It’s time to re-think the assumption that Africa’s Serengeti grasslands are pristine works of nature. It turns out that the amazing biodiversity they host today might owe more to ancient cowpats deposited by livestock corralled in overnight pens by Stone Age nomadic herders. Far from despoiling the Serengeti, nomadic farmers have helped its unique ecosystems develop over several millennia.
“Our findings show that African savannahs thought of as ‘untouched’ environments stretching back millions of years are more biodiverse as a result of the spread of the earliest herders,” says Fiona Marshall of Washington University in St Louis.
Researchers already knew that cowpats from livestock provide hotspots of nutrients in otherwise barren grasslands, enabling much richer ecosystems to evolve in and around these human-made biological oases.
The assumption was that, on the Serengeti, these dung-rich oases – called grassy glades – date back roughly 1000 years. To explore whether they had an earlier history, Marshall and her colleagues sampled layers of earth from depths up to a metre below five previously identified ancient pastoral sites in Narok County in southwest Kenya.
They were easily able to distinguish layers rich in the remains of dung. Radiocarbon dating revealed the dung deposits were between 1550 and 3700 years old. In other words, nomadic herders have been shaping the Serengeti for almost 4000 years.
Within the dung they found abundant remains of plants the livestock had eaten, including phytoliths, microscopic silica-based fragments that survive after plants rot. They also found spherulites, fragments of calcium carbonate that survive in dung, plus rich stores of nutrients vital for life including phosphorus, nitrogen, magnesium and calcium.
As controls, Marshall and her colleagues also sampled sites close to each grassy glade. They found that the ancient glades were 10 to 10,000 times more fertile, depending on the site and its nutrient and mineral profile.
Experts on Serengeti botany say that nomadic farmers continue to benefit the savannahs today, but only if they remain highly mobile. “If large numbers of livestock are confined to one area for more than a few days or weeks, you get overgrazing and depletion of nutrients,” says Arne Witt of CABI Africa in Nairobi, Kenya, who studies how invasive plant species disrupt the Serengeti.
But he warns that the nomadic farmers’ freedom of movement is receding with the growth of human and cattle numbers, more private land ownership, more roads and more fences.
Journal reference: Nature, DOI: 10.1038/s41586-01800456-9
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