The evolution of arid ecosystems in eastern Africa

Abstract

The present aridity of Africa contrasts with the lush environments that existed over most of the continent in the early Cenozoic. The extinction of large terrestrial herbivores at the end of the Mesozoic, and relatively warm global climatic conditions in the early Cenozoic contributed to the spread of forests and woodlands in regions today occupied by grasslands and deserts. The increase in aridity after the Eocene climate optimum has been complex, characterized by multiple reversals and modulated by rifting in eastern Africa. The paleobotanical evidence indicates that ecological differentiation within the continent existed early in the Cenozoic, with some areas dominated by moist forests and others by drier Acacia woodlands. C4 grasslands began to spread during the Late Miocene, and became more prominent during the Pleistocene. In parallel to the spread of grassland mosaics during the Cenozoic there was an increase in the diversity of large herbivorous mammals (with body mass >350 kg). This diversity in megafauna peaked in the Pliocene. One of the key ecological roles of the megafauna was to create and maintain complex mosaics that included open habitats. Faunal evidence of paleoenvironments in the Turkana Basin of Kenya corroborates conclusions derived from other lines of evidence, but raises new questions. Hypsodont and cursorial bovids increased in abundance in the Late Miocene about 6 Ma, in the Pliocene after 3 Ma, and again in the Plio-Pleistocene after 2 Ma. But this faunal evidence also demonstrates that not all parts of the Turkana Basin responded in the same way to climatic changes. The lower Omo valley of Ethiopia, a northern extension of the Turkana Basin, remained significantly more forested than the areas near the western margins of the basin. Major river valleys like the Omo served as refuges and centers of endemism during intervals of significant climatic fluctuations. A similar role was played by the coastal and montane forests of eastern Africa, which were separated from the Central African forests by an arid corridor that stretched from northeast Africa to Namibia, and probably originated in the Miocene. The complex mosaic of environments in eastern Africa today continues to support an immensely diverse range of plants and animals, many of them found nowhere else on earth.