Bats of Southern and Central Africa. Ara Monadjem

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Figure 24. The extent of the elevated expanse of the Kalahari Plateau is depicted in this computer-generated Digital Elevation Model (DEM). Situated south of the Congo basin, the Kalahari Plateau is bounded by flanking coastal escarpments, and has been greatly altered by the East African Rift System (notably the Gwembe-Luangwa valleys) (Generated from the SRTM-3 (Shuttle Radar Topographic Mapping 3) dataset compiled by NASA, with annotations, including locations of principal cities; © F. P. D. Cotterill, AEON).

      These dynamic drainage systems are landforms of central significance to palaeobiological and geological research, because their history provides invaluable keys to decipher the complexities of landscape evolution. Although comparatively subtle, these changes have profoundly influenced the distribution and evolution of Africa’s fauna, including bats (Cotterill 2003, 2004, 2005, Moore et al. 2009, 2012, 2020).

CLIMATES PAST AND PRESENT

      The climate of southern Africa is dominated by a characteristic seasonality, with a cool-dry season interfacing with a hot-dry season, and then a hot-wet season, through the annual cycle. The southwestern Cape region experiences a winter-rainfall regime with hot, windy summers. A gradient of increasing rainfall trends from west to east; precipitation is both erratic and lowest in the deserts of the Namib, Namaqua, and the southwestern Kalahari. Adiabatic rainfall is not infrequent along the windward margins of the coastal escarpments and mountains. Prevailing mean annual temperatures and mean annual rainfall also exhibit an overall increase from south to north, with precipitation under increasing influence of the Inter-Tropical Convergence Zone and the Congo Air Boundary. The Monsoon rainfall system also influences the climate of northeastern Mozambique (Tyson and Preston-Whyte 2000).

      The evolution of southern Africa’s landscapes and, especially, its vegetation has been influenced strongly by palaeoclimatic dynamics. Empirical evidence for past conditions across the hinterland is very poor, especially for the time before the Late Pleistocene (120,000 years ago). Nevertheless, the widespread fossil dune systems across the central and northern Kalahari Plateau testify to widespread arid conditions that prevailed across a vast portion of the subregion, presumably during the Late Cenozoic (de Menocal 2004, Goudie 2005). Such extreme periods of aridity were interspersed with equally humid phases, when mesic vegetation expanded south and west of the Congo basin (Benson and Irwin 1965).

      Several lines of evidence suggest these expansions were focused along drainage lines in the Upper Congo and Zambezi systems. The significance of this interplay of both geomorphological evolution and palaeoclimatic dynamics in exercising dominant controls over palaeo-environments, is illustrated by the modern habitat mosaic characterising the Great Equatorial Divide. Here, Late Cenozoic drainage evolution has seen the overall migration of the Congo’s headwaters southward, by their repeated captures of Zambezi tributaries. These piracy events have culminated in the Great Equatorial Divide shifting south by ∼400 km through the Late Cenozoic (F. P. D. Cotterill, unpublished data). This has contributed significantly to a ‘shuffling’ juxtaposition of forest species and savanna species across these landscapes. It explains interesting biogeographical patterns in fishes (Schwarzer et al. 2012), mammals, birds (Cotterill 2002a, b, 2006) and snakes (Broadley et al. 2003), which are congruent with the high endemism and species richness of plants documented across this region (Linder 2001). This history is invoked to explain vicariant speciation of forest-adapted vertebrate species with their ranges centred on mushitu (gallery forest) and fringes of mesic miombo savannas in Katanga and northern Zambia. Alongside vegetation shifts that accompanied major changes in the palaeoclimate through the Plio-Pleistocene, this geomorphological evolution plausibly explains why the Ikelenge Pedicle (northwestern Zambia) and Katanga in the DRC are zones of high endemism. At least one horseshoe bat species, Rhinolophus sakejiensis, is known only from the Ikelenge Pedicle (Cotterill 2002a, b, 2006, Broadley et al. 2003). It is this forest–savanna mosaic that dominates the vegetation of the southern Congo basin and northern Angola, and justifies inclusion of such a vast portion of south-central Africa into this book (see Geographical Scope).

      Figure 25. Biomes of southern and Central Africa, based on climate and plant architecture (modified after White 1983). The bold line at 4°S depicts the northern limit to the biogeographical region that underpins this synthesis of chiropteran diversity.

      Figure 26. Ecoregions of southern and Central Africa (after Olson et al. 2001) are derived from the phytochoriological classification of White (1983). The bold line depicts the northern limit to the study area, which at 4°S approximates the southern limit of the belt of Congo closed forest, and the northern extent of the Guinea-Congolia/Zambesia Transition Zone.

VEGETATION

      Vegetation can be classified in markedly different ways. Prominent classifications include the phytochoria (regions of floristic endemism) of White (1983), which were subdivided into habitats (e.g. grasslands and forests). More recently, Olson et al. (2001) reclassified the phytochoria in White’s The Vegetation of Africa into ecoregions. This book relies mainly on biomes, whereby the vegetation of southern Africa is classified into several principal vegetation types based on the growth form or architecture of plants (grasses, shrubs, trees) and habitat structure (such as forest compared to woodland) that has evolved across the landscapes of the region. Examples of these vegetation types are forest, savanna, grassland, semi-desert, desert and sclerophyllous shrubland. The distribution of these vegetation types is controlled to a large extent by the interplay of abiotic and biotic determinants (annual rainfall and its seasonality, soils, ambient temperature, and disturbance regimes, notably the incidence of fire and herbivory). It is the interplay of these ecological determinants that controls whether a landscape is dominated, for example, by grasslands or savanna woodlands. At a global scale, these determinants also largely explain the convergence in vegetation structure in different parts of the world with similar climatic conditions, despite their extreme isolation. An example is the occurrence of similar sclerophyllous vegetation in areas with a Mediterranean climate: known as fynbos in South Africa, chaparral in California, matorral in Chile, kwongan in Australia and maquis or macchia in southern Europe. СКАЧАТЬ