Geoscience Reference
In-Depth Information
Basin remains the last large remnant of a once continuous and vast expanse of trop-
ical rainforest.
Origins of the African Rainforest
The tropical rainforests of Africa reflect the history of the continent. Tens of mil-
lions of years have shaped the African Rainforest. Underlying a large part of the
region is one of the oldest rock formations in the world, the 500-million-year-old
Precambrian Shield. This ancient basement rock occurs in pieces in Africa, as well
as in Asia and South America. Significant tectonic events and severe climatic epi-
sodes have influenced the current distribution of the rainforest. Ancient plant fami-
lies known only from the fossil record in Africa have living relatives in South
America, Asia, and Australia, providing evidence of the Gondwanan connections
that once existed among these continents.
The modern African rainforest appeared about 35 mya. At that time, rainforests
covered a substantial portion of the continent, stretching north to the Arab Repub-
lic of Egypt and Libya and east to the Indian Ocean, in areas now covered by desert
and savanna. Fossil pollen found in those areas shows the presence of tropical for-
est species. After separating from South America, Africa was isolated and a unique
group of plants and animals evolved. As the African continent continued to move
northward, it collided with Asia, about 20 mya. It remained isolated, however, due
to climatic barriers imposed by large deserts.
The rainforest reached its largest extent during the Miocene (35-10 mya), and
then global cooling brought cooler and drier climates to Africa. This resulted in a
decrease in the extent of the African rainforest and an expansion of the savannas
and woodlands seen in Africa today.
Another period of global cooling occurred about 2.5 mya and brought a cooler,
drier, and more seasonal climate that further restricted the African rainforest.
Cycles of glaciations continued throughout the Pleistocene Epoch. The last major
glacial expansion ended about 18,000 years ago. Studies of fossil pollen (palynol-
ogy) have found species more accustomed to drier environments, lowered lake lev-
els, and increased dune activity during these times.
The effects of past global climate change on rainforest distribution in Africa
were severe. Researchers estimate that the African rainforest was reduced to as lit-
tle as 10 percent of its largest, Miocene, extent. It only remained in small isolated
blocks with higher rainfall and along river margins. These areas became refugia
where once widespread animals or plants were able to survive.
Madagascar was isolated from Africa around 90 mya and developed its own
unique tropical rainforest along its eastern coast. Although limited in extent, Mada-
gascar is home to some of the richest rainforests on Earth, and more than half of all
Madagascar's rare and wonderful plants and animals are found in these forests.
