Ecoregions

East African halophytics

Content Cover Image

Natron Lake southern side. Source: Creative Commons

The East African Halophytics ecoregion is composed of two hypersaline lakes situated within Tanzania, along the Great Rift Valley. These lakes constitute an inhospitable environment for most plants and animals, since the waters are saline, elevated in temperature, and subject to rapid salinity changes following rains. In fact, Lake Natron, exhibits one of the highest salinities of any lacustrine body on Earth, attaining super-saturated salt levels of 340 parts per thousand.

However, the lakes are home to huge flocks of lesser and greater flamingos that breed on the mudflats which surround the lakes. The lesser flamingos filter the blue-green algae from the salty waters, and the greater flamingos feed on copepod larvae within the shallow lake waters. The East African Halophytics habitat is threatened by active human development and is not protected in any way. As a consequence, the flamingo populations that use the lakes as a principal breeding ground are also under pressure. The ecoregion is classified within the Afrotropics Realm and is considered an element of the Flooded Grasslands and Savanna Biome.

Location and general description

The East African Halophytics encompass two saline (soda) lakes, Lake Natron and Lake Bahi, both situated in the eastern arm of the Rift Valley in Tanzania. Lake Natron is the larger of the two lakes, measuring a maximum dimension of 57 kilometers (km) long and 22 km wide. It is situated in Arusha District of northern Tanzania (along the Kenyan border) at 36 °E longitude and 2° 30’ S latitude. Lake Bahi is about half the size of Lake Natron and is located in Singida District of central Tanzania, near the major town of Dodoma, at about 35 °E longitude and 6 °S latitude.

caption Source: World Wildlife Fund

Both of these saline lakes are located in Tanzania’s low elevation semi-arid region. This climatic zone receives erratic rainfall below 800 millimetres per annum, with substantial annual variation. There is a significant relationship between elevation and rainfall in this area, with every 100-metre rise in elevation corresponding to an increase of 35 mm in annual rainfall. Both of the lakes are situated at low elevation. Lake Natron is situated at 610 m on the Rift Valley escarpment, to the east of the Saleh Depression. Most of the precipitation in this locale occurs between December and May, followed by a protracted dry season. Daily temperatures at the lakes frequently exceed 40 °C. Such hot, arid conditions expose the lakes to high evaporation rates.

Long-term climatic desiccation in the area has caused these formerly large freshwater lakes to shrink to their present remnants, which are now highly saline. The continuation of this process threatens their long-term existence as permanent surface water bodies; there are already a number of other generally dry saline lake beds in the area which previously were classified as year around lakes. The lakes of this ecoregion are significantly smaller than they were during the pluvial period about 3000 to 4000 BC, and unless the rainfall rates increase substantially, they will continue to shrink until they become pans and eventually saline grasslands.

The Great Rift Valley was formed at a time when volcanic activity was more prevalent than today, spreading wide sheets of lava and volcanic ash over the ancient land surface. In more recent times, rapid weathering of the many volcanic hills and mountains along the Rift Valley has continued to deposit volcanic ash in topographic depressions, resulting in deep sodium-rich soils. These highly sodic soils form the poorly drained beds of the lakes.

Lake Natron is principally fed by the Ewaso Ngiro River, which has its catchmentCatchment is the entire area of a hydrological drainage basin. in the central Kenyan highlands. The lake is also fed by hot, mineral-laden springs that bubble up on the periphery of the lake, bringing soluble salts to the surface. Lake Bahi is fed by the Bubu River, which originates in the Mbulu Highlands of central Tanzania. Water loss from both lakes occurs solely through evaporation, as neither lake has an outlet (surface or subterranean). The high evaporation rates and the ongoing input of salts from the saline substrate result in extremely saline water. Lake Natron is the ultimate alkaline lake; water in this lake is so rich in dissolved sodium carbonate that it is frequently viscous to touch.

Floral biodiversity

The lakes are devoid of macrophytic vegetation, but are very productive in terms of algae, with blue-green algae (Cyanophyta) such as Spirulina spp. dominating the saline waters. The extensive saline mudflats surrounding the waters are generally considered to be saline deserts devoid of macrophytic vegetation. These salt-encrusted flats are covered in a layer of blue-green algae during the rainy season. A few halophytic plant species are able to survive on the saline soils fringing the lakes. The dominant halophytes surrounding the lakes are Smooth flatsedge (Cyperus laevigatus); grasses of the Crowfoot genus (Dactyloctenium spp.; Seaside rush with needle-sharp stems (Juncus maritimus); Toothbrush tree (Salvadora persica), adapted to semi-desert conditions; Sporobolus spicatus; Sporobolus robustus; Suaeda monoica; and Triplocephalum holstii. The grass Sporobolus spicatus can tolerate much higher salinities than S. robustus. The slightly less alkaline plains surrounding the lakes are dominated by a number of  grass species and by Egyptian riverhemp (Sesbania sesban), with scattered  Fever-tree thorn (Acacia xanthophloea) trees. The saline vegetation surrounding Lake Natron grades into the Serengeti Volcanic Grassland to the south and elsewhere into Acacia-Commiphora Bushland and Thicket. Lake Bahi is totally surrounded by Acacia-Commiphora Bushland and Thicket.

Faunal biodiversity

The waters of these saline lakes are extremely inhospitable, with a saturated salt solution exhibiting a pH ranging from 9 to 10, and temperatures that can attain 41 °C near the mineral springs. In addition, these conditions are not constant, but change abruptly during the rains. Heavy rains can temporarily flood the lakes with cold water and reduce the pH dramatically because the rain is slightly acidic. The mudflats surrounding the permanent water are just as inhospitable to most biota. Temperatures are extreme, winds are strong, vegetation is scarce and movement across this thick muddy environment is impractical for most mammals. As a result of these extreme and highly variable environments, species richness is low. However, the population sizes of the few species adapted to these environments are large.

Fish

A single species of fish known as the Magadi, White-lipped or Alkaline tilapia (Oreochromis alcalica) is found in abundance in the waters of these lakes. This small fish (about ten centimetres in length) inhabits aquatic edges of the hot spring inlets, where the water temperature is between 36 and 40 °C. It is endemic to the saline lakes of the Rift Valley and is adapted to tolerate the high temperatures and salinity of the lakes as well as the vicissitudes engendered by seasonal rains.

caption Flamingo species (Phoenicopterus sp.), Arusha National Park, Tanzania. Source: David Olson

Birdlife

Lakes Natron and Bahi have distinctively large populations of wetland birds, most notably flamingos. Lake Natron manifests the highest concentration of flamingos in East Africa. Both the greater and the lesser flamingo (Phoenicopterus ruber and Phoeniconaias minor) are found at both of these lakes, with the lesser flamingo outnumbering the greater by 100 to one.

Lake Natron is the major breeding ground for flamingos in East Africa and is the only regular breeding site for the lesser flamingo in Africa. Lesser flamingos bred at Lake Natron in nine out of the fourteen years from 1954 to 1967. Only one other breeding site was recorded during those years, Kenya's Lake Magadi in 1962. The Magadikgadi Pan in Botswana is the principal breeding site for the greater flamingo in Africa, with Lake Natron also being an important breeding site. Between 1951 and 1971 the greater flamingo bred at Lake Natron in five out of twelve years. While Lake Natron is an essential breeding site, it is not a focal feeding site for flamingos. The major feeding sites are Lakes Nakuru and Bogoria in Kenya, which are slightly less saline and have a greater abundance of small crustaceans and blue-green algae.

Africa’s flamingo populations are not isolated, and flamingos migrate between the soda lakes of East Africa and the Etosha and Magadikgadi Pans in southern Africa. Therefore, flamingo conservation stretches across many political boundaries, and threats to all key habitat need to be considered in an attempt to conserve the African flamingos. An Africa-wide assessment of greater flamingo populations in 1975 estimated 165,000 greater flamingos in Africa (75,000 in southern Africa, 50,000 in East Africa and 40,000 in northwest Africa). Present estimates indicate a decrease of 48 percent, to only 85,000 birds. The decrease has been most significant in southern Africa where numbers have decreased from about 75,000 to 47,500 birds. This dramatic decline may be a result of poor breeding success in the Etosha Pan over the past forty years.

Mammals

The area surrounding Lake Natron has few motorable tracks; consequently, the fauna has not been well studied. This area is included in the Masai ecosystem, the western boundary of which is formed by the Rift Valley Escarpment. There are a number of unusual features to the fauna in this ecosystem. The Blue wildebeest (Connochaetus taurinus hecki), for example, is a light tawny colour in this area, while that of the Serengeti blue wildebeest to the west (Connochaetus taurinus albojubatus) is darker in coat colour. According to local information from hunters and nomads, Burchell's Zebra (Equus quagga burchelli) and gazelles do not cross the escarpment, and the Thomson’s gazelle (Gazella thomsoni) is divided into an eastern lighter race and a western darker race. Animals are known to move up and down the escarpment within the Masai ecosystem. Limited studies show that the Blue wildebeest migrate between Lake Natron and Lake Manyara National Park. Other large mammals found in this ecosystem include the endangered African elephant (Loxodonta africana), the critically endangered Black rhino (Diceros bicornis), Giraffe (Giraffa camelopardalis), Cape buffalo (Syncerus caffer), Impala (Aepyceros melampus) and Grant’s gazelle (Gazella granti). There are no terrestrial predators in the lakes or on the surrounding mudflats, since it is not possible for a large terrestrial animal to move across the thick mud. The usual savanna predators, such as lion (Panthera leo), and Cheetah (Acinonyx jubatus), are present on the plains and surrounding the lakes.

Amphibians and reptiles

caption Senegal running frog. Source: African Amphibians Lifedesk, R.C.Drewes There is a paucity of amphibian taxa in this ecoregion. One representative is the Senegal running frog (Kassina senegalensis). Reptilan species found here include the Bell's hingeback tortoise (Psammobates pardalis), Helmeted turtle (Pelomedusa subrufa), the Leopard tortoise (Psammobates pardalis). Tomier’s Leaf-toed Gecko (Hemidactylus squamulatus) is a specialist to the vegetation surrounding the soda lakes of the Rift Valley, which include the lakes of this ecoregion.

Current status

Both Lake Natron and Lake Bahi fall outside of the protected areas network and are offered no formal protection. The area surrounding Lake Natron falls within a large game controlled area, an area in which hunting is allowed but is regulated. This offers some protection to wild animals in the area, but no protection to the ecosystem as a whole.

Ecoregion threats

A major threat to this ecoregion is the hydroelectric power scheme advanced by the Kenyan government for the Ewaso Ngiro River, which is the chief source of water inflow to Lake Natron. Altering its flow in any fashion could potentially change the hydrology and ecology of the lake, threatening the world’s largest and most secure breeding site of the lesser flamingo.

Lake Natron is also the site of small-scale sodium bicarbonate (baking soda) extraction plant, which directly impacts a small portion of the lake. These impacts are largely related to the infrastructure associated with the plant.

Lastly, uncontrolled tourism is a potential threat to the ecoregion. At present, the area is not on a popular tourist route, since it is difficult to access and is almost devoid of infrastructure and tourist facilities. Lake Natron is, in fact, so inaccessible that it was only discovered in 1954. Because of this, however, more adventurous four-by-four motorists may visit the area without permission, and there is the threat that they may not behave in an ecologically sensitive manner. Driving vehicles onto the mudflats may, for example, disturb breeding flamingos, causing them to desert their eggs and young. Such disturbances have been recorded at Etosha Pan.

Justification of ecoregion delineation

The linework for the two polygons comprising this ecoregion includes the halophytic vegetation around Bahi and Lake Natron. Experts chose to merge a number of the other halophytic areas in the region in with the adjacent ecoregions; these included Lakes Eyasi, Manyara and Amboseli. The East African halophytics ecoregion is given the ecocode AT0901 by the World Wildlife Fund.

Additional information on this ecoregion

  • For a more terse summary of this ecoregion, see the WWF WildWorld profile for the East African hydrophytics

References

  • Berry, H. H. 1971. Flamingo breeding on the Etosha Pan, South West Africa, during 1971. Madoqua series, 1(5):5-31.
  • Brown, L. H. 1957. The Mystery of the Flamingos. Collins, London.
  • Brown, L. H., and A. Root. 1971. The breeding behavior of the Lesser Flamingo Phoeniconaias minor. Ibis, 113:147-172.
  • Brown, L. H., D. Powell-Cotton, and J. B. D. Hopcraft. 1973. The breeding of the Greater Flamingo and Great White Pelican in East Africa. Ibis, 115:352-374.
  • Finlayson, C. M., and M. Moser. 1991. Wetlands. International Waterfowl and Wetlands Research Bureau. Facts on File, New York.
  • Greenway, P. J., and D. F. Vesey-Fitzgerald. 1969. The vegetation of the Lake Manyara National Park. Journal of Ecology, 57:127-149.
  • Hogan, C. M., curator. 2014. Salvadora persica. Encyclopedia of Life
  • Johnson, A. R., and L. Bennun. 1994. Lesser Flamingo: concern over Lake Natron. International Waterfowl Research Bureau News, 11:10-11.
  • Kenya Birds. 1993. Flamingos face uncertain future. Kenya Birds, 2:6.
  • Lott, C. editor. 1992. The Spectrum Guide to Tanzania. Camperapix Publishers International, Nairobi, Kenya. ISBN: 1874041350
  • Mwalyosi, R. B. B. 1977. A count of large mammals in Lake Manyara National Park. East African Wildlife Journal, 15:333-335.
  • Prins, H. H. T. 1987. Nature Conservation as an integral part of optimal land use in East Africa: The case of the Masai Ecosystem of Northern Tanzania. Biological Conservation, 40:141-161.
  • Simmons, R. E. 1995. Population declines, viable breeding areas and management options for flamingos in southern Africa. Conservation Biology, 10:504-514.
  • Stuart, C. and T. Stuart. 1998. Africa’s Great Wild Places. Southern Book Publishers, Halfway House, South Africa. ISBN: 1868126706
  • White, F. 1983. The vegetation of Africa, a descriptive memoir to accompany the UNESCO/AETFAT/UNSO Vegetation Map of Africa (3 Plates, Northwestern Africa, Northeastern Africa, and Southern Africa, 1:5,000,000). UNESCO, Paris. ISBN: 9231019554
  • Woodworth et al. 1997. IN TEXT.
  • Zimmerman, D. A., D. A. Turner, and D. J. Pearson. 1996. Birds of Kenya and Northern Tanzania. Russel Friedman Books, Nairobi, Kenya. ISBN: 0691010226


 

Disclaimer: This article contains certain information that was originally published by, the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

 

Glossary

Citation

Fund, W. (2014). East African halophytics. Retrieved from http://www.eoearth.org/view/article/151863

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