The African Wintering Areas

South of the Sahara, Africa is a vast continent, extending over some 21 million km2 and comprising a wide range of habitats from hot desert to humid forest. Ecological conditions within Africa are determined much more by rainfall than by temperature, so that vegetation communities vary according to the amount of rain and the relative durations of wet and dry seasons. In fact, rainfall is the main factor governing the biological productivity of almost the whole continent.

1Separate estimates are available for particular types of birds. Thus, more than 2.5 million Palaearctic ducks winter in Africa, although the numbers vary greatly from year to year (Scott & Rose 1996). In addition, more than two million Palaearctic raptors winter in tropical and south temperate Africa (Chapter 7).

The alternation of wet and dry seasons is driven by the annual north-south movement of the Inter-Tropical Convergence Zone (ITCZ), a west-east zone of low pressure that forms where the northeast trade winds meet the southeast trade winds, and along which rain falls on a broad front (Jones 1995). During the northern summer, the ITCZ moves from the equator slowly northward, bringing rain to the northern tropics, and then retreats southwards again in autumn (Figure 24.2). The wet season thus begins later and ends sooner further north, so that the length of the wet season and the total annual rainfall decline from the equator northwards - in West Africa from more than 2000 mm near the equator to less than 50 mm (failing altogether in some years) near the southern fringe of the desert. In addition, as the mean annual rainfall declines, the more variable is the amount from year to year.

In autumn, the ITCZ crosses the equator southwards, so that from September onwards, southern Africa enters its wet season and the rains continue throughout the austral summer (coincident with the northern winter), ending in the austral autumn (northern spring). Although the rainfall pattern is more complex in southern Africa than north of the equator, in broad terms the rainfall distributions are mirror images of one another, with the wet seasons six months out of step. As in the northern tropics, the rains in southern Africa generally begin later and end sooner with increasing latitude.

Figure 24.2 Rainfall patterns in Africa. Rainfall determines the vegetation, from rainforest (wettest), through woodland and savannah to desert (driest), giving equivalent vegetation belts both north and south of the equator (see text). ITCZ, Inter-Tropical Convergence Zone. From Jones (1995).

Figure 24.2 Rainfall patterns in Africa. Rainfall determines the vegetation, from rainforest (wettest), through woodland and savannah to desert (driest), giving equivalent vegetation belts both north and south of the equator (see text). ITCZ, Inter-Tropical Convergence Zone. From Jones (1995).

Geographical exceptions to a single rainy season are relatively minor, and include the region between 5°N and 5°S in West Africa, where rain falls year-round and forest flourishes; a region between about 7°N and 7°S in East Africa where two rainy seasons occur, one around May and another minor one around October; and the western Cape and southern coasts where rain falls year-round but more in winter than in summer. Everywhere else the single rainy season is followed by months of unbroken drought, the 'dry season', equivalent in its ecological effects to the northern winter.

Because the total rainfall and duration of the wet season both decline with distance north and south from the equator, the natural vegetation zones also mirror each other in the northern and southern tropics. The region of almost year-round rainfall in equatorial West and Central Africa supports rainforest. Then, with decreasing rainfall towards both north and south, the vegetation becomes progressively shorter and sparser, as forest gives way to broad-leaved deciduous woodland and then to increasingly sparse acacia savannah and eventually desert (Figure 24.2). Where the rainforest has been cleared, it has given rise to a 'derived savannah' of remnant secondary forest and broad-leaved trees akin to the adjacent zones to the north and south, known as Guinea savannah in the north and miombo (Brachystegia) woodland in the south. Further from the equator occur mixed broad-leaved/Acacia woodlands (known as the Sudan zone in the northern tropics), giving way to increasingly arid Acacia savannahs (the Sahel in the northern tropics), which reach the margins of the northern (Sahara) and southern (Kalahari/Namib) deserts.

The greatest exception to this broad latitudinal vegetational pattern within the continent is East Africa, where very varied topography and twin rains combine to produce a complex mosaic of vegetation types from rainforest to semi-arid Acacia savannah. This complexity makes it harder in this region to discern clear latitudinal trends in rainfall, vegetation zones or bird migration patterns. Most importantly for the migrants, however, the seasons of maximum plant productivity and associated insect life are six months out of step between the northern and southern tropics.

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