Boundaries

The Karstveld sinkhole lakes and caves are distinctive subterranean habitats, supporting a substantial number of endemic aquatic species within several taxonomic groups. The Karstveld is the name given to extensive dolomite and limestone formations located to the southeast and east of the Etosha Pan in Namibia. The area covers the Otavi Mountains and continues westward through the northern parts of Outjo District as far as Otjovasandu (Irish 1992).

Freshwater habitats

Sinkhole lakes and caves are karst features characteristic of dolomite that are formed as water penetrates cracks in the dolomite and leaches away the rock. This process forms water-filled underground caves, and sinkholes result when the roof of a cave collapses. The karstic waters are permanent and mainly underground (Irish 1992; Barnard 1998; Curtis et al. 1998).

The Karstveld groundwater system consists of a number of distinct water bodies including subterranean cave lakes, cenotes, and hemi-cenotes. Cave lakes occur in laterally developed horizontal cave systems. Cenotes are water-filled sinkholes, and hemi-cenotes are cenotes with openings that are small relative to the size of the water surface below ground. The sinkhole lakes are moderately clear and water temperatures average 19-27°C (Stuart et al. 1990).

Terrestrial habitats

Mountainous savanna dominates the vegetation in the Karstveld ecoregion.

Description of endemic fishes

Clarias cavernicola, endemic to Aigamas Cave, is southern Africa’s only true cavefish (Skelton 1993). This depigmented fish has reduced eyes and feeds on bat droppings, animal carcasses, and insects that fall into the water from overhead (Skelton 1994). The species is endangered and may have a population of less than 200 individuals (Irish 1992; Barnard 1998). The endangered polychromatic Otjikoto tilapia, Tilapia guinasana, is endemic to Lake Otjikoto (Skelton 1987; Irish 1992).

Justification for delineation

The boundaries of this ecoregion are defined by the extent of Karstveld, the extensive dolomite and limestone formations located to the southeast and east of the Etosha Pan in Namibia. The biogeographic affinities of the Karstveld fauna are Zambezian, and a number of taxa are shared with the Okavango, Cunene, and Zambezi Rivers. The lengthy period of isolation that the Karstveld fauna have experienced, as well as the particularly stable conditions within the lakes, have led to the evolution of a number of endemic species (Skelton 1987, 1990).

Level of taxonomic exploration

Fair. Several of the caves and their lakes within this ecoregion have been mapped and explored (Irish 1991, 1992; Marais & Irish 1997), although further studies are warranted. These studies should determine the status of isolated populations and the effects that water abstraction has had on these systems. Irish (1991) recommends the collection of information on the cryptic fauna of springs, the stygobiotic fauna (aquatic species living permanently underground in the dark zone of caves) of unsampled subterranean waters, and more detailed geohydrological information.