AFROALPINE HIGHLANDS► Isolation & Endemism
The Ethiopian dome began to rise some 75 million years ago and eventually split open and fractured through Great Rift Valley. During the Ice Ages the entire Ethiopian dome would have been less like Africa and more like the tundras of Eurasia, with the highest areas covered by glaciers. Formerly interconnected in glacial times, the present Afroalpine habitats (above about 3,000m) are distributed in isolated pockets on the top of mountains. Still, Ethiopia contains the largest extent of Afroalpine habitat in the African continent (see altitudinal map of Ethiopia).
Ethiopia’s unique environment for its region, together with its isolation, was a potent stimulus for rapid speciation. Most highland colonists have been African species that were sufficiently plastic to cope with the area’s many unique traits. Formidable obstacles, such as the grassy foodplains of the White Nile and the deserts of Northern Kenya, inhibited immigration of high altitude forms from most other directions. This isolation has resulted in a important number of endemisms of plants and animals. Some of this endemic species are in danger of extinction, like the Ethiopian wolf, and the status and biology of many others are unknown.
An array of organisms that colonized Ethiopia in glacial times are now isolated at high elevations and in many cases became new species. Nineteen of the 30 mammals currently to be endemic to Ethiopia live in the high montane area. Of these nineteen, eleven are shrews or rodents. Rodents are the dominant herbivores on the Afroalpine grasslands with densities up to 25kg/ha in parts of the Bale Mountains. In most areas, the endemic Ethiopian wolf prey on endemic rodents.
Two large and critically endangered ungulates and one primate are also high altitude specialists. The walia ibex (Capra ibex walie) is a southern offshoot of the Nubian ibex. It is now restricted to a small number of escarpments in northen Ethiopia. The world population may not exceed 400, of which over half live in the Simien Mountains.
South of the Rift Valley the mountain nyala (Tragelaphus buxtoni) survives only in the Bale and Arsi Mountains and in parts of the Chercher Mountains further East. This large and sturdy member of the kudu family weights up to 300kg. In Bale it is more numerous in montane forest and grasslands around 3,000m. The Bale Mountains Narional Park contains the largest numbering perhaps 1,500.
The gelada baboon (Theropithecus gelada) is mostly restricted to the highlands north of the Rift Valley where it remains widespread in grassland areas.
Fifteen of Ethiopia’s 26 endemic bird species live at high elevantions. Notable species include the blue-winged goose (Cyanochen cyanoptera), which occurs throughout the Ethiopian plateau including the Afroalpine zone. The largest bird of high elevantions is the northern wattled crane (Grus carunculatus), a distinctively marked and genetically isolated race of a species that occur at low elevations in southern Africa.
There are estimated to be 24 amphibians endemic to Ethiopia of which nearly half are from the high elevations. The invertebrates remain largely unstudied.
The higher plants of the Ethiopian mountains are still being catalogued. There may be between 100-150 high altitude endemics. The most conspicuous is the giant lobelia (Lobelia rhyncopetalum) whose sentinel forms dot the Afroalpine landscape.
The Palearctic connection
The high mountains of Ethiopia form an important biogeographic link with the temperate lands of Eurasia. During the glatial periods, tenuous links formed between northern Ethiopia and the Mediterranean region through the escarpments that flank the Red Sea, a route travelled by many animals and plants. Successful immigrants with certain Palaearctic ancestors are the walia ibex and the Starck’s hare Lepus starcki.
During the temperate winter the mountains of Ethiopia provide habitat for an array of paleartic migrants. Three Paleartic species, the chough (Pyrrhocorax pyrrhocorax), the ruddy shelduck (Tadorna ferruginea) and the golden eagle (Aquila chrysaetos) have their only known breeding sites outside the north temperate zone in the Aafroalpine one of Ethiopia.
The tops of African mountains have environmental conditions that differ dramatically from the lowlands below. Plants and to a lesser degree animals have evolved some peculiar adaptations in response to the Afroalpine climate. Plant gigantism is the most striking example. Giant lobelias, tree heather (Erica arborea) and giant Saint John’s wort (Hypericum revolutum) are important elements of the high altitude flora of Ethiopia. The everlasting flowers (Helichrysum sp) are conspicuous in mountain habitats, their high reflective silvery levaes and dry, papery flowers allow them to survive the desiccating winds up to 4,300m.
The most peculiar of the mammals is the giant molerat (Tachyoryctes mcrocephalus). This animal is retcricted to the Afroalpine regions of the Bale Mountains where it is common. It is seeral times larger then the related common molerta (T. splendens) of lower elevations. It also takes it food from above ground vegetation. Its eyes have migrated to the top of his head so that it can scan for predators while exposing as little of its body as possible. The extensive burrowing and mound building activites of the moelrtas are dominant ecological forces in the Afroalpine grasslands of the Bale Mountains.
See Afroalpine ecosystem literature.