Читать книгу Bats of Southern and Central Africa - Ara Monadjem - Страница 39

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. Similarly, forest occurs in areas with a high-rainfall tropical climate (van Wyk and Smith 2001).

Figure 27. The intricate spatial associations between patches of gallery forest within the savanna matrix that comprises the Guinea–Congolia/Zambesia Transition Zone of White (1983). The spatial structure of this habitat mosaic across the landscape is closely associated with the drainage topology across the southern Congo basin, central and northern Angola, and northern Zambia. Edaphic control and protection from fires are equally important determinants of where evergreen forest patches persist in the landscape. (a) View south at 10.5°S from mesic miombo woodland on a Kalahari sand ridge reveals floodplains and fringing gallery forest along the Upper Kwango River, on the Angola–Congo border; (b) dense fringing gallery forest along the Kwango River; (c) a forested tributary within savanna grassland with scattered trees on Kalahari sediments (© F. P. D. Cotterill).

Figure 28. Riparian fringe along the Angwa River below the Zambezi escarpment, northern Zimbabwe, looking north towards the Chewore Hills. The flora of this region is diverse. The vegetation at intermediate altitudes is mainly mixed woodlands (including Diplorrhynchus condylocarpon and Combretum spp.), with the valley basin dominated by mopane (Colophospermum mopane) and jesse thickets (mixed Commiphora–Combretum). Baobabs (Adansonia digitata) are abundant. Hollows in these trees – especially in cathedral mopane – provide daylight domiciles for many chiropteran species, as do riparian emergents (e.g. Diospyros mespiliformis and Berchemia discolor) (© F. P. D. Cotterill).

Figure 29. Open woodland on the floodplain of Mana Pools National Park, northern Zimbabwe, is dominated by Faidherbia albida. The bat fauna has been well studied (Rautenbach and Fenton 1992). The animal-eating bat Nycteris grandis is one of several species that roosts in the hollow boles of these large trees (© F. P. D. Cotterill).

Figure 30. Mesic miombo woodland covers large portions of Katanga (DRC) and the northern regions of Angola and Zambia. Hollows in large trees provide daylight roosts for several species of bats, including Scotophilus dinganii and Mops niveiventer. This photograph was taken near the type locality of Rhinolophus sakejiensis, which was in the dense understorey of thick gallery forest along a stream (© F. P. D. Cotterill).

Figure 31. Southern miombo woodland dominated by Brachystegia glaucescens on rugged granitic terrain in the Matobo Hills supports a rich bat fauna. Species include the vesper bats, Eptesicus hottentotus, Myotis tricolor and Laephotis botswanae, and the horseshoe bats, Rhinolophus darlingi and R. mossambicus (© F. P. D. Cotterill).

Figure 32. Moremi Game Reserve, Botswana. Cathedral mopane woodland, Colophospermum mopane, illustrating incidence of elephant impacts on habitat structure, with fallen, damaged and standing dead trees, and leading to a ‘homogenisation’ (reduction of heterogeneity) of the habitat. Gaps under bark, hollows, and crevices in mature mopane trees provide daylight roosts for many species of bats, including Chaerephon chapini, Scotophilus leucogaster and several other small vespertilionids, notably Nycticeinops schlieffeni (© F. P. D. Cotterill).

Figure 33. A stark contrast between agro-ecolandscapes and relatively intact southern miombo woodland on Kalahari sand in the Sebungwe Region, Zimbabwe. The conservation implications of widespread loss of arboreal habitat for biodiversity are exemplified in this aerial view comparing Gokwe Communal Land (left) and the Mafungabusi State Forest (right) (© A. J. Loveridge).

To a large extent, the structural features of a vegetation type also determine its associated fauna. The preference shown by birds and mammals for a specific vegetation type, for example forest or savanna, is determined mainly by the structural features of the dominant plants, not by their taxonomic identity. In the case of bats, vegetation structure has two very important controls on the bat assemblages associated with a biome: these govern the nature of foraging habitat (see Echolocation for more detail) and the characteristics of roosts and food. An area that is more or less uniformly covered by one of these vegetation types usually represents a major biotic zone and is often called a biome (van Wyk and Smith 2001). The major biomes of southern Africa are fynbos, desert, succulent Karoo, Nama Karoo, savanna, forest, and grassland. In contrast, the classification of vegetation into phytochoria describes the biogeographical affinities of constituent plant species (principally endemics) across Africa as a whole. Congruent patterns in animals (including vertebrates, such as birds) (Dowsett et al. 2008) are also explained by an association with phytochoria. Examples include the association of Rhinolophus sakejiensis with relictual Guineo-Congolian forest, and Plerotes anchietae with moist miombo woodlands of the Zambesian phytochorion.

The fynbos biome, also known as the Cape Floristic Region, is situated at the southern tip of Africa. Centred on the Cape Fold Belt mountains, with its origins in the late Oligocene, this biome is recognised as one of the main centres of plant diversity and endemism in Africa (van Wyk and Smith 2001, Hoffmann et al. 2015). While this biome supports a remarkably high endemism of plants and some animals, its bat fauna is distinctly depauperate, and only one species, Rhinolophus capensis, can be considered a partial endemic to this region.

To the west of the subcontinent, arid and semi-arid biotas characterise the Namib Desert, succulent Karoo and the Nama Karoo (often referred to as semi-arid scrub vegetation). The southern African arid region hosts at least 17 bat species, representing eight families, of which three are endemic to the region (Rhinolophus denti, Laephotis namibensis and Cistugo seabrae) and one is vagrant (the fruit bat Eidolon helvum) (Monadjem et al. 2018a). Local-scale landscape features (e.g. habitat structure) might be more important than aridity in driving bat species richness, and an unknown factor (possibly temperature limiting the availability of insects flying high above the ground) may restrict the diversity of the open-air foragers throughout the region (Monadjem et al. 2018a).