Boundaries

Tree-lined fossil river beds and ephemeral, endorheic pans dot the semi-arid landscape of the Karoo ecoregion. The pans include the Verneukpan-Grootvloer system (Barnes 1998a), one of a series of four major pan systems found in southern Africa (Baard et al. 1985). The ecoregion as a whole includes two distinct terrestrial habitat types (Lowe-McConnell 1996): the succulent Karoo, which contains the region known locally as Namaqualand, located on the west coast of South Africa; and the more easterly Nama Karoo (Branch & Braack 1989). The ecoregion is located almost entirely within the Northern Cape province of South Africa.

Freshwater habitats

Pans such as Verneukpan and Grootvloerpan reach depths of up to 1.2 m during wet periods, though this happens rarely. For example, Verneukpan contained substantial water only five times during the period from 1885 to 1985 (Baard et al. 1985). In the succulent Karoo, surface flow in the coastal-draining rivers is also intermittent, as most of the little runoff that the rivers receive is absorbed by the sandy river beds (Heydoorn & Grindley 1981). 

Across the ecoregion, water in the rivers and many of the pans tends to brackish or even saline conditions, particularly toward the end of the wet season (Channing 1987). Closed for years at a time, the estuaries of rivers such as Groenrivier are saline to hypersaline, becoming fresher only after being scoured open by occasional floods (Heydoorn & Grindley 1981). 

Terrestrial habitats

In the succulent Karoo, the winter rainfall regime prevents the widespread growth of trees and promotes an abundance of sclerophyllous shrubs (Harrison et al. 1997). The vegetation of the Nama Karoo, by contrast, is characterised by dwarf shrubs and grasses (Lovegrove 1993). Vegetation on pans such as Verneukpan include species such as Rhigozum trichotomum, Zygophyllum retrofractum complex, Lycium schizocalyx, L. oxycarpum, Pteronia mucronata, Stipagrostis spp., and Eriocephalus aspalathoides (Lloyd & Le Roux 1985).

Dry for most of the year (Barnes 1998b), riverbeds in the Nama Karoo descend sharply from escarpments to meander across the flat plains of the Central Plateau. Lined by belts of riverine Acacia karoo thicket, the riverbeds create a network of riparian habitats that extends across the landscape (Barnes 1998b). Other riparian species include Tamarix usneoides and Euclea, Ozoroa,and Acacia shrubs (Barnes & Anderson 1998). 

Justification for delineation

This semi-arid ecoregion is defined by the succulent Karoo and Nama Karoo regions and is characterized by a depauperate aquatic fauna with a southern temperate (Cape) ichthyofauna (Skelton et al. 1995). The rivers of the Karoo ecoregion flow through an area interpreted as the relict channels and overbank deposits of large meandering mixed-load rivers. These rivers had their headwaters in the wetter Gondwanide mountain catchments in the south some 255 million years ago (Smith 1987). The floodplains supported a diverse fauna of mammal-like reptiles, many of which have been preserved in fossilized alluvial deposits (Smith 1993). The more recent paleo-history of the Karoo ecoregion can be interpreted from its fish fauna. The Orange River system forms a focus of distribution for several near-endemic lineages, with the distributions of Barbus aeneus, B. anoplus,and Labeo umbratus overlapping into the Karoo ecoregion (Skelton 1986b, 1994). Species such as B. anoplus also overlap into the Olifants catchment, and Skelton (1986a) suggests that the faunal association between the Orange and Olifants Rivers probably relates to the period when the Upper Vaal-Orange drained via the Olifants mouth (from Paleogene to late Oligocene-early Miocene times), rather than via the present Orange mouth. In addition, the escarpment edge and its associated micro-climate forms an important biogeographic refugium, with relict populations of taxa that are associated with the moist montane grassland of the escarpment edge or regions of deep alluvial sands along the old river courses. Examples of these include amphibians like Cacosternum namaquense (Branch & Braack 1989).