The Angolan Escarpment Zone

Abstract

The Angolan Escarpment is a physical feature that forms part of the Great Escarpment of Southern Africa. It comprises elements of all the major biomes of Angola. This chapter focuses on the biogeographical importance of the Escarpment Zone, which runs south from the Maiombe forests of Cabinda to the arid mountains rising above the Cunene River.

Key Concepts and Questions: This Chapter Will Explain

• Why the Escarpment Zone is considered a physiographic concept rather than a single ecoregion.

• How diverse environments are reflected in the habitats and biota of the Escarpment.

• How the forests of the Escarpment Zone and the Angolan Highlands relate to the Guineo-Congolian and Afromontane biomes.

• Why the avifauna of the Escarpment Zone and adjoining highlands provides such important insights into the evolution of African forests.

Context: An Escarpment Zone Extending from Rain Forest to Desert

In the preceeding chapters of Part 4, the ecological characteristics of the six Angolan biomes have been described. Each of these biomes have specific structural and functional attributes that distinguish them from one another, and justify their recognition as representatives of global biomes. Cutting across these biomes, from north to south, is the Angolan Escarpment, a geographical spine that separates the coastal lowlands from the interior plateaus. The vegetation that covers the Escarpment landscapes range from tropical rain forest to desert, and includes five ecoregions. For this reason, the description of the Angolan Escarpment Zone is presented as the concluding chapter of Part 4, given its complexity and need for a specific format.

The Angolan Escarpment Zone forms part of the 5000 km-long Great Escarpment of Southern Africa, which lies between 50 and 200 km inland of the coasts of Mozambique, South Africa, Namibia and Angola. The Great Escarpment rises from 500 to 2000 m over most of its range, with the highest peaks of mountains above the Escarpment attaining 2620 m in Angola, and 3450 m in Lesotho. Across its 5000 km length, climates range from tropical summer rainfall to warm temperate winter rainfall, and from 250 to 1600 mm per year. The extensive Great Escarpment of Southern African and associated highlands are estimated to host over 8500 plant species, of which 1460 species are endemics. It has at least 126 endemic vertebrate species (Clark et al., 2011).

The Angolan Escarpment Zone (Figs. 4.10 and 18.1) extends across 1400 km from northern Cabinda to the Cunene River. The strong latitudinal gradient in rainfall, and longitudinal gradient in altitude, with accompanied diversity of habitats, flora and fauna, accounts for the inclusion of part or all of eight ecoregions within the Zone. The Angolan Escarpment Zone is bounded on the west (below ca. 500 m) by the coastal lowlands, and to the east by the upper reaches of the Marginal Mountain Chain and the Ancient Plateau (senso Diniz, 1991). The central Angolan highlands rise above the Escarpment, from 1700 to 2620 m, and drop eastwards across the interior plateau (planalto). The Escarpment Zone includes several prominent scarps, where the landscape is broken by abrupt increases in altitude over a few km (Fig. 4.3). It is also characterised by the presence of many inselbergs – isolated geomorphological features that rise as steep-sided hills or mountains formed of resistant bedrock, surrounded by lower-elevation lowlands, such as Serra da Neve. Rising to 2489 m, Serra da Neve is isolated both physically and geologically from the main escarpment but with similarly interesting endemic species and habitat diversity (Fig. 15.6). A variant of an inselberg that is particularly common in the Escarpment Zone is known as a bornhardt, a domed-shaped hill, often of Basement Complex granite (Fig. 4.2).

Fig. 18.1

The Highlands and Escarpment Zone of Angola. The Angolan Escarpment Zone is broadly outlined by the yellow line, and includes the highlands and inselbergs (red dots) that characterise the rugged topography of the Escarpment Zone. Also indicated are the areas of land rising above 1800 (blue) and above 2000 m (white), where isolated remnants of Afromontane Forest might be found. The names of key peaks are given. To the east of the Escarpment Zone, the montane grasslands of the high plateaus are indicated by the green line. Cartography John Mendelsohn

The climate of the Angolan Escarpment Zone varies widely, from humid in the north, with up to 1600 mm precipitation per annum, to arid in the south where it forms the interior margin of the Namib Desert, receiving less than 250 mm. Rainfall is confined to the summer months, with low clouds and fog supplementing rainfall in the forests of the steep, seaward-facing slopes of the Escarpment.

The geology is very mixed, but mostly comprises crystalline Precambrian rocks—gneisses, quartzites, schists, granites, gabbros, amphibolites and migmatites. Soils include ferallitic and paraferralitic ferralsols and well-drained ferralitic nitisols, and in some areas, more fertile lixisols and cambisols.

The Angolan Escarpment cannot be regarded as a single ecoregion (as in Burgess et al., 2004), but rather as a series of ecoregions within an Escarpment Zone. Chapter 2 maps and outlines the ecoregions that occur along the escarpment: (1. Guineo-Congolian Rain Forests, 2. Western Congolian Forest/Savanna Mosaics, 6. Angolan Escarpment Savannas, 12. Angolan Mopane Woodlands, and 13. Namib Savanna Woodlands), and the complex of Arid Savanna, Mesic Savanna and Afromontane ecoregions that lie to their west and east.

1 Sectors of the Angolan Escarpment Zone

The paucity of geo-referenced data on Angolan plant and animal species precludes an objective and statistically-based classification of biogeographic sectors of the Angolan Escarpment Zone. However, the Zone can be divided into four sectors, based on general physiographic and climatic characteristics. The abbreviated outlines which follow need refinement through active data collection in the field, especially to test the faunistic and floristic patterns relating to the physiographic sectors.

Escarpment of Cabinda

• From Gabon southwards, and inland of the coast, low hills give way to steep ridges of a narrow escarpment belt which ends just north of the Congo River.

• In Cabinda, these hills form the southern extension of the Mayombe Massif, which is known as the Alto Maiombe in Cabinda, reaching 780 m at its highest point (Ecoregion 1).

• The closed forests comprise Guineo-Congolian species, and are surrounded by tallgrass savannas of Western Congolian Forest/Savanna Mosaic communities (Ecoregion 2).

2 Sectors of the Angolan Escarpment Zone

The paucity of geo-referenced data on Angolan plant and animal species precludes an objective and statistically-based classification of biogeographic sectors of the Angolan Escarpment Zone. However, the Zone can be divided into four sectors, based on general physiographic and climatic characteristics. The abbreviated outlines which follow need refinement through active data collection in the field, especially to test the faunistic and floristic patterns relating to the physiographic sectors.

Escarpment of Cabinda

• From Gabon southwards, and inland of the coast, low hills give way to steep ridges of a narrow escarpment belt which ends just north of the Congo River.

• In Cabinda, these hills form the southern extension of the Mayombe Massif, which is known as the Alto Maiombe in Cabinda, reaching 780 m at its highest point (Ecoregion 1).

• The closed forests comprise Guineo-Congolian species, and are surrounded by tallgrass savannas of Western Congolian Forest/Savanna Mosaic communities (Ecoregion 2).

Northern Escarpment, from the Congo River to the Cuanza River

• South of the Congo River, the landscape comprises a highly dissected relief with hills and low mountains rising eastwards to 1300 m.

• The width of this sector decreases from 200 km in the north to 100 km in the south.

• Mean annual precipitation increases from 500 to 1600 mm, from the coast to the interior.

• Several large forest blocks and numerous smaller fragments of semi-deciduous Guineo-Congolian forest occur below and along the escarpments and in riverine galleries (Ecoregion 1).

• The forests are dispersed within a matrix of Western Congolian Forest/Savanna Mosaics of tall grasslands with fire-tolerant woody species and with moist forest patches in valleys and on hills (Ecoregion 2).

• The flora is predominantly Guineo-Congolian and Zambezian, with an absence or low frequency of Afromontane elements.

• Both flora and fauna has a species richness of all taxa, with dominance of Guineo-Congolian species in forests and Zambezian species in savannas (Ecoregion 6).

Central Escarpment, from the Cuanza River to the Coporolo River

• The Central Escarpment forms a low, narrow but stepped landscape from about 200 to 400 m along the base, rising within 50 km from the coast to 1000 m (and to 1690 m at highest points on Serra Njelo).

• A second, broken, escarpment lies between 100 and 150 km from the coast, rising above 1600 m along the Marginal Mountain Chain, and to 2620 m at Mount Moco, descending eastwards across the Planalto.

• Mean annual precipitation increase from 500 to 1000 mm from the coast to the highlands.

• Arid savannas, woodlands, thickets and dry forests occur as a mosaic along its base, transitioning into mixed Mesic savannas above 500–800 m; with Acacia, Annona, Cochlospermum, Erythrina, Piliostigma savannas up to 800 m and Brachystegia, Julbernardia, Combretum, Terminalia woodlands along the upper escarpment (Ecoregion 6).

• Guineo-Congolian elements are mixed with Zambezian elements in the fragmetas of rain forest at Amboim-Gabela and Seles-Cumbira, at altitudes from 500 to 800 m. These represent species-poor outliers of Ecoregion 1).

• Above 800 m a transition to medium-height grassland and in some places, ravine forests with Afromontane elements.

• The rain forests and adjoining dry thickets are of biological and conservation important for their high richness of endemic and narrow-range bird species, and seven primate species.

Southern Escarpment, from the Coporolo River to the Cunene River

• The Escarpment becomes increasingly narrow, very steep and very high 100 km inland of the coast at Moçâmedes. The scarp face rises from 500 to 2000 m over a distance of 10–40 km, due west of Humpata, where it is well defined by the Chela quartzites (Fig. 6.1).

• Southwards from the Serra da Chela the escarpment becomes more diffuse and broken as it follows arid mountains east of Virei and as it approaches the Cunene River.

• Mean Annual Precipitation decrease from 150 to 850 mm, from the base of the Escarpment to the Humpata Plateau.

• The Southern Escarpment includes an exceptional altitudinal diversity of habitats, as described in Sect. 5.7.

• Arid woodlands and thickets, including Acacia, Adansonia, Colophospermum occur along the foothills, with Albizia, Brachylaena, Combretum, Commiphora, Tarchonanthus woodlands and thickets on the higher levels. With increasing aridity, species composition and structure changes southwards to semi-desert towards the Cunene River (components of Ecoregions 6, 12, 13).

• Very restricted deciduous forest patches/galleries in lower areas with a few Guineo-Congolian floristic elements including Piptadeniastrum and Newtonia.

• The upper crest, at 1800–2000 m, leads to plateau grasslands and ravine forests of Afromontane elements including Protea, Faurea, Nuxia and Podocarpus on the Humpata and Huíla plateaus (Ecoregion 4).

• The Humpata and Huíla plateaus, comprising Wet Miombo (Ecoregion 7) has many endemic ‘highland’ elements in the short grasslands (Ecoregion 5).

• Endemic birds include species of rocky outcrop, grassland and thicket habitats.

The combination of high rainfall and good soils on the Cabinda, Northern and Central Escarpments have enabled high agricultural productivity, most specifically for the production of ‘robusta’ coffee Coffea canephora during the colonial period, and for bananas and other cash crops thereafter. As a consequence, deforestation has severely reduced the cover of the moist forests of the Escarpment Zone, while invasive plant species (Inga vera and Chromolaena odorata) have infested disturbed sites. Along the Southern Escarpment, in more arid areas, the cactus Opuntia stricta has invaded large areas (Rejmánek et al., 2016). Across the length of the Angolan Escarpment, hunting for bushmeat and the felling of trees for the production of timber, firewood or charcoal have had serious impacts on biodiversity.

In the absence of formally designated and effectively managed conservation areas, the extremely rich biota and ecosystems of the Angolan Escarpment Zone are currently vulnerable to irreversible degradation.

3 Vertebrate Fauna of the Escarpment Zone

As already noted, the Angolan Escarpment Zone and adjoining montane highlands need intensive surveys to provide a detailed characterisation of their fauna and flora. It is nevertheless possible to provide an indicative listing of vertebrate species with distributions centred on the Northern and Central Escarpments (Table 18.1). In common with the Great Escarpment of southern Africa, the Angolan Escarpment Zone and its adjoining highlands are of considerable biogeographic and evolutionary interest, as outlined in Box 18.1.

Table 18.1 Vertebrate species typical of the Angolan Northern and Central Escarpments (excluding Cabinda)

Box 18.1: The Evolutionary Importance of the Angolan Escarpment and Highlands

Understanding patterns in the distribution and abundance of species is a recurrent theme of ecological enquiry. Biomes and ecoregions provide a coarse-grained outline of Angola’s biodiversity structure. At a finer geographic scale, the birds of Angola are better documented than are plants or even vegetation, and provide valuable insights into evolutionary relationships of the highland biota. Birds are often the trigger of natural history interest among students, and an accessible entry point to the wider spectrum of biodiversity knowledge. Michael Mills, a South African ornithologist, has done much to document and popularise the avifauna—both common and rare species—of Angola. In his guidebook to the ‘special’ birds of the country, Mills (2018) lists 70 species that are notable for their rarity or endemicity. The list indicates that most restricted-range species are from forests, or from the thickets, shrublands and grasslands associated with forests of the Angolan Escarpment and Afromontane ecoregions. Of the country’s 29 endemic bird species, 12 are confined to the Escarpment and 8 more are narrow endemics that occur along the Escarpment but are not restricted to it. Similar narrow distributions, limited to the Escarpment Zone, are found among reptiles, amphibia, butterflies and dragonfly groups (Huntley et al., 2019 and references therein).

These patterns of endemism and rarity have long attracted the attention of biologists, first to collect, describe and document diversity, and more recently, to explain evolutionary relationships. In 1957, Patricia Hall of the British Museum, London, collected 250 species of passerine birds along the Angolan Escarpment and adjoining landscapes. Based on her field knowledge and on museum specimens, she published a paper titled The Faunistic Importance of the Scarp of Angola (Hall, 1960) in which she proposed a series of hypotheses regarding the evolution of the patterns of bird distribution that she had studied. She defined the ‘Escarpment Zone’ as:

• “A wedge of richer vegetation lying between the drier areas of thornveld or grassland and brachystegia woodland. In the north it is wide with considerable patches of forest; in the south it tapers to a very narrow belt along the escarpment with forest patches rarer and less extensive.”

This characterisation agrees with the definitions of the Angolan Escarpment Zone, Coastal Arid Savanna and Wet Miombo ecoregions used in this volume. Hall excluded the Afromontane Forests and Grassland ecoregions from her ‘Escarpment Zone’. This distinction is upheld by the composition of plant and animal communities (despite the general physiognomic similarity) of Escarpment and Afromontane forests.

Hall (1960) proposed that the faunistic importance of the escarpment relates to three speciation processes:

• Firstly, the role of the escarpment as a centre of speciation. This she illustrated by a number of examples in which birds found in the Escarpment Zone differ in some striking character from all others of the same species throughout Africa, and which do not apparently interbreed with their relatives in other zones.

• Secondly, the Escarpment Zone is of importance in providing a barrier of some distance between members of a species present in the two drier zones (Acacia thornveld and Brachystegia woodland), but with the species absent from the Escarpment Zone, allowing subspecies to develop in each of the two geographically separate and drier zones.

• Thirdly, the Escarpment Zone is important in containing a number of ‘old’ endemic and near endemic species.

Hall was able to demonstrate numerous cases that supported her three hypotheses. The vast majority of the 250 species she examined were found in only one of the three habitats—arid coast, moist escarpment forests, or mesic miombo. Only five species were found in all three zones. Her explanation of speciation processes proposed successive wetter and drier climatic episodes, with forests retracting and expanding, and with allopatric speciation taking place in the three distinct zones where the escarpment zone provided both a barrier and a refuge. The persistence of moist conditions and stable habitats and corridors along the escarpment was possible due to the orographic fog and higher rainfall sustained under the influence of the Benguela Current, even during arid episodes.

More than 60 years after the publication of Hall’s paper, the Angolan Escarpment and Afromontane forests continue to attract both professional and amateur biologists. Hall’s (1960) hypotheses, based purely on the morphological and plumage characters of her study specimens, and on speculations about climate change, preceded the extensive literature on the evolution of African rain forests and the biogeography of Afromontane species. Hall focused on the relationship between Escarpment species with those of the adjoining coastal lowlands and plateau highlands, with limited reference to the disjunct bird faunas of the ‘sky islands’ of the Afromontane Archipelago.

Since Hall’s paper, the Escarpment and Afromontane forests have been identified as critical areas for conservation—the Western Angola Endemic Bird Area. This Endemic Bird Area includes 14 restricted range birds which are considered ‘endemic birds of global conservation concern’ (Stattersfield et al., 1998). Nine of these are found at just one of the escarpment forests—Cumbira. Cumbira also holds the highest number of Angolan endemic species (11 of 17). These findings (Mills, 2019) demonstrate the scientific and conservation importance of this seriously threatened forest. The endemic bird species include: Gabela Akalat, Pulitzer’s Longbill, Gabela Bushshrike, Gabela Helmetshrike, Monteiro’s Bushshrike, Grey-striped Francolin and White-fronted Wattle-eye.

A detailed study of the distribution of forest bird species of the Angolan Escarpment (Mills, 2010) revealed three distinct bird communities occupying different forest habitats. Of the 91 forest species surveyed, 47 were identified as being limited to or having isolated populations on the escarpment. The moist forest types of the northern escarpment (Cazengo, Dembos, Uíge) are richest in Congolian species, but have only one endemic—Braun’s Bushshrike. The central (Amboim/Cumbira) escarpment forests at between 500 and 1000 m are drier, more fragmented, and are surrounded by thickets and arid savanna, especially at altitudes below 300 m. Interestingly, the low-altitude dry forests have more threatened endemic species than the moister mid-scarp forests, which are richer in Congolian species. Mills (2010) suggests that the endemic and range-restricted species of the escarpment evolved during phases of aridity during glacial maxima when the moist forests contracted and were isolated from the Congo Basin, at which time the forests would have been similar to the present drier fringing habitats. He infers that the endemic taxa might have moved to the periphery of the escarpment when the main forests became moister, with better connections to the Congo Basin. He proposed that molecular phylogenetic studies would provide greater clarity to the processes of speciation along the escarpment.

Rapid advances in research approaches to studying the evolution of the rare birds of Angola, such as electronic and georeferenced databases, molecular phylogenetics, climate change modelling and ecological niche models are now available. These tools were used by Portuguese student Bruno Vaz da Silva to study the evolutionary history of the birds of the Angolan highlands, focusing on the forests of Mount Moco and Mount Namba. Vaz da Silva (2015) used five species as study subjects: Evergreen Forest Warbler, Thick-billed Seedeater, Bocage’s Akalat, Red-faced Crimsonwing and African Hill-babbler. All five have relatives in the Afromontane ecosystems of Cameroon, Ruwenzori and the Eastern Arc mountains of Tanzania. These sites are all more than 2000 km distant from Angola’s montane forests (Fig. 18.2).

Fig. 18.2

The present distribution of five Afromontane bird species. From Vaz da Silva (2015). Bird figures from Lynx Edicions and maps from BirdLife International and Handbook of the Birds of the World (2021), with permission

Vaz Silva’s study aimed at establishing a comparative phylogeography of the species and to describe the niches currently occupied. This would help understand the dynamics of disjunction and reconnection—where fragmentation occurred and where corridors between isolated habitats existed. The results of the study provided strong evidence of ecologically suitable habitats existing as corridors between Angola and East Africa via Zambia during the last 120,000 years of the Pleistocene, with weaker ecological connections to Cameroon (Fig. 18.3). Stepping stones for such forest species still exist across Angola in the gallery forests of the upper reaches of the Congo and Zambezi basins, and the escarpment forests above the Baixa de Cassange at Quela and Tala Mungungo, at the waterfalls at Dala in Lunda-Sul and across the Zambian highlands.

Fig. 18.3

Species distribution modelling results for African Hill-babbler: Left—the beginning of the last glacial period (120 thousand years ago), Centre—the Last Glacial Maximum (21 thousand years ago), compared with Right—the projection for the present. The existence of a corridor across Angola via Zambia during the Last Glacial Maximum was suggested for all five species studied. From Vaz da Silva (2015). Bird figures from Lynx Edicions and maps from BirdLife International and Handbook of the Birds of the World (2021), with permission

The phylogenies indicate that speciation was driven by arid periods with habitat fragmentation, followed by reconnection, going back to the Pliocene (3.1 Ma), with recurrent events through the Pleistocene. The timing and frequency of genetic divergence in the study species, at 3.1, 2.7 and 1.7 Ma coincide with episodes of aridity. The evidence suggests that both dispersal and vicariance (geographical separation preventing gene exchange) processes might have played important roles in shaping montane bird communities (Fjeldså & Bowie, 2008). Most interestingly, the results indicate that Angola may be an old and stable Afromontane centre, from which other centres may have been recolonised following extinctions (Vaz da Silva, 2015). The study confirms the critical importance of conserving Angola’s Escarpment and Afromontane forests, as reported nearly 50 years ago (Huntley, 1974).