Using a new mechanistic model of evolution on Earth, researchers can now better explain why the rainforests of Africa are home to fewer species than the tropical forests of South America and Southeast Asia. The key to high species diversity lies in how dynamically the continents have evolved over time.

One might assume that all tropical rainforests are about equally diverse due to their stable warm and humid climate and their geographical location around the equator — but this is not the case. Compared to South America and Southeast Asia, the number of species in Africa’s humid tropical forests is significantly lower for many groups of organisms.

Palms with few species

A good illustration of this uneven distribution — what researchers refer to as the pantropical diversity disparity (PDD) — is palm trees: of the 2,500 species worldwide, 1,200 occur in the Southeast Asian region and 800 in the tropical forests of South America, but only 66 in African rainforests.

Why this should be so is debated among biodiversity researchers. There is some evidence that the current climate is the cause of the lower species diversity in Africa’s tropical forests. The climate in Africa’s tropical belt is drier and cooler than that in Southeast Asia and South America.

Other evidence suggests that the different environmental and tectonic histories of the three tropical forest regions over tens of millions of years had an impact on the differing levels of biodiversity. Such environmental changes include, for example, the formation of mountains, islands, or arid and desert areas.

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Materials provided by ETH Zurich. Original written by Peter Ruegg. Note: Content may be edited for style and length.