In a remote area of the Namib Desert, rough grasses eke out a living from the region’s meager rainfall.
The growth of so many grasses in such a harsh environment is impressive, but also mysterious. The meadow is dotted with millions of strange circles, each devoid of grass or other vegetation, which together form an eerie polka dot pattern of “fairy circles” across the landscape.
Located 80 to 140 kilometers (50 to 87 miles) inland from the Namibian coast, this belt of circular gaps in the grassland is visible from miles around, the study authors note, and exhibits “a remarkable degree of spatial order.”
A typical fairy circle has a diameter of 2 to 10 meters, separated from the rest by a distance of up to 10 meters.
Scientists have made steady progress in debunking Namibian fairy circles, with the leading theories falling into two main camps.
One theory holds that the cycles are caused by termites feasting on roots, while the other suggests that grasses self-organize to maximize water availability.
Studies have lent credence to each theory, and some research has shown that both termites and self-organization may be behind fairy circles. But that explanation became more difficult after similar cycles were reported in Australia in 2016, with no clear connection to termites.
Recent research has more consistently shown self-organization, in which grasses form fairy circles to make the most of minimal rainfall, but without necessarily excluding termites.
In 2020, research led by Stephan Getzin, from the Department of Ecosystem Modeling at the University of Goettingen in Germany, added further support to the water scarcity scenario, which Getzin and colleagues described as an example of a Turing pattern.
In their latest study, Getzin and a team of researchers returned to Namibia in hopes of finding even more fascinating evidence, investigating fairy circles at 10 sites in the Namib Desert.
Rainfall is rare and irregular in this area. Grasses sometimes appear inside the fairy circles immediately after rain, but usually die soon after, the researchers note, while the grass between the circles survives.
Getzin and his colleagues monitored sporadic rains in the 10 sites, examining the grasses, their roots and shoots, as well as any possible termite damage to the roots.
They studied the conditions around grass dying after rainfall and set up soil moisture sensors in and around fairy circles to record data at half-hourly intervals, starting in the 2020 dry season and continuing through the end of the 2022 wet season.
Ten days after the rain, the inside of the fairy circles had very little growth, the study found, and the new grass that had grown was already dying. Twenty days after the rain, any grass inside the circles was dead, while the surrounding grass was “green and soft.”
Roots from dead grass inside the circles were as long as—or even longer than—roots outside the circles, suggesting the plants were investing heavily in root growth to seek water. The researchers found no evidence of termites feeding on the roots, they report.
“The sudden absence of grass for most areas within the circles cannot be explained by termite activity because there was no biomass for these insects to feed on,” says Getzin. “But more importantly, we can show that termites are not responsible for why grasses die immediately after rainfall without any sign of root-feeding creatures.”
The soil sensors revealed a slow decline in soil moisture both inside and outside the circles after an initial rainfall, the researchers report, when the grasses were not yet established.
Once the surrounding grasses were strong, however, the soil moisture quickly disappeared everywhere – even inside the fairy circles, despite the lack of grass there to absorb the water.
“Under the intense heat in the Namib, grasses are permanently transpiring and losing water. Therefore, they create soil moisture gaps around their roots and water is drawn towards them,” says Getzin.
“Our results are in complete agreement with those of researchers who have shown that ground water diffuses rapidly and horizontally in these sands even over distances greater than seven meters.”
This is an incredible example of an “ecohydrological feedback,” the researchers write, in which barren circles become essentially reservoirs that help sustain edge grasses.
This research could have implications elsewhere, Getzin points out, as this kind of self-organization appears to protect plants from increasing drought – a problem that is already worsening in some places due to climate change.
“By forming strong landscape patterns of uniform fairy circles, grasses act as ecosystem engineers and directly benefit from the water resource provided by vegetation gaps,” says Getzin.
“In fact, we know of related self-organized vegetation structures from various other harsh arid regions of the world, and in all these cases the plants have no other chance to survive than to grow in precisely such geometric formations.”
The study was published in Perspectives in Plant Ecology, Evolution and Systematics.