What causes Namibian fairy circles?

About 100 kilometres from the Atlantic coast, the Namibian desert is dotted with millions of “fairy circles”: circular gaps in the grassland, a few metres in diameter.

There is some resemblance to Australian spinifex circles, but there the connection ends.

Scientists have been debating the origins of the Namibian fairy circles for decades.

Researchers have theorised that they’re caused by termites, or that the plants were somehow being prompted to grow like this on their own.

A new study has thrown weight behind the self-organised plant theory, suggesting that the thirsty plants themselves might be the culprits.

Published in Perspectives in Plant Ecology Evolution and Systematics, the study draws on observations from two very high rainfall seasons, in 2020 and 2021.

The international team of researchers chased rain around the Namib desert over two years, taking soil moisture samples in the fairy circles at regular points after rainfall, and closely examining the grasses for termite damage.

Small grasses did start growing at the centres of fairy circles immediately after rainfall, but they were dying off just 10 days later. After 20 days, grasses inside the circles were dead. By contrast, grasses around the edge of the circles stayed green and healthy.

Crucially, the researchers couldn’t find any evidence that termites were munching on the grasses while they were dying.

“The sudden absence of grass for most areas within the circles cannot be explained by the activity of termites because there was no biomass for these insects to feed on,” says lead author Dr Stephan Getzin, Department of Ecosystem Modelling at the University of Göttingen, Germany.

“But more importantly, we can show that the termites are not responsible because the grasses die immediately after rainfall without any sign of creatures feeding on the root.”

So if it’s not termites killing off these circular patches of grass, what’s doing it? The researchers think it has to do with the spread of water.

“Under the strong heat in the Namib, the grasses are permanently transpiring and losing water. Hence, they create soil-moisture vacuums around their roots and water is drawn towards them,” says Getzin.

“Our results strongly agree with those of researchers who have shown that water in soil diffuses quickly and horizontally in these sands even over distances greater than seven meters.”

The researchers believe that the grasses are self-organising, growing in these circular patches to share water out appropriately.

“By forming strongly patterned landscapes of evenly spaced fairy circles, the grasses act as ecosystem engineers and benefit directly from the water resource provided by the vegetation gaps,” says Getzin.

“In fact, we know related self-organized vegetation structures from various other harsh drylands in the world, and in all those cases the plants have no other chance to survive except by growing exactly in such geometrical formations,” says Getzin.

The researchers believe that this information is particularly important in the face of increasing aridification.

The results are not globally uniform. As Cosmos reported in 2021, spinifex rings in Australia’s arid zone start as a solid clump and then change to the familiar ring shape due to microbial activity.