The tropical rainforests are hotspots of biodiversity, but there are differences. Researchers have now used a newly developed computer model to investigate why the rainforests of Africa are home to fewer species than the tropical forests of South America and Southeast Asia. In doing so, they discovered that the tectonic development of the continents and their dynamics play a role for biodiversity that is often underestimated.
Tropical rainforests are the most biodiverse habitats on earth. They house a huge number of different plants, animals, fungi and other organisms. These evergreen lungs of the earth stretch like a lush ribbon in the tropical latitudes around the earth. Rainforests can therefore be found in South America, for example in the Amazon basin, as well as in Central Africa and distributed over the islands of Southeast Asia.
Not equally rich in species everywhere
The main factors that favor the rich biodiversity of the rainforests are their location around the equator and the stable, warm, humid climate. They promote plant growth and the formation of richly structured habitats. The interesting thing, however, is that because this applies equally to the tropical forests of South America, Southeast Asia and Africa, one could assume that they are also about the same species-rich – but that is not the case. Compared to South America and Southeast Asia, the number of species in the humid tropical forests of Africa is significantly smaller for many groups of organisms. For example, of the 2500 palm species worldwide, 1200 are found in Southeast Asia and 800 in the tropical forests of South America, but only 66 in African rainforests.
But what is the reason for this so-called pantropical diversity disparity (PDD)? So far there is no clear answer to this. Some researchers see the climate as the cause: Today there are slight, but possibly decisive, climatic deviations between the tropical forests. In Africa’s tropical belt, it is on average a little drier and cooler than in Southeast Asia and South America. Other scientists suspect the reasons more in the past. Accordingly, fewer different species developed in Africa because the environment and tectonics have changed less than in the other tropical regions over the past millions of years.
Species formation in the computer model
So far, however, it has hardly been possible to determine how large the share of these two parameters is in the current biodiversity of a region – after all, every habitat and every ecosystem is the result of both past and present influencing factors. That is why a team headed by Loïc Pellissier from ETH Zurich investigated this question with the help of the “gen3sis” computer model that they had specially developed for this purpose. This model simulates the primary framework conditions such as climate and geological-tectonic development and also takes four basic ecological parameters into account. These include the ecological niche of every species, evolution, speciation and spread.
“With these four basic rules, we can simulate the population dynamics of organisms against the background of changing environmental conditions,” explains Pellissier. “This also allows us to explain very well how the organisms came about.” With the help of this model, they have now investigated the case of the tropical forests. The result: the events of the past were more influential in the emergence of the biodiversity differences than is the climate today. “Our model confirms that differences in the dynamics of the prehistoric environment brought about the unequal distribution of biodiversity and not current climatic factors,” says Pellissier. “Geological processes and global temperature flows determine where and when species arise or become extinct.”
Africa was tectonically too inactive
In concrete terms, this means: In South America and Southeast Asia, plate tectonics has led to a major reshaping of the landscapes in the last 100 million years: In South America, the Andes piled up and thus changed the climate and vegetation of the lower-lying areas as well. In Southeast Asia, continental drift and the volcanic activity it triggered resulted in islands being formed and land bridges being created or destroyed. All of this led to the creation of new ecological niches and new species to emerge.
It was different in Africa: This continent is not in an active subduction zone and has changed little tectonically over the last 110 million years. In addition, the rainforests of Africa were previously framed by arid regions in the north and south. This sometimes drastic transition from tropical forest to savannah limits the possibilities for species to spread: “Species from rainforests can hardly adapt to the conditions in the surrounding arid areas,” says Pellissier. Over time, all of this has resulted in the biodiversity in the tropical forests of Africa lagging behind that of the forests in South America and Southeast Asia, the team explains.
Pellissier and his colleagues are now in the process of refining their model. They also want to use further simulations to understand how biodiversity came about in other species-rich regions, such as the mountains of western China.
Source: Swiss Federal Institute of Technology Zurich; Technical article: PLoS Biology, doi: 10.1371 / journal.pbio.3001340