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. 

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). 

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).

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.