Background information fact; by edgardowelelo@yahoo.com, Master of the Game
Africa is the largest and, in terms of its global position, the least mobile fragment left over from the break – up, between 270 million and 200 million years ago, of the super – continent Gondwanaland. Its other fragments, South America, Australia, India, the Middle East, South – East Asia and Antarctica, are now scattered across the SOUTHERN HEMISPHERE. The continent’s major aspect is of vast alluvial basins and plains textured by branching streams. Down the eastern side is the GREAT RIFT VALLEY SYSTEM derived from several phases of uplift, the most recent upheavals being 22 million, 6 million and 2.5 million years ago. The brittle crust bulged up in domes, the largest in ETHIOPIA being 400km wide. Aligned over thousands of kilometres in a jagged course from north to south the uplifted face has split along its axes to form RIFT VALLEYS that average about 40km in width. PLATEAUX have been lifted and associated volcanics have spewed out lava over extensive areas and thrown up volcanoes such as Kilimanjaro, which is 5,895m high. Earlier volcanoes have eroded away and deposited deep sediments. Cycles of erosion and deposition have created dramatic landscapes of mountains, deep valleys and gorges. Recent volcanics and the tipping of land surfaces have changed the direction of rivers and diverted waters from one basin to another. Among the most dramatic and consequential of developments is the filling of the RIFT VALLEY and other basins with lakes. Some of these have been EPHEMERAL. Lake CHAD in the SAHARA has expanded and contracted more than once. LAKE VICTORIA (NYANZA) began as a result of the blockage of two large tributaries of the RIVER ZAIRE. Due to uplift in what is now RWANDA and BURUNDI the waters of these rivers ponded back creating the present very shallow lake about 1 million years ago. All these lakes have been important as barriers to the spread or mixture of mammal populations. In DRC a very large lake persisted for several million years until the late PLIOCENE. Then, cut – back by a small ATLANTIC RIVER created a new drainage line, emptying the DRC basin of its huge lake. This relatively recent event has left a pattern of distribution that makes the RIVER ZAIRE / CONGO one of the most consistent and major biogeographic boundaries in AFRICA (in spite of the vegetation north and south of the river being superficially indistinguishable). Overspill from the former Lake Zaire was north of its present drainage. Its most likely course was down the OGOOUE VALLEY, which is disproportionately deep and wide for its contemporary catchment area. Another major biogeographic boundary, along the River Cross in E Nigeria, may also find its explanation in there – routing of a major river. The RIVER NIGER is thought, formerly, to have emptied east of its present delta, at the mouth of the RIVER CROSS. Other geological events may have shaped recent distributions. In the VIRUNGA VOLCANOES AREA, major eruptions about 22,000 years ago blocked and deepened LAKE KIVU. Subsequent eruptions, one as recent as 9,000 years ago, are thought to have depleted fish and reptile fauna and it is possible that release of vast accumulations of methane gas in LAKE KIVU combined with tremors and eruptions to send a poisoned fireball down over the slopes west of LAKE KIVU. In any event this area has many unusual anomalies of mammalian distribution which require research and explanation. In contrast with these areas of recent volcanic activity are rarer OLD GONDWANA SURFACES. Here and there blocks of ancient rocks have been thrust up and cut back. Among these are the SAHARAN MASSIFS, rift – Edge Mountains between Ruwenzori and Lake Malawi (Nyasa) and the crystalline mountains of EASTERN TANZANIA. Chairs of ancient mountains run on southwards, reaching over 3,480m in the DRAKENSBERG and ending up in the eroded landscapes of the KAROO and CAPE. These undisturbed, geologically ancient landscapes support ancient or conservative mammal types, such as insectivores, golden moles, blesmols and galagos. Africa’s links with EURASIA have pivoted on periods of connection with ARABIA. Sometimes the land masses (continents) touched at the south end of the RED SEA but mostly it has been in the north at SUEZ. The land bridges between MOROCCO and SPAIN, across the Straits of GIBRALTAR and between TUNISIA and ITALY over the Straits of SICILY , might have been more tenuous and appear to have been less influential for mammals. The most important period of connection, during which there was a significant exchange of fauna and flora, was about 30 million years ago (following many millions of years of isolation). The better – known emigrants were African apes and elephants, while early ungulates, hares, some Eurasian rodents and carnivores were immigrants. At a more local and contemporary level, altitude, latitude, natural barriers and soils all influence mammals in ways that are touched upon in the profiles of species.
CLIMATE
Africa is a relatively dry continent and its rain derives from evaporation off seas that warm and cool with the seasons. As a result the equatorial belt is the only region that enjoys two wet seasons. Because cooler conditions bring less evaporation, Africa is dry north of the equator during the northern winter and dry to the south in the southern winter. In the tropics this means ‘’summer’’ rains; in the subtropics conditions are more generally hot and dry. The mountains have their own climatic pattern, with marked temperature extremes between day and night, heavy rainfall, especially in the equatorial and tropical belts and generally cooler temperatures overall. The Cape and Mediterranean littorals have a different pattern of dry summers and wet winters. Excluding the equator, coastal strips and mountains are the main beneficiaries of rain coming in off the ATLANTIC and INDIAN OCEANS and heavy rains in all tropical uplands create long river systems that often flow across otherwise arid lands. The archetypal river of this sort is the NILE but many less well – known rivers show a similar pattern (in SW Tanzania foreign technocrats are impounding waters in the upper reaches of the RUAHA for rice – growing schemes, thus drying out lower reaches of the river where large populations of mammals are declining as their pastures and water disappear. The irregularity of seasons, especially in the subtropics and in the dry belt that runs from NE to SW Africa, often cause floods followed by droughts. In shallow basins, such as the RUKWA DEPRESSION and LAKE CHAD, massive evaporation and empty feeder – streams cause periodic drying up of these lakes with attendant die – offs of large antelope and hippopotamus populations. Very arid hinterlands often have narrow coastal strips where nocturnal fogs are sufficient to support some vegetation. On the Namibian foreshore, the Somali coast, up the RED SEA and along the MEDITERRANEAN and ATLANTIC COASTS these dew thickets or fog – fed grasses support uniquely adapted plant and animal communities that survive on ‘’islands’’ bounded by sea on one side and desert on the other (mammal examples are the silver dikdik in Somalia, Grant’s golden mole and a hairy – footed gerbil in Namibia). The SAHARA and SOMALI DESERT AREAS are part of a much larger zone, stretching to the HIMALAYAS, where dry air subsides and clouds rarely form. Throughout Africa it is drought that inhibits plant growth not absence of light nor low temperatures. Snow and frost are regular features only on the very highest mountains and at the temperate extremities of Africa. Surface drinking – water is the single most limiting climatic influence for the majority of mammals. Those species that can extract sufficient moisture from their food alone are the classic desert rodents, antelopes and carnivores.
AFRICAN VEGETATION
African vegetation is dominated by an equatorial belt of rain – fed forest and three principle desert areas: the SAHARA, the HORN (Somalia) and SW Africa (Namibia). Between – these extremes are moist forest– Savannah mosaics, woodlands (dominated by leguminous trees, called Miombo in the south – east and Doka in the north – west), various wooded grasslands or savannahs, often dominated by Acacia bush or scrub, and verging on subdesert or semidesert in places. The desert graduates from bare sand dunes (erg) and rocky pavements or screes (hammada) through various conditions in which ephemeral grasses or herbs, scattered shrubs and small trees modify the desert sufficiently to permit various mammals to survive. MONTANE AREAS also range from nearly bare screes on the top of Mt Kilimanjaro through various Afro – alpine habitats to montane grasslands, moorlands and forests. In the Cape and Karoo there are unique shrublands, moors, grasslands, and semi – deserts subject to frequent summer fires and sustained by winter rains. There are few places where the vegetation communities have not been affected by human settlement, felling, frequent fires and large herds of livestock. Nonetheless national parks have often succeeded in maintaining relatively healthy and representative communities of indigenous animals and plants.
HABITATS (THE PAST)
The broad outlines of modern habitats, deserts, forests and various intermediates, were already well developed many millions of years ago. However, their relative importance has fluctuated widely, following huge swings in global climate that go back more than 15 million years but are best known for the last 1 million years. Today’s extensive rainforests and warm savannahs are exceptional and we live in a period close to the wettest and warmest end of the scale. The last major spell of cold, dry weather peaked in Africa about 19,000 years ago, coinciding with the last global Ice Age. This was the most recent of more than 20 Ice Ages, each of which brought dry weather to most of Africa. At such times lowland rainforests would have retreated to especially favoured spots in Liberia, Cameroon, E. DRC and Tanzania, all close to the equator. The communities of animals and plants that are now restricted to cool, relatively dry belts on African mountains would then have spread over very extensive areas and mammals that are now restricted to temperate or cool areas (like springbuck in Southern Africa or gelada in Ethiopia) are known from fossils to have once been abundant in tropical areas. The gross pattern of climatic change is a simple one. Dry habitats have tended to expand or contract from north to south and vice versa. Warm wet periods on the other hand cause forests to pulse on an equatorial east – west axis. The result has been repeated fragmentation of both wet and dry habitats and, with that fragmentation, populations of animals and plants have suffered repeated bouts of isolation. While relatively stable species have simply pulsed with their habitats, many others have made local adaptations and speciated or subspeciated. This mechanism helps to explain the extraordinary diversity of species and subspecies, especially among equatorial mammals. A glance at the distribution patterns of many primates, squirrels and small carnivores suggests that climatic changes must have been very important because many of these species only live in restricted regions, often identifiable as refuges, within much more extensive forests. Amid all the changes there have been identifiable areas of stable climates. Somalia has always been a hot, dry spot, Namibia a cool one, Cameroon consistently warm and wet, Ethiopia high and dry, and Ruwenzori high and wet. The coast and mountains of equatorial EAST AFRICA have always caught rain from the INDIAN OCEAN while Liberia and Cameroon have caught it from the ATLANTIC. At the centre, the RUWENZORI MOUNTAINS are moistured from both east and west. Where major variables meet along consistent boundaries a great variety of biological niches becomes possible. Wet meets dry; high meets low, cold meets warm. Faces of hills, mountains or escarpments tilt towards rain or rain – shadow, towards moist or desiccating winds. Many localised species survive in these narrow zones, which are found on mountain slopes in S and SW, NE, E and central Africa. These narrow corridors and the stable foci, both wet and dry, are ‘’ centres of endemism’’ and they give a special interest to areas such as the Cape, the equatorial coasts and mountains, Ethiopia and Somalia as natural ‘’reserves’’ for rare and conservative species.
