres long. The flies are the exclusive pollinators of 28 different plant species in Namaqualand.
The vegetation of the Succulent Karoo is in somewhat better shape than in other hotspot. A sparse human population of only 300,000 people and the fact that more than 90 percent of the hotspot is used for natural grazing (a form of land use that is, theoretically, compatible with maintenance of biodiversity) have eased the conversion pressures on this region as compared to the other hotspots. It is difficult to arrive at accurate estimates of the amount of vegetation remaining intact, but at least five percent has been irreversibly lost to overgrazing, mining and agriculture, with about two-thirds of the land having been seriously overgrazed, especially in Namaqualand. Consequently, only an estimated 30,000 km, or 29 percent, of the hotspot remains in a relatively pristine state.
Diamond mining has had a very heavy impact on the Namaqualand coastline and alluvial terraces of the lower Orange River Valley. Approximately two-thirds of the South African coastline, and almost all the Namibian coastline in this hotspot, has been mined for diamonds. This mining is now supplemented by the large-scale extraction of heavy minerals, including gypsum, marble, monazite, kaolin, ilmenite, and titanium, which threatens to vastly increase the impact of mining on the region's biodiversity.