Boundaries

The Namib ecoregion is one of the most arid areas on earth but boasts a diversity of wetland ecosystems. It is situated along the Atlantic and includes the arid Skeleton Coast and the hyper-arid Namib Desert. The ecoregion extends north of the Cunene River to include the Curoca River and extends south to, but excludes, the Orange River. The ecoregion is primarily within Namibia and extends into southwestern Angola.

Topography

The desert within the ecoregion consists of a dune sea, gravel plains, and granite inselbergs. It is bordered in the east by an escarpment along most of its length. Elevations range from sea level to over 2,400 m.

Freshwater habitats

Despite the aridity, there are several types of wetlands in the Namib Desert (Logan 1960; Goudie 1972; Day 1990). Ephemeral systems include rain-fed endorheic pools or pans, rivers that flow for short periods, and the pools that remain in the riverbeds after flow has ceased (Day 1997). Endorheic pans range in size from large shallow lakes like Sossusvlei, a huge clay-bottomed lake in the midst of the Namib dune field south of the Kuiseb River, to tiny depressions in granite inselbergs. Ephemeral rivers include a number of westward flowing “sand rivers” situated along the coast (Jacobson et al. 1995). These rivers are vital to the biological functioning of the entire ecoregion.

There are twelve major ephemeral rivers in the Namib coastal ecoregion and their catchments vary in size from less than 2,000 km2 (the Khumib River) to over 30,000 km2 (the Swakop River) (Loutit 1991). The ephemeral rivers are dependent on water from upstream to replenish their underground aquifers. Recharge of the aquifers tends to be slow due to the presence of clay and silt lenses in the riverbeds and is only completed after a few days flooding (Bate & Walker 1993). Rainfall is highly seasonal and river discharges may range within an order of magnitude from the dry to the wet season (Day 1997).

The perennial Cunene River flows over the rim of the continental plateau onto the Atlantic coastal slope; at the junction of these two formations is the 122 m high Ruacana Falls. The Cunene below the falls is included in this ecoregion and throughout this reach the river’s flow is punctuated by a series of rapids (Roberts 1975). The lower 340 km of the Cunene has a steep gradient and flows swiftly in a narrow gorge with no floodplains or associated standing water and little vegetation (Curtis et al. 1998).

A number of the ephemeral rivers have permanent or semi-permanent wetlands at their mouths. These wetlands tend to be vegetated by species of sedge and reeds, such as Typha, Scirpus, Phragmites, Cyperus, and Juncellus, with large stands of Suaeda and Tamarix (Jacobson et al. 1995). Sandwich Harbor, about 50 km south of Walvis Bay, contains a freshwater marsh that is fed at least partially by fresh subsurface water. A number of estuaries, including the Uniab, have extensive reedbeds supported by semi-permanent trickles and pools of relatively fresh water (Day 1997). The estuaries of the other sand rivers are normally blocked by sand but are opened to the sea during occasional flooding events.

Terrestrial habitats

Linear oases of riparian vegetation line the river channels that cross the otherwise sparsely-vegetated desert. Because of permanent underground flow, trees grow along the river courses where groundwater is sufficiently close to the surface (Jacobson et al. 1995). In the Kuiseb River, the four main species found in the riverbed are Acacia albida, A. erioloba, Euclea pseudebenus, and Tamarix usneoides. These species grow in approximately that order away from the river bank and decrease in size accordingly. They provide an extremely important forage supply for many domestic and wild animals during times of drought (Bate & Walker 1993; Jacobson et al. 1995).

Description of endemic fishes

The lower Cunene supports six endemic fish within this ecoregion. The endemic fish are Chetia welwitschi, Kneria maydelli, Orthochromis machadoi, Thoracochromis albolabris, T. buysi, and Sargochromis coulteri (Skelton 1993). Permanent seeps along the tributaries of the Cunene are critical for supporting certain of these species, like Kneria maydelli (Barnard 1998).

Justification for delineation

This ecoregion is defined by the coastal and largely ephemeral rivers and wetlands of the Skeleton Coast and Namib Desert, from above the Orange River north to the Curoca River. Paleohydrological studies of this ecoregion reveal that the climate has been highly variable over time and that it was substantially wetter (albeit temporarily) about 20,000 and again roughly 6,000 years before present (Jacobson et al. 1995). As rainfall decreased and evaporation increased at the beginning of each dry climatic period, many wetlands changed from permanent to temporary systems. Only those aquatic organisms that were adapted to periods of desiccation would have survived in these systems. Similarly, there must have been many localized species extinctions and adaptations in permanent springs and pools that gradually became hyper-saline as evaporation exceeded precipitation. Speciation (and thus endemism) was encouraged by the island-like nature of the wetlands in the ecoregion, which are separated by huge arid “seas,” thus minimizing the movement of genetic material between wetlands. The fauna of the upper Cunene River has affinities with the Zambezi River and thus, the headwaters of the Cunene are included in the Zambezian Headwaters [555] ecoregion. The present day Cunene is thought to have formed through a coastal drainage capturing the headwaters of the Cunene that previously connected to the Zambezi system (Skelton 1994).

Level of taxonomic exploration

Poor. Wetlands in arid regions such as the Namib ecoregion are poorly researched. These wetlands could, however, prove to be regionally significant and of immense conservation value (Breen 1991). Little is known about the ecology and life history of most of the wetland biota of the ecoregion (Jacobson et al. 1995), and even basic quantitative information and monitoring data on many taxa are inadequate or nonexistent due to a lack of staff and funds (Day 1997).