/ 26 November 2015

SA scientists reconstruct ancient ecosystems

Scientists are using their knowledge of Africa’s large herbivores
Scientists are using their knowledge of Africa’s large herbivores

Africa’s national parks are like windows into ancient landscapes and, using this information, South African scientists have reconstructed what our ecosystems looked like a thousand years ago.

This has important implications for climate-change modelling because, by understanding large herbivore numbers and populations, scientists will be able to refine their understanding of Earth’s climate.

Africa was the only place where this research, published in the prestigious academic journal Science, could be undertaken because it has had “far fewer extinctions” of large plant-eating animals than any other, says lead author Gareth Hempson, a postdoctoral researcher at the University of the Witwatersrand.

“Animals matter and ecologists across the world are starting to realise that many ecosystems cannot be understood without including animals and their impact into their thinking,” Hempson says.

“The problem is that in most places natural wildlife populations are extinct. The challenge that we took up was to try and put animals back into those landscapes [in terms of predicting which, and how many, animals would live where]. Africa turns out to be the place to do that. It has had far fewer extinctions and [has] these amazing game reserves that have pretty much intact populations.”

By compiling census data from the continent’s national parks, the team – which included Hempson, professors Sally Archibald from Wits and William Bond from the University of Cape Town – extrapolated what ­ecosystems around Africa would have looked like in terms of their large-herbivore numbers a thousand years ago.

“[We] analysed how factors like rainfall, soil fertility and vegetation types influenced the abundance of different species,” Hempson says.

“With that information and the knowledge about what rainfall, soils and vegetation used to be like, we were able to predict how many animals of each species there were in all the places that are now so radically transformed.”

Through the looking glass
Archibald says that the continent’s national parks are like “peeking through a looking glass back in time”. “When you go to the Kruger [National Park], you can see the impact these animals have on the landscape. Africa is the only place you can see that,” she says.

She cites the examples of Australia and North America, which no longer have the large herbivores – such as rhino, giraffe and elephant – that Africa does. “Humans arrived in Australia about 40 000 years ago, and shortly after that all the mega-herbivores were exterminated,” Archibald says. “Our results are exciting because their herbivores were exterminated tens of thousands of years ago [and we can now reconstruct their herbivore populations].”

The team’s insights into ancient African ecosystems are then used to infer what other parts of the world would have looked like, she says.

In Africa’s very wet and very dry ecosystems, there were few herbivores. But the in-between areas – wet and dry grasslands – were filled with life. “They were your classic African savannahs,” Hempson says.

The drier savannahs were “teeming” with smaller herbivores such as zebra and antelope, Archibald says. But large plant-eaters, such as rhino and elephant, inhabited the wetter savannahs.

“We’ve been … comparing animals and fire to see which one is more important [in controlling landscapes and ecosystems],” Archibald says. “We understand a lot more about fire because we can map it around Africa. Now we can map animals in the same way.”

But African ecosystems have fundamentally changed: sheep, cattle and goats have replaced many of the large herbivores in nonprotected ecosystems (that are not national parks).

The next step in the research is to understand “how livestock has replaced wild populations”, Hempson says. “To what extent do they substitute for wild populations, and how do the ecosystems change?

“We did not have a way to fit animals into our global ecosystem models, and these are the models that we use to try and understand where planet Earth is headed,” he says.

“Fitting animals into [global models], especially in places like Africa, is crucial … It’s become clear that we need to include animals in our thinking to understand global processes.”