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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

552: Namib

Major Habitat Type:

xeric freshwaters and endorheic (closed) basins

Author:

Belinda Day, Freshwater Research Institute, Zoology Department, University of Cape Town, Cape Town, South Africa

Reviewers:

Paul Skelton, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa and Peter Jacobson, Department of Biology, Grinnell College, Grinnell, Iowa, USA

Countries:

Angola; Namibia; South Africa

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.

Main rivers or other water bodies:

The ecoregion contains a single perennial river, the Cunene; all others are ephemeral.

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.

Climate:

The climate of the ecoregion is extremely harsh and variable. As in most arid areas, rainfall is not only very low, but is also extremely unpredictable and evaporation rates are high (Loutit 1991; Jacobson et al. 1995). There is a strong climatic gradient trending from the east, where the ephemeral rivers have their headwaters, to the west where they meet the sea. Rainfall decreases in these westward flowing catchments from approximately 500 mm in the east to between 0 and 25 mm per annum at the coast (Jacobson et al. 1995). Rainfall tends to be intense and of short duration, with the result that soil infiltration rates are low. On average, only 2% of rainfall becomes surface-runoff, 1% recharges aquifers, 14% is lost through evapotranspiration, and 83% evaporates (EEU: Environment Evaluation Unit at University of Cape Town 1990). Evaporation reaches a maximum of about 3,400 mm per year at the mouth of the Cunene River (Barnard 1998).

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).

Other noteworthy aquatic biotic elements:

The linear oases formed by the ephemeral rivers are vital for supporting both aquatic and terrestrial biodiversity within the ecoregion. Flooding in these rivers brings enormous quantities of silt, nutrients, organic matter, and seeds downstream. The silt and organic debris are quickly colonized by algae, fungi, seedlings, and a host of invertebrates, which form the basis of a complex food chain that sustains the longitudinal ecosystems (Jacobson et al. 1995; Jacobson et al. 1996; Jacobson et al. 1999, 2000a, 2000b). These riverine corridors, as well as freshwater springs and pools, allow large herbivores (such as antelope, elephant, and rhinoceros) and carnivores (including lions, black-backed jackals, and leopards) to move westwards through the desert to the coast during periods of prolonged rains (Jacobson et al. 1995).

The Cunene supports about two hundred species of aquatic molluscs, crustaceans, insects, amphibians, and fish throughout its length. The lower Cunene is the southernmost extent of the freshwater shrimp, Macrobrachium vollenhoveni, and the freshwater oyster, Etheria elliptica (Curtis et al. 1998). Over 300 species of birds depend on the riverine corridor of the Cunene River and the highly localized Cichladusa ruficauda and Estrilda thomensis are associated with the riverine palms and thickets (Simmons et al. 2001).

Outside the Cunene, the ecoregion’s waters are depauperate for most aquatic taxa and host few endemics. Only about 144 species of aquatic molluscs, crustaceans, insects, amphibians and fish are known from the ephemeral rivers of Namibia, and the majority of these are insects (about 90 species) (Curtis et al. 1998). Although relatively large populations of wetland birds congregate at the mouths of the ephemeral rivers, there are no endemic waterbirds. Similarly, there are no wetland mammals and only a few species of freshwater fish, none of which is endemic.

Although poorly known, the invertebrate fauna of the temporary ponds and hyper-saline springs is extremely diverse and often highly specialized (Irish 1991). A number of aquatic invertebrate taxa, including the branchiopods and the ostracods, contain some endemics (Curtis et al. 1998). The microinvertebrate fauna in the permanent underground streams below the ephemeral sand rivers is completely unstudied, but is likely to be distinctive. Further investigation of these ecosystems is likely to reveal new species and endemics.

Despite its aridity, the ecoregion is rich in frog species (Channing & Griffin 1993). About seventeen aquatic-obligate frogs live along the waterbodies of this ecoregion. Most species live in the vleis (wetlands), pools, backwaters, and wetland vegetation that develop in the riverbeds after rain. Many frogs survive by burying themselves in the sand or mud during times of drought. Others are found in non-riverine temporary waterbodies, rock pools, vleis, and pans that develop after heavy rains, and still others are found only in permanent springs. One species, Breviceps macrops, appears to be endemic to the fog belt of the coastal dunes (Channing & Griffin 1993).

Sandwich Harbor is an extremely important bird habitat; counts have exceeded 300,000 birds in some years, and congregations of 50,000 wetland birds in summer and 20,000 in winter regularly occur (Simmons et al. 2001). The Walvis Bay wetlands, including the Kuiseb estuary, cover over 35 km2 and support a large proportion of southern Africa’s flamingo populations as well as many migratory species (du Toit & Scuazzin 1995). The mouth of the Cunene River also hosts up to 56 wetland bird species and is a stopover on the migratory bird flyway to and from southern Africa (Simmons et al. 1993).

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).

References/sources:

Barnard, P. (1998). "Biological diversity in Namibia" Windhoek, Namibia: Namibian National Biodiversity Task Force.

Bate, G. C.,Walker, B. H. (1993). "Water relations of the vegetation along the Kuiseb River, Namibia" Madoqua 18(2) 85-91.

Breen, C. M. (1991). "Are intermittently flooded wetlands of arid environments important conservation sites?" MADOQUA 17(2) 61-65.

Channing, A.,Griffin, M. (1993). "An annotated checklist of the frogs of Namibia" MADOQUA 18(2) 101-116.

Curtis, B., Roberts, K. S., et al. (1998). "Species richness and conservation of Namibian freshwater macro-invertebrates, fish and amphibians" Biodiversity and Conservation 7(4) 447-466.

Day, J. A. (1990)"Environmental correlates of aquatic faunal distribution in the Namib Desert" In Seely, M.K. (Ed.). Namib ecology: 25 years of Namib research. (pp. 99-107) Pretoria: Transvaal Museum.

Day, J. A. (1997). "The status of freshwater resources in Namibia" Namibian Directorate of Environmental Affairs Research Discussion Paper 22

du Toit, D.,Scuazzin, T. (1995). "Sink or swim… water and the Namibian environment" Windhoek: Desert Research Foundation of Namibia.

Goudie, A. (1972). "Climate, weathering, crust formation, dunes and fluvial features of the Central Namib Desert near Gobabeb, southwest Africa" Madoqua 1(15-31)

Irish, J. (1991). "Conservation aspects of karst waters in Namibia" Madoqua 17(2) 141-146.

Jacobson, P. J., Angermeier, P. L., et al. (1996). "The conservation significance of ephemeral rivers in northwestern Namibia" Ecological Soc. Of America Abstracts 77(3) 214.

Jacobson, P. J., Jacobson, K. M., et al. (2000). "Hydrologic influences on soil properties along ephemeral rivers in the Namib Desert" Journal of Arid Environments 45 21-34.

Jacobson, P. J., Jacobson, K. M., et al. (2000). "Variation in material transport and water chemistry along a large ephemeral river in the Namib Desert" Freshwater Biology 44 481-492.

Jacobson, P. J., Jacobson, K. M., et al. (1999). "Transport, retention, and ecological significance of woody debris within a large ephemeral river" Journal of the North American Benthological Society 18 429-444.

Jacobson, P. J., Jacobson, K. M., et al. (1995). "Ephemeral rivers and their catchments: Sustaining people and development in western Namibia" Windhoek: Desert Research Foundation of Namibia.

Logan, R. F. (1960) "The central Namib Desert, southwest Africa, National Research Council Publication 758". Washington, D.C.. National Academy of Sciences.

Loutit, R. (1991). "Western flowing ephemeral rivers and their importance to wetlands in Namibia" Madoqua 17(2) 135-140.

Roberts, T. R. (1975). "Geographical distribution of African freshwater fishes" Zool. J. Linn. Soc. 57 249-319.

Simmons, R. E., Boix-Hinzen, C., et al. (2001)"Namibia" In Fishpool, L.D.C.;Evans, M.I. (Ed.). Important Bird Areas in Africa and associated islands: Priority sites for conservation. (pp. 639-660) Newbury and Cambridge, UK: Pisces Publications and BirdLife International (Birdlife Conservation Series No. 11).

Simmons, R. E., Braby, R., et al. (1993). "Ecological studies of the Cunene River mouth: Avifauna, herpetofauna, water qualtiy, flow rates, geomorphology and implications of the Epupa Dam" Madoqua 18(2) 163-180.

Skelton, P. H. (1994). "Diversity and distribution of freshwater fishes in East and Southern Africa" Annals of the Royal Central Africa Museum (Zoology) 275 95-131.

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