Ecoregion Description

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Species Richness

# of Endemic Species

Threats

553: Etosha

Major Habitat Type:

xeric freshwaters and endorheic (closed) basins

Author:

Lucy Scott, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa

Reviewers:

Paul Skelton, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa; Holger Kolberg, Directorate Scientific Services, Ministry of Environment and Tourism, Windhoek, Namibia; a

Countries:

Angola; Namibia

Boundaries:

This ecoregion is defined by the Cuvelai River drainage basin, which includes the ephemeral Etosha Pan. The seasonally wet oshanas and pans of the Etosha ecoregion provide habitat for a surprisingly diverse freshwater fauna.

Main rivers or other water bodies:

Etosha’s drainage system, which straddles the border of Angola and Namibia, includes the ephemeral shallow streams, inter-linked water courses locally known as oshanas (grass covered temporary water channels) and pans (circular shaped depression temporarily holding water, often endorheic) of the Cuvelai River. The main source of water to Etosha Pan is from the Cuvelai drainage system. The Cuvelai River originates in Angola, and its catchment lies between the Cunene basin in the west and the Okavango basin in the east (van der Waal 1991; Barnard 1998).

Topography:

The Etosha ecoregion has a relatively arid climate. Rain falls mainly in summer and precipitation increases slightly from west to east and decreases greatly from north to south. The average annual rainfall is 400-550 mm and falls mostly from December to March (Barnard 1998).

Climate:

Freshwater habitats:

A number of ephemeral rivers, some of them with origins as far north as the Angolan highlands nearly 400km north of Etosha Pan, at altitudes of up to 1,450 m above sea level, feed the oshanas and pans of this ecoregion. At the highest altitudes the source streams are perennial and well-defined, fed by annual rainfall of over 1,000 mm on average (van der Waal 1991). Moving south from the headwaters of this system, the land becomes flatter and the oshanas meander towards the Namibian border. Rainfall declines to less than 500 mm per year and the anastomizing channels eventually connect to form a large inland inverted delta within northern Namibia with a southward gradient of more than 1:10 000. The oshanas receive local rain water on average two out of three years and drain an area of 50,000 km² to flow via the westernmost Oshana Etaka, Cuvelai and Oshigambo into the interconnected Omadhiya pans (including Lake Oponono). The Ekuma River flows south from this series of lakes to drain into the Etosha Pan, a flat, saline depression roughly 7,000 km²(Miller 1997). On average, large floods in the Cuvelai drainage system reach the Etosha Pan only once every 7-10 years (Barnard 1998; Mendelsohn et al. 2000). In some wet years, the Etosha Pan receives additional water from the Omuramba Owambo, an oshana entering the Etosha Pan from the east through Fischer’s Pan (van der Waal 1991; Mendelsohn et al. 2000). A massive build-up of fish numbers and a subsequent mass migration of young fish take place down the rivers into the oshana region during good rain years when the dense local human population in the oshana region harvests the abundant fish life.

Terrestrial Habitats:

The landscape in the Etosha ecoregion consists of saline desert with dwarf savanna fringe around the Etosha Pan. Within the savanna fringe is a mosaic of grasslands, shrublands, and woodlands. The annual grass Sporobolus salsus grows on the pan itself, after good rains or flooding and blue-green algae cover the surface of the pan during the rainy season. The sedge Cyperus marginatus is also common on the pan margins, as are several perennial grasses (Mendelsohn et al. 2000). Halophytic vegetation lines the edge of the pan consisting principally of Sporobolus spicatus, S. ioclados, S. tenellus, Odyssea paucinervis, and the small shrubs, Suaeda articulata and Salsola tuberculata (Mendelsohn et al. 2000). Atriplex vestita and Sporobolus tenellus are also present as are the occasional patches of annuals such as Chloris virgata, Diplachne fusca, Dactyloctenium aegyptium, and Eragrostis porosa (White 1983).

The Etosha Pan is a salt pan, lying on a basement of impermeable limestone that has been eroded away by wind to form a depression into which floodwater flows and then evaporates, leaving salt deposits. Clay pans in the area are much smaller depressions in sandy areas that are permeable to water, and the soils are rich in nutrients (Barnard 1998; Mendelsohn et al. 2000).

The Oshana region lies as an inverted delta of more than 100km wide north of Etosha and stretches into southern Angola. This region has higher lying Mopane savanna but the watercourses are open grass covered during the dry season dominated by Echinochloa, Elytrophorus and the sedge Fuirea. During the wet part of the annual cycle, aquatic plants including Marsilea, Aponogeton, Utricularia, Nymphaea, Nymphoides, Ipomoeia, Ottelia are found. Pans genearally are more saline and Sporobolus spicatus and Odyssea paucinervis dominate (Clarke 1998).

Other noteworthy aquatic biotic elements:

When the ephemeral Etosha and Cuvelai pans receive floodwaters, they are transformed into important habitats for numerous wetland species. The ecoregion has several endemic aquatic species, including 16 crustaceans (all Ostracods) and 4 insects (Curtis et al. 1998). About 150 plants, 40 crustaceans, and 19 fish have been recorded in the oshanas (van der Waal 1991; Barnard 1998). Etosha National Park supports at least 340 birds (Simmons et al. 2001) and about 40% of the wetland birds within the Cuvelai drainage are included in the Namibian Red Data Book (Kolberg et al. 1996).

The biodiversity of this ecoregion is entirely dependent on seasonal flooding events. Amphibians and aquatic invertebrates, such as fairy shrimp, that are adapted to long dry spells come to life when the floodwaters enter the ephemeral pans and oshanas (Mendelsohn et al. 2000). The Etosha Pan and oshanas are one of two regular breeding sites in southern Africa for lesser and greater flamingos, Phoenicopterus minor and P. ruber, respectively. Historically, the combined population of these birds was known to reach 1 million individuals in some years; in more recent times, numbers have only reached about 20,000 (Barnes 1998). Breeding success has been limited and the population here is no longer considered a viable breeding population (Simmons 1996; Simmons et al. 2001). There are also large breeding colonies of great white pelican (Pelecanus onocrotalus) and chestnut-banded plover (Charadrius pallidus), and the Etosha Pan supports the only breeding population (only about 60 individuals) of vulnerable blue cranes (Grus paradisea) outside of South Africa (Simmons et al. 2001).

Justification for delineation:

The biogeographic affinities of the Etosha ecoregion are Zambezian due to past river connections to the east (Skelton 1993, 1994). The Owambo basin, in which the greater Etosha/Cuvelai drainage is situated, formed 70 million years ago. Since then it has been filling up with sand, silt, and clay washed in and blown from higher ground. Large rivers once drained into the basin during periods of higher rainfall to form large, shallow lakes that often dried up quickly, leaving behind concentrations of salts that are responsible for the brackish quality of much of the groundwater. The Cunene River flowed into the Owambo Basin until about 12 million years ago, when continental uplift changed the slope of the land, causing the Cunene ultimately to flow west from the Ruacana Falls to the Atlantic. The Oshana Etaka to the west of the Etosha system is a relatively deep watercourse and may represent part of the original course of the Cunene River (Mendelsohn et al. 2000).

Level of taxonomic exploration:

Fair. The state of knowledge is moderate and research could be directed toward the study of the ecological functioning of the system.

References/sources:

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

Barnes, K. N. (1998)"Important Bird Areas of South Africa - an introduction" In Barnes, K.N. (Ed.). The important bird areas of southern Africa. (pp. 27-45) Johannesburg, South Africa: BirdLife International.

Barnes, K. N. (1998)"Important Bird Areas of the North-west Province" In Barnes, K.N. (Ed.). The Important Bird Areas of southern Africa. (pp. 93-101) Johannesburg, South Africa: BirdLife International.

Clarke, N. V. (1998). "Guide to the common plants of the Cuvelai wetlands" Windhoek, Namibia: Southern African Botanical Diversity Network.

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.

Kolberg, H., Griffin, M., et al. (1996)"The ephemeral wetlands of central northern Namibia" In Hails, A.J. (Ed.). Wetlands, biodiversity and the Ramsar Convention. (pp. 40-42) Gland, Switzerland: Ramsar Convention Bureau.

Mendelsohn, J., el Obeid, S., et al. (2000) A profile of north-central Namibia. (pp. 79) Windhoek.: Gamsberg Macmillan Publishers.

Miller, R. M. (1997)"The Owambo Basin of Northern Namibia" In Selley, R.C. (Ed.). African basins. (pp. 237-268) Amsterdam, The Netherlands: Elsevier.

Simmons, R. E. (1996). "Population declines, viable breeding areas, and management options for flamingos in southern Africa" Conservation Biology 10(2) 504-514.

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.

Skelton, P. H. (1993) A complete guide to the freshwater fishes of Southern Africa. South Africa: Southern Book Publishers, Halfway House.

van der Waal, B. C. W. (1991). "Fish life of the Oshana delta in Owambo, Namibia, and the translocation of Cunene species" Madoqua 17(2) 201-209.

White, F. (1983) "The vegetation of Africa, a descriptive memoir to accompany the UNESCO/AETFAT/UNSO vegetation map of Africa, Natural Resources Research 20: 1-356". Paris, France: UNESCO.