Succulent Karoo

Arid lands tend to have highly endemic floras and faunas, but are usually relatively species poor. The Succulent Karoo is an extraordinary exception, and is one of only two arid regions in the world that can be considered a true biodiversity hotspot (the other being the Horn of Africa, which is recognized here as a new hotspot). It harbors a staggering 6 356 plant species (in 168 families and 1 002 genera) (Driver et al. 2003) and the richest succulent flora in the world (Van Jaarsveld 1987).

This predominantly winter-rainfall desert, which occupies 102 691 km² on the arid fringes of the Cape Floristic Region, has been divided by biogeographers into two zones, the strongly winter-rainfall Namaqualand-Namib Domain of the Cape west coast and southern Namibia (generally known as Namaqualand) and the Southern Karoo Domain, where rainfall often peaks in the spring and autumn months (Jürgens 1991). In broad phytogeographical terms, the Succulent Karoo forms part of the Karoo-Namib Regional Center of Endemism (White 1983). However, the area is largely transitional between the Cape Floristic Region and the Nama-Karoo, and shows a particularly strong floristic affinity 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 (Jürgens 1991; J. Born, P. Linder, and P. Desmet, unpubl.)

The typical vegetation of the Succulent Karoo is a dwarf shrubland dominated almost entirely by leaf succulents in the Mesembryanthemaceae, Crassulaceae, Asteraceae and Liliaceae families (Milton et al. 1997). Some 1 700 out of a total of 1 843 estimated succulents are leaf succulents in the Succulent Karoo, and this dominance is unique among the deserts of the world (Jürgens 1986). The recent and explosive diversification of the Mesembryanthemaceae, the largest group, has been described as an event unrivaled among flowering plants (Ihlenfeldt 1994; Desmet et al. 1998; Klak et al. 2004). Indeed, the exceptionally high vascular plant diversity and endemism (2 539 species, or 40%, and 80, or 8% of genera) of the Succulent Karoo is associated with massive speciation within the predominantly succulent and bulbous genera. Stem succulents, comprising some 130 species, include species of Euphorbia, Tylecodon, Othonna, Pelargonium, and numerous stapeliads. Seasonal bulbs (1 143 species) and annuals (390 species), which appear in the open spaces between the shrubs, provide magnificent spring displays, especially in Namaqualand.

The quartz gravels of the Knersvlakte, as well as similar habitats elsewhere in Namaqualand and in the Little Karoo, support a unique vegetation type characterized by an extremely sparse cover of locally endemic Mesembryanthemaceae and other succulents. Nearly all of these are stemless, embedded forms (stone plants). The Succulent Karoo is home to about 700 such species belonging to the genera Argyroderma, Bulbine, Conophytum, Crassula, Haworthia, and Lithops. Another unusual feature of much of the Succulent Karoo is the presence of heuweltjies or mima-like mounds (Dean and Milton 1999). The mounds are of zoogenic origin, originally created by termites (Microhodotermes viator) and often colonized by a wide variety of burrowing animals. As a result, nutrient-enriched subsoil is brought to the surface, creating a substratum physically and chemically very different from the intervening matrix. Thus, heuweltjies support a flora that is markedly distinct in appearance and composition from that of the surrounding vegetation.

Among the most famous of all the attractive succulent species found in the Succulent Karoo is the halfmens (“half human”) (Pachypodium namaquanum), a stem succulent endemic to the Richtersveld and that grows to a height of up to 4 m. The stems of these plants are inclined to the north, thus creating an uncanny resemblance to humans peering northwards to the distant plains of southern Namibia. Khoi-khoi folklore has it that the plants are the remnants of an ancient tribe, banished by warfare from their original home in Namibia, which gaze nostalgically across the Orange River to their homeland. The scientific explanation for this unusual orientation is equally engaging (Rundel et al. 1995). Growing on shaded slopes, the plants 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 fauna of the Succulent Karoo is rich in endemic species, especially among arachnids, hopliniid beetles, aculeate Hymenoptera, and reptiles (Vernon 1999). Estimates from available data suggest that endemism in insects is especially high (>50%) for groups examined (Hymenoptera [Apoidea] and Isoptera) (Driver et al. 2003). The predictable rainfall appears to have resulted in many resident forms of invertebrates and small vertebrates, which has led to isolation and speciation. Several invertebrate groups are particularly well represented, especially those that reach high levels of diversity in arid regions, such as scorpions (with 18 of the 70 scorpions in the Succulent Karoo endemic; Driver et al. 2003). Monkey beetles (Rutelinae: Hoplini), a group largely endemic to southern Africa, are concentrated in the Succulent Karoo, where some genera are important pollinators of daisies and mesembs.

Among the vertebrates, there are, at present, 74 mammals recorded from the Succulent Karoo, of which two are endemic: De Winton's golden mole (Cryptochloris wintoni, VU) and the Namaqua dune mole-rat (Bathyergus janetta). Birds are represented by 227 regularly occurring species, including one endemic, the recently described Barlow's lark (Certhilauda barlowi). In addition, the black harrier (Circus maurus, VU), with its distinctive black-and-white plumage, has the most restricted range of the world's 13 harrier species (Harrison et al. 1997) and is frequently observed hunting over the Namaqualand plains. Amphibians are poorly represented, with only 29 species of frogs, one of which, the desert rain frog (Breviceps macrops, VU) is endemic, occurring on the Namaqualand coast of South Africa north to Luderitz in coastal southwestern Namibia. Reptiles are very diverse, with 94 species, of which 15 are endemic. These include 58 lizard and gecko species with 15 endemics (the genera Cordylus and Pachydactylus are especially rich, with eight and 12 species and six and two endemics, respectively) and 29 snakes, of which none is endemic. The freshwater fish fauna includes 26 indigenous species, though none are endemic (modified from D. Impson et al., unpubl.).

Considering this wealth of unique plant species, and high endemism, it is encouraging that the Succulent Karoo is one of the more intact hotspot systems. Over 90% of the region is used for natural grazing, a form of land use that is, theoretically, compatible with maintenance of biodiversity and ecosystem processes. It is difficult to make accurate estimates of the amount of intact vegetation. It is known that at least 5% of the biome has been irreversibly lost to mining (3%) and cropping agriculture (2%), and about 0.1% to urbanization (Driver et al. 2003). The amount transformed or degraded by overgrazing is unknown, although one estimate states that approximately 70 000 km², especially the communally owned parts of Namaqualand, have been seriously overgrazed (Davis et al. 1994). Thus, optimistically, only some 30 000 km² of the Succulent Karoo, or about 29%, exist in a relatively pristine state, although this figure is probably much lower.

A number of threats are likely to take on greater importance in this region in the future. The rise of the ostrich farming industry has resulted in the degradation of thousands of hectares of veld in the Little Karoo (Driver et al. 2003). Also, the expected increase in the extent of communal land in Namaqualand and elsewhere in the region will undoubtedly accelerate desertification, too (Hoffman et al. 1999), while dryland farming in Namaqualand is practiced where the annual rainfall is as low as 150 mm per year. And, of course, diamond mining has had a major impact on the Namaqualand coastline and alluvial terraces of the lower Orange River Valley (Davis et al. 1994). Approximately 65% of the South African, and almost all Namibian Atlantic coastline of the Succulent Karoo, has been mined for diamonds. The extraction of heavy minerals, an equally destructive process, continues to develop as a major industry on the coast of southern Namaqualand.

Remarkably, for a country like South Africa renowned for its well-established game parks and reserves, protected area coverage in this hotspot is poor. According to the World Database on Protected Areas, only 2.5% of the area has some form of protection (with only 1.8% in IUCN categories I to IV). Regional information proves the situation is not much better, with only 3.5% of the Succulent Karoo formally protected in category I reserves; these are statutory reserves managed primarily for biodiversity conservation, and include National Parks and Provincial Nature Reserves (Driver et al. 2003). An additional 2.3% of the biome is protected in category II reserves. These are statutory and non-statutory reserves managed for biodiversity conservation and/or other land uses. A statutory reserve managed for biodiversity conservation and other land uses would be classified as a category II reserve rather than a category I reserve. This is the case with the Richtersveld National Park, a contractual national park in which both conservation and other land uses occur. Other category II reserves include municipal reserves and conservancies (voluntary agreements among private landowners). Considering that the region has more than 936 Red Data Listed plant species (Driver et al. 2003), the reserve system for the Succulent Karoo is grossly inadequate for conserving the region's biodiversity. It is estimated that 59% of the region's quarter-degree grid cells would be required in a reserve system to represent each plant species at least once (Lombard et al. 1999).

Recently, there have been two positive developments regarding the conservation status of the Succulent Karoo. First, the reserve system is being expanded, although at the current rate of expansion it will take 130 years to achieve the reservation targets set by the Succulent Karoo Ecosystem Program (SKEP) project (P. Desmet, unpubl.). The creation of the Namaqua National Park (ca. 600 km²) in the central uplands of Namaqualand is a positive development. This park is set to expand westwards to encompass Sandveld habitats on the coastal plain as well as the marine zone. Secondly, the SKEP project has created much awareness in the region, as well as interest in conserving biodiversity, and fortunately funds have become available recently to fuel this awareness and interest. The Critical Ecosystem Partnership Fund (CEPF) has allocated $8 million for this hotspot over five years. Already action is taking place in numerous geographic priority areas, with technical assistance in project design and proposal preparation provided to numerous civil society groups, and grant funds being disbursed in support of conservation initiatives throughout the region. Fortunately, the situation in the Succulent Karoo is such that timely action can still ensure that a strong, representative cross section of its rich and diverse desert landscapes will survive into the future.

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