Ecoregion Description

569: Okavango

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

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

Reviewers:

Paul Skelton, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa and Ketlhatlogile Mosepele, University of Botswana – HOORC, Maun, Botswana

Countries:

Angola; Botswana; Namibia; Zimbabwe

Boundaries:

This ecoregion encompasses the floodplains of the Okavango Delta. The ecoregion is situated in northern Botswana and includes one of few major inland endorheic deltas on the continent and one of the most important wetlands in southern Africa.

Drainages flowing into:

The Okavango is an endorheic basin.

Topography:

The Okavango River system, which has an average altitude of 900-1200 m, rises in the central highlands of Angola on the Benguela Plateau. It shares its watershed divide with the westward-flowing Cunene and Cuanza, and it flows south and east into the mainly arid Okavango basin to diverge into the many arms of the Okavango swamps (Beadle 1981).

Climate:

The climate of the ecoregion is subtropical, with a summer rainfall and precipitation increasing slightly from west to east. An average annual rainfall of about 650 mm falls mostly from December to April (Jubb & Gaigher 1971). Due to the ecoregion's location on the fringe of the semi-arid Kalahari Desert, 96% of the inflow to the delta is lost through evapotranspiration (Ellery & McCarthy 1998).

Freshwater habitats:

The Okavango Floodplains ecoregion contains the largest expanse of wetlands in southern Africa and supports a range of habitats and a rich bird fauna. The Okavango delta is one of the largest inland deltas in the world (Allanson et al. 1990). The permanent Okavango swamp covers 6,000-8,000 km2, but annual floods from the highlands of southern Angola inundate the floodplains, expanding the seasonally inundated area to 12,000-15,000 km2. Near the tributaries, the swamps are intersected with numerous deep, perennial channels, shallow floodwater channels, and backwaters or lagoons. Stony rapids are absent from the swamps and the water is clear throughout the year. The swamps in the delta vary in character mainly according to the perennial or intermittent occurrence of floodwaters. In the northwestern section of the delta, which is permanently flooded, the swamps are mostly covered with a dense growth of papyrus (Cyperus papyrus), stands of reeds (Phragmites), and bullrush (Typha). In the lower part of the delta, where the floodwater is more seasonal, the dry depressions are periodically filled with water and generally only have reeds and papyrus around the edges (Jubb & Gaigher 1971).

Terrestrial Habitats:

A region of thorn scrub / mopane (Colophospermum mopane) savanna is located in the north. It includes stretches of dense dry Baikiaea plurijuga forest, mopane and Pterocarpus woodland savanna, thorn scrub (Acacia/Sclerocarya) savanna, and semi-arid grassland. Forest savanna and dry woodland predominate in the drier southern portion of the ecoregion (Stuart et al. 1990).

Fish Fauna:

The extensive floodplains support a moderately rich aquatic fish fauna when compared with other floodplain systems in Africa. About 80 species of fish have been recorded from the rivers and swamps of the Okavango. Three fish species with restricted distributions in scarce rocky habitats are the ocellated spiny-eel (Aethiomastacembelus vanderwaali), largemouth squeaker (Synodontis macrostoma), and the broad-headed catfish (Clariallabes platyprosopos) (Skelton 1993). The seasonal floods of the Okavango are the most important driver of ecological processes in the region and the distribution of fishes is generally governed by the flood cycle. The migration of fish out of the main channels into adjacent flooded areas and swamps coincides with the first floods each year in November and December (Merron & Bruton 1995). Floodplain spawning offers the advantages to juvenile fish of abundant food and well-oxygenated conditions.

Other noteworthy aquatic biotic elements:

A total of 450 bird species have been recorded in the delta. Among these are Pelecanus onocrotanus, P. rufescens, 18 species of heron, and breeding Rhynchops flavirostris. Large mixed roosts of herons, egrets, storks, and ibis occur in selected localities in the delta. Among the many Anatidae, Nettapus auritus has the largest congregations with up to 15,000 breeding pairs and 40,000 non-breeding visitors recorded (Tyler & Bishop 2001). The Okavango floodplains are important for several threatened species. They are the most important breeding site for the vulnerable slaty egret (Egretta vinaceigula). Breeding and non-breeding individuals of the vulnerable wattled crane (Grus carunculatus) also occur in the delta (Tyler & Bishop 2001).

Ecological phenomena:

Lake Ngami in the Okavango Floodplains is noted for its global importance for wetland birds (Thieme et al. 2005). During times of flooding, more than one million birds have been documented at this site (Fishpool & Evans 2001).

Justification for delineation:

This ecoregion is defined by the boundaries of the Okavango Basin, excluding the upper reaches of the river, which were historically connected with the river systems of the Upper Zambezi ecoregion [816]. The biogeographic affinities of the aquatic fauna of the Okavango are Zambezian, although it also shares a number of species with the southern tributaries of the Congo River, especially the Kasai. The faunal affinities of the region can be best explained by the theory that present day headwaters of the Congo and Cuanza were formerly part of the Okavango/Zambezi (Skelton 1994). As a result of tectonic faulting, the Okavango was impeded and deflected eastward to the Upper Zambezi and was ultimately captured by the middle Zambezi River (Beadle 1981). Skelton (1994) considers the Okavango to be a western-sector subunit of the Zambezian Province and to be the heart of the western Zambezian evolutionary arena.

Level of taxonomic exploration:

Good. The state of ecological knowledge is reasonable, and the area is well known taxonomically. In June 2000, Conservation International completed a rapid ecological assessment of the delta (Alonso & Nordin 2003). In terms of biological investigation, fish assessments and investigations into how the system functions as a whole are needed (R. Bills, South African Institute of Aquatic Biodiversity, personal communication).

References/sources:

Allanson, B. R., Hart, R. C., et al. (1990)"Inland waters of southern Africa: An ecological perspective" In Dumont, H.J.;Werger, M.J.A. (Ed.). Monographiae Biologicae 64. The Netherlands: Kluwer Academic Publishers.

Alonso, L. E.,Nordin, L. (2003) "A rapid biological assessment of the aquatic ecosystems of the Okavango Delta, Botswana: High water survey. RAP Bulletin of Biological Assessment 27". Washington, DC. Conservation International.

Beadle, L. C. (1981). "The inland waters of tropical Africa" England: Longman Group Limited.

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.

Ellery, W. N.,McCarthy, T. S. (1998). "Environmental change over the decades since dredging and excavation of the lower Boro River, Okavango Delta, Botswana" Journal of Biogeography 25(2) 361-378.

Fishpool, L. D. C.,Evans, M. I. (Ed.) (2001). "Important Bird Areas in Africa and associated islands: Priority sites for conservation" Newbury and Cambridge, UK: Pisces Publications and BirdLife International (Birdlife Conservation Series No. 11).

Jubb, R. A.,Gaigher, I. G. (1971). "Check list of the fishes of Botswana" Arnoldia Rhodesia 5(7) 1-22.

Merron, Glenn S.,Bruton, M. N. (1995). "Community ecology and conservation of the fishes of the Okavango Delta, Botswana" Environmental Biology of Fishes 43 109-119.

Porter, J. W.,Muzila, I. L. (1989). "Aspects of swamp hydrology in the Okavango" Botswana Notes and Records 21 73-91.

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

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.

Smec (1986) "Southern Okavango integrated water development Phase 1. Report to the Ministry of Mineral Resources, Water and Energy". Gaborone, Botswana. SMEC.

Stuart, S. N., Adams, R. J., et al. (1990) Biodiversity in Sub-Saharan Africa and its islands: Conservation, management and sustainable use, Occasional Papers of the IUCN Species Survival Commission No. 6. Gland, Switzerland: IUCN.

Thieme, M. L., Abell, R., et al. (2005). "Freshwater Ecoregions of Africa and Madagascar: A Conservation Assessment" Washington, D.C., USA: Island Press.

Tyler, S. J.,Bishop, D. R. (2001)"Botswana" In Fishpool, L.D.C.;Evans, M.I. (Ed.). Important bird areas in Africa and associated islands: Priority sites for conservation. (pp. 99-112) Newbury and Cambridge, UK: Pisces Publications and BirdLife International (Birdlife Conservation Series No. 11).