Angolan scarp savanna and woodlands
Red buffalo (Syncerus cafer nanus).Kissama NP,. By: Brian Huntley
The Angolan scarp savanna and woodlands ecoregion is a complex area where several major African ecological zones meet, and where topographical features have resulted in a high diversity of vegetation types and significant levels of endemism. Biologically, the most important portion of the ecoregion is the west-facing scarp that supports rainforest at higher altitudes. This forest holds a significant number of endemic birds, and some other endemic animals and plants. The long period of insurrection, foreign mercenary activity and civil instability in Angola have contributed to the outcome that these forests and other parts of the ecoregion have never been adequately surveyed biologically, and hence more endemics can be expected with further study. However, the highly unstable civil war means that all biological investigation, management of protected areas, or other kinds of conservation interventions are virtually impossible at the present time. This lack of access and management may be negatively affecting habitats and their biological values.
Location and general description
This ecoregion comprises a long narrow strip of land running from about 6° to 14° S latitude between the Atlantic Ocean, the Southwest Arid biome of Angola and the top of the scarp face of the Central African Plateau. The area is characterized by steep topographic, climatic and vegetation gradients along both the east-west and north-south axes. These gradients have resulted in high biological diversity and endemism, and are of great evolutionary significance. This is illustrated by the fact that the ecoregion lies at the convergence of all six of White’s phytochoria that are found in Angola. Three of these phytochoria are represented within the ecoregion: the Guineo-Congolian and the Zambezian centers of endemism; and the Guineo-Congolia/Zambezia regional transition zone. Three further phytochoria border on the ecoregion: the Kalahari-Highveld regional transition zone and the Karoo-Namib regional center of endemism abut it in the more arid southwest, while the Afromontane archipelago-like regional center of endemism lies along its southeastern border at the highest elevations. The vegetation is highly varied and ranges from dry woodland and wooded grassland to humid mist forest.
Though narrow, the ecoregion covers an elevational range from sea level to about 1000 metres (m). It includes two main geomorphologic regions: the Coastal Belt and the Transition Zone. The Coastal Belt varies in width from 200 kilometres (km) in the north to less than 40 km before the ecoregion continues inland, and does not exceed 300 m elevation. The Transition Zone is a discontinuous escarpment belt formed through erosion of the ancient massif, which runs more or less parallel to the coast. It rises sharply in the south, while the increase is more gradual farther north, forming a series of steps. Major rivers draining the ecoregion are the M’Bridge, Loge, Cuanza, and Cuvo.
The ecoregion overlies two main geological divisions: the coastal sediments and the ancient massif. The coastal sediments are made up of marine sediments (marls and limestones) and recent sands laid down in different sedimentary basins. Inland these are replaced by gneisses, gneissic granites, and metamorphosed sediments of the Precambrian Basement Complex that forms part of the ancient massif. A great variety of soil types, some very fertile, occur in the ecoregion, and their distribution is determined by geology and climatic factors. Soil types encountered include heavy black cotton soils, red or reddish sandy soils, calcareous patches on the coastal plain and lower escarpment, and the fertile paraferralitic and ferralitic soils that are the substrate of the moist escarpment forests.
The ecoregion has a tropical climate with summer rain. Along the coast, the cold Benguela Current strongly influences the climate so that humidity is high year-round while annual rainfall is low, ranging from 400 millimeters (mm) to 800 mm. Offshore, the Benguela Current meets warm equatorial waters and produces mists that are precipitated by the escarpment. A narrow belt on and below the escarpment thus combines the high summer rainfall of the inland areas with the year-round humidity of the coastal plains. Total precipitation in some of these areas is among the highest in Angola and is thought to exceed 1600 mm. Temperatures vary within the ecoregion, depending on elevation and latitude. The highest mean annual temperatures are encountered along the inner margin of the Coastal Belt north of the Cueve River, where they exceed 25° C. The lowest mean annual temperatures in the ecoregion occur along the border with the highlands of the Angolan Montane Forest-Grassland Mosaic ecoregion and are approximately 20° C.
Huambo province, Angola (Source: [http://[WWF]]/Brian Huntley) The ecoregion is comprised of three main vegetation zones. The first, north of the Cuanza River, is a mosaic of tall, tropical gallery forest and tall grassland, interdigitated by mangrove and swamp communities along the major rivers and their mouths. The second is a discontinuous series of semi-deciduous humid forest patches along the higher slopes of the escarpment. The third, south of the Cuanza River, includes arid to semi-arid undifferentiated woodlands and wooded grasslands on the Coastal Belt and lower slopes of the escarpment. Two azonal vegetation types, mangrove communities at the major river mouths and swamp vegetation in floodplains on the lower reaches of the larger rivers, are also found within the ecoregion.
Human population density in the ecoregion is highly variable, being densest in and around Luanda, the country’s capital city. More than 1.5 million people live in Luanda, almost one-tenth of Angola’s entire population. The city’s population is increasing at a high rate as people move to the larger cities as a result of wartime instability and lack of security. The southern parts of the coastal belt are very thinly populated, while the fertile, high-rainfall parts of the escarpment have relatively high population densities.
Biodiversity features
Red buffalo (Syncerus cafer nanus) in Kissama National Park, Angola (Source: WWF/Brian Huntley) The escarpment of Angola forms a tension zone among the surrounding biomes. Its evolutionary importance was recognized by Hall in her study of Angolan birds and supported with mammalogical evidence by Cabral. Firstly, the escarpment allows subspecies to develop in the drier southwest arid and Brachystegia biomes by forming a barrier between them. Secondly, the escarpment zone with its great range of elevations and relatively high humidity has provided refugia for forest species in periods of climatic desiccation. This has allowed species which remained in these refugia islands to survive climate changes and has facilitated the evolution of new species when their ancestral forms became isolated in these escarpment forests.
The vegetation in the ecoregion is highly varied with many unique plant communities, which in turn support distinctive faunas. The escarpment forests are important centers of endemism and are listed as critical sites for biodiversity conservation in Angola. Despite being of great biological interest, the vegetation and fauna of these areas have been poorly studied. For example, no plant lists exist for any of the ecoregion’s many vegetation communities. Plant diversity, species richness and endemism are high, but no reliable data exist at present. Of the ecoregion’s fauna, only birds have been studied and documented in any detail.
Thirteen bird species are either strictly endemic or near-endemic to this ecoregion, of which seven are threatened according to IUCN. These include the grey-striped francolin (Francolinus griseostriatus VU), red-crested tauraco (Tauraco erythrolophus), Angola helmetshrike (Prionops gabela EN), white-fronted wattle-eye (Platysteira albifrons), Angola slaty-flycatcher (Dioptrornis brunneus), Gabela akalat (Sheppardia gabela EN), Angola cave-chat (Xenocopsychus ansorgei), Pulitzer’s longbill (Macrosphenus pulitzeri EN), golden-backed bishop (Euplectes aureus), orange breasted bush shrike (Laniarius brauni EN), Gabela bush shrike (Laniarius amboimensis EN), and Monteiro’s bushshrike (Malaconotus monteiri DD), the last of which is also found in Cameroon. Many of these species are included in the restricted range species listed for the Western Angola Endemic Bird Area by Stattersfield et al., although they included montane forests, which we have separated into another ecoregion.
There are also a number of endemic bird subspecies, the brown-chested alethe (Alethe poliocephala hallae), yellow-necked greenbul (Chlorocichla falkensteini falkensteini), grey-backed camaroptera (Camaroptera brevicaudata harteri) and two subspecies of Lühder’s bushshrike, Laniarius luehderi brauni and L. l. amboimensis. The latter two are considered full species by some taxonomists.
There are two strictly endemic reptile species, Barboza’s Leaf-toed Gecko (Hemidactylus bayonii) and Monopeltis luandae, a member of the Wedge-snouted Worm Lizard genus. Moreover, there are four strictly endemic amphibian species, Quanza Reed Frog (Hyperolius punctulatus), Congulu Forest Treefrog (Leptopelis jordani), Quissange Forest Treefrog (Leptopelis marginatus), and the Congolo Frog (Hylarana parkeriana). The Spiny Striped Reed Frog (Afrixalus dorsalis) occurs here in the bushland and in forest outliers within the humid savanna. As with the birds, most of the strict endemics are found in the Angola scarp forests, although some are also restricted to the drier areas in the lowlands.
In the northernmost part of the ecoregion, between the Zaire and Cuanza Rivers, a complex vegetation mosaic is found on highly dissected topography. Large areas of tall grassland are interspersed with gallery forests, which represent the southwestern corner of the Guineo-Congolia/Zambezia regional transition zone. The forest patches are 20 m to 40 m tall and are dominated by tree species such as Piptadeniastrum africanum, Milicia excelsa, Ceiba pentandra, and Musanga cecropioides. The forest fauna is rich, though better represented in the extensive Western Congolian Forest-Savanna Mosaic ecoregion. Mammals include Beecroft’s flying squirrel (Anomulurops beecrofti), giant forest squirrel (Protoxerus stangeri), forest elephant (Loxodonta africana cyclotis), golden potto (Arctocebus aureus), Bosman’s potto (Perodicticus potto), bay duiker (Cephalophus dorsalis), and water chevrotain (Hyemoschus aquaticus). The grasslands are 2 m to 4 m high with few scattered woody plants which are limited to species that tolerate the annual intense grass fires. Examples of these include Hymenocardia acida, Erythrina abyssinica, Piligostigma thonningii, and Cussonia angolensis. Mangrove and swamp vegetation forms large tongues inland from the coast along the larger rivers.
On the upper slopes of the escarpment, where rain and mist provide year-round moisture and the fertile soils favor tree growth, mist (or cloud) forest occurs in a discontinuous band one kilometre to 15 kilometres wide. These forest patches total between 1300 km2 and 2000 km2 and are most extensive in the Gabela and Amboim areas. They are also known as coffee forests, as they have been used extensively for coffee cultivation. These semi-evergreen humid forests are of Guineo-Congolian affinity and are thought to be impoverished outliers of Congolian rain forest with tall macrophyll or mesophyll canopy trees, extensive growth of epiphytes and lianes and many fruiting species. Dominant tree species include Celtis prantlii, Morus mezozygia, Albizia glaberrima, A. gummifera, Ficus mucuso, and F. exasperata, and canopy height rarely exceeds 30 m. Two wild coffee species, Coffea canephora and C. welwitschii, are among the understory species.
Among the mammals naturally occurring in the escarpment forest are yellow-backed duiker (Cephalophus sylvicultor), black-fronted duiker (C. nigrifrons), blue duiker (C. monticola), tree pangolin (Phataginus tricuspis), and red buffalo (Syncerus caffer nanus). Grazing antelopes are largely absent. Besides the narrow endemics listed above, characteristic birds of the escarpment forests and woodlands include red-crested turaco (Tauraco erythrolophus), red-backed mousebird (Colius castanotus) (both endemic to Angola), mottled spinetail (Telacanthura ussheri), batlike spinetail (Neafrapus boehmi), naked-faced barbet (Gymnobucco calvus), red-tailed palm thrush (Cichladusa ruficauda), and yellow-bellied wattle-eye (Dyaphorophyia concreta).
South of the Cuanza River, the Zambezian component of the ecoregion comprises a mosaic of closed woodlands, grasslands and palm savanna, which is found along the lower and drier slopes of the escarpment and along the coast. The distribution of the different vegetation communities within this zone is strongly influenced by soil type. The woodlands are floristically rich and are more easily defined by the absence of mopane and miombo dominants than by their own floristic composition. Despite their small area, they are composed of many more tree species than either miombo or mopane and include several endemic species. Dense dry thicket and scrub forest are the typical woodland physiognomies, with Sterculia setigera, Euphorbia conspicua, Strychnos spp., Acacia welwitschii, and baobab (Adansonia digitata) being the dominant woody species. Between the closed woodlands, there are grasslands dominated by Andropogon gayanus, Heteropogon contortus, and Hyparrhenia spp. Extensive sand plateaus nearer the coast are occupied by Hyphaene gossweileri palm savanna, where the grass layer is dominated by Eragrostis superba, Schizachyrium semiberbe, and Digitaria milanjiana. To the north and south of Luanda, a remarkable grassland occupies large areas on smoothly rounded hills of marine sediments with heavy black and dark brown clays. This is dominated, to the virtual exclusion of all other species, by Setaria welwitschii, which forms dense swards 1 to 1.5 m high.
The large mammal fauna in this area includes roan antelope (Hippotragus equinus), red buffalo (Syncerus caffer nanus), elephant (Loxodonta africana), reedbuck (Redunca arundinum), bushbuck (Tragelaphus scriptus) and eland (Taurotragus oryx), which are represented in the Kisama National Park. Large predators used to occur in the coastal plains but their populations, like those of all large mammals, have been severely reduced if not entirely eliminated as a result of uncontrolled hunting. Many of the bird species that occur in the escarpment forests are found in the woodlands here. The white-fronted wattle-eye (Platysteira albifrons) is endemic to the northernmost part of the coastal belt in this ecoregion. Large populations of the woolly-necked stork (Ciconia episcopus) occur on grassy plains and gallery forest along the ecoregion's rivers.
The Angolan Coast, the northern half of which is part of the ecoregion, includes several specialized maritime and estuarine ecosystems which do not form part of any of the biogeographic divisions outlined above, but which are of great biological interest. These communities are described in the Central African mangroves ecoregion.
Ecoregion status
Due to its low agricultural potential, the dry coastal belt and lower escarpment are relatively sparsely settled, and large areas remain unfragmented. For instance, large stretches of completely undisturbed habitat have been reported in Kisama National Park. Around the larger urban centers, particularly Luanda, human settlement and activities such as woodcutting and livestock overgrazing have had considerable, though mostly localized, impacts on the vegetation and soils.
The escarpment forests were almost entirely utilized for coffee production that peaked between 1950 and 1970, when an estimated 95 percent of the forest area was affected. This involved clearing the understory while leaving canopy trees for shade. Coffee-berry disease in the 1950’s, a drop in coffee prices in the mid-1970’s, and the upheaval of the civil war since 1974 have resulted in the abandonment of many of the coffee plantations, with the result that much of the forest has recovered today. However, subsistence cultivation on the fertile forest soils is increasing and Hawkins estimated that some 30 percent of the forest areas were affected, especially around Gabela where many of the ridges and valley bottoms have been cleared.
Two areas in the ecoregion are protected, with three more areas proposed for protection but not yet established. The large Kisama National Park is bordered by the Atlantic Coast and the banks of the Cuanza and Longa Rivers and is listed by many to be among critical sites for biodiversity conservation. Marine, estuarine, floodplain, grassland and thicket habitats are represented here. Ilheu dos Passaros Integral Nature Reserve is a small offshore island with mangrove communities and mud flats which are of great botanical interest and provide key breeding habitats for water birds.
The proposed Gabela and Chingoroi Strict Nature Reserves, both representing patches of escarpment forest, are needed to protect the area’s rare endemic bird species, because their small forest habitats are declining due to agricultural activities. Without these two protected areas, the escarpment forest vegetation and its fauna are unprotected despite their vulnerability and great biological interest. The Pungo Andongo Natural Monument comprises a series of large rocky outcrops between Gabela and the coast, which should be protected for their biological and aesthetic value.
The almost continuous civil war in Angola, since Soviet financed Cuban mercinaries invaded in 1974, has led to great instability, poor security, economic depression, massive displacement of the rural population and a lack of infrastructure and basic services. Its effects on conservation, particularly that of large mammals, have been devastating. Most of Angola’s protected areas have been abandoned as their wardens were forced to leave for economic and security reasons and have become unpatrolled areas for poachers and settlers.
Types and severity of threats
The most immediate and important threat to the ecoregion’s biodiversity is the encroachment of subsistence agriculture in the fertile escarpment forest areas. These are, at present, not even nominally protected. This threat is expected to escalate once stability returns to Angola and displaced rural people return to farming areas. It is also possible that coffee plantations will be re-established in the escarpment forests.
Hunting is virtually uncontrolled in most of Angola, including the protected areas. It is estimated that 21 species of larger mammals are close to extinction in Angola, including lion, cheetah, West African manatee and forest elephant. There are no data on the extent and impact of subsistence hunting on populations of smaller mammals and birds, but these species may be an important source of protein in the more populated rural areas. Marine turtles and their eggs are also harvested along the coast.
It seems that commercial exploitation of timber has not taken place so far, probably due to the importance of canopy species for coffee production. However, the network of wide tracks established for coffee plantations means that it would be very easy to start timber exploitation, especially if coffee production is not resumed on the previous scale.
Justification of ecoregion delineation
This ecoregion is primarily based on Birdlife’s Western Angola Endemic Bird Area. It encompasses portions of ‘North Zambezian undifferentiated woodland’ and ‘mosaic of lowland forest and secondary grassland,’ extending inland 1 km to 15 km along the Angola escarpment, which forms its eastern border. Although included in the Western Angola EBA, the Bailundu Highlands have been separated into the Angolan Montane Forest-Grassland Mosaic ecoregion since the characteristic elements of the ecoregion’s fauna and flora are more closely related to other Afromontane areas than to the surrounding Angolan biomes.
Further reading
- Barbosa, L.A.G. 1970. Carta Fitogeografica de Angola. IICA, Luanda.
- Dean, W.R.J. 2000. The birds of Angola: An annotated checklist. BOU Checklist No. 18. British Ornithologists’ Union, Herts, U.K.
- Hall, B.P. 1960. The faunistic importance of the scarp of Angola. Ibis 102: 420-442
- Hawkins, F. 1993. An integrated biodiversity conservation project under development: The ICBP Angola Scarp Project. Proceedings of the VIII Pan-African Ornithological Congress: 279-284
- Hilton-Taylor, C. 2000. The IUCN red list of threatened species. Gland, Switzerland and Cambridge, U.K. ISBN: 2831705657
- Huntley, B.J. 1974a. Outlines of wildlife conservation in Angola. Journal of the southern African Wildlife Management Association 4: 157-166.
- Huntley, B.J. 1974b. Ecosystem conservation priorities in Angola. Ecologist’s Report No. 28. Servicos de Veterinaria, Luanda. 21 pp.
- Huntley, B.J., and E.M. Matos. 1994. Botanical diversity and its conservation in Angola. B.J.Huntley, editor. Botanical Diversity in Southern Africa. Strelitzia 1: 53-74. National Botanical Institute, Pretoria. ISBN: 1874907250
- Huntley, B.J., and G.C. Matos. 1992. Biodiversity: Angolan environmental status quo assessment report. IUCN Regional Office for Southern Africa, Harare.
- Moyo, S., P. O’Keefe, and M. Sill. 1993. The southern African environment: Profiles of the SADC countries. Earthscan, London.
- Stattersfield, A.J., M.J. Crosby, A.J. Long, and D.C. Wege. 1998. Endemic bird areas of the world: priorities for conservation. BirdLife International, Cambridge, UK.
- Stuart, S.N., and R.J. Adams. 1990. Biodiversity in sub-Saharan Africa and its islands. Occasional Papers of the IUCN Species Survival Commission No. 6. ISBN: 2831700213
- Texeira, J.B. 1968. Angola. I. Hedberg, and O. Hedberg, editors. Conservation of vegetation in Africa south of the Sahara. Acta Phytogeographica Suecica 54: 193-197.
- Werger, M.J.A., and B.J. Coetzee. 1978. The Sudano-Zambezian Region. M.J.A. Werger, editor. Biogeography and Ecology of Southern Africa. W. Junk, The Hague.
- 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
- White, F., and M.J.A. Werger. 1978. The Guineo-Congolian transition to southern Africa. M.J.A. Werger, editor. Biogeography and Ecology of Southern Africa. W. Junk, The Hague.
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