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Ecoregion Description
554: Karstveld Sink Holes
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
Countries:
Namibia
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).
Main rivers or other water bodies:
Within the Karstveld ecoregion, the main water bodies are the cave lake of Aigamas Cave (33 km northwest of Otavi); Dragon’s breath cave, the largest underground lake in the world with an approximated volume of 1,140,000 m3; the hemi-cenotes Harasib (63 m deep) and Aikab (58 m deep); and the cenotes Lake Otjikoto (58 m deep) and Lake Guinas (130 m deep) (Irish 1992).
Climate:
The Karstveld ecoregion receives an average rainfall of 600-700 mm annually, with distinct wet and dry seasons including summer rains (Curtis et al. 1998).
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.
Fish Fauna:
The Karstveld systems have low species richness but a high level of endemism, even between water bodies within the system. There are only three native fish species known to the Karstveld, but two of those are endemic. Pseudocrenilabrus philander is known from lakes Otjikoto and Guinas and is widespread elsewhere in southern Africa (Skelton 1993).
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).
Other noteworthy aquatic biotic elements:
At least six stygobiotic amphipods of the family Ingolfiellidae, four isopods, and two endemic fish are known from the waters of this ecoregion. Among the amphipods, Trogloleleupia dracospiritus occurs only in Dragon’s Breath Cave and T. gobabis is found only in Arnhem Cave. The aquatic isopod Namibianira aikabensis occurs in the Aikab hemi-cenote and is limited in distribution to Namibian caves (Marais & Irish 1997). Many of the water bodies of the Karstveld remain unexplored biologically and future sampling would likely reveal new species (Irish 1992; Curtis et al. 1998).
Dragon’s Breath Cave contains the largest subterranean lake in the world. At least 16 invertebrate species, including the endemic amphipod Trogloleleupia dracospiritus and one bat, Hipposideros caffer,inhabit the cave. This cave supports a detritus-based system, with all input to the food web coming from allochthonous sources, largely from dried bat guano (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.
References/sources:
Barnard, P. (1998). "Biological diversity in Namibia" Windhoek, Namibia: Namibian National Biodiversity Task Force.
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.
Irish, J. (1991). "Conservation aspects of karst waters in Namibia" Madoqua 17(2) 141-146.
Irish, J. (1992). "Cave investigations in Namibia: I. Biospeleology, ecology, and conservation of Dragon's Breath Cave" Cimbebasia 13 59-67.
Marais, E.,Irish, J. (1997). "Cave investigations in Namibia IV. Aikab hemicenote, and other karst phenomena in the Etosha National Park" Madoqua 20(1) 81-90.
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.
Skelton, P. H. (1990). "The status of fishes from sinkholes and caves in Namibia" Journal of the Namibia Scientific Society 42 75-83.
Skelton, P. H. (1987). "South African Red Data Book - Fishes" South African National Scientific Programs Report 40 199.
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.
