FAIRY
Aerial view on the Australian fairy circles which spread homogeneously over the Pilbara landscape. Kevin Sanders

Fairy circles found in Namibia and distant Australia are identical in their pattern and distribution, according to research done at Helmholtz Centre for Environmental Research (UFZ) in Leipzig. The regularly spaced patches of bare soil amidst grasslands seen in Australia are believed to be the self-organising response of plants to water scarcity.

After Namibia, the strange 'fairy circles' on land were sighted in Australia in 2014. Typically, six bare patches four metres across, 10 metres from each other and centred around a point form a hexagonal honeycomb. Groups of such circles were chanced upon in a small mining town of Pilbara in western Australia.

Analysis of aerial photographs and spatial patterns of vegetation by the team, published in the Proceedings of the National Academy of Sciences, revealed Australian and African fairy circles are almost identical.

'Fairy circle' expert Dr. Stephan Getzin from UFZ has supported the theory that the barren circles arise on their own under certain conditions, most probably resulting from competition for water in arid regions. "The interesting thing about fairy circles is that they are spread with great regularity and homogeneity, even over vast areas, but they occur only within a narrow rainfall belt," he explains.

He and his co-authors Hezi Yizhaq and Ehud Meron from Ben-Gurion University of Negev in Israel have also confirmed their theory with computer simulations.

While it has been speculated that the fairy circles are caused by ants or insects nibbling away at the plants' roots, or by underground bubbles of carbon monoxide rising to the surface, the Pilbara circles told a different story. Analysis of the temperature and permeability of the soil showed that water flowed across the bare patches of hard baked crusty soil to where spinifex grasses grew at the edge.

The centre being hard remains impenetrable by water. In places where the grasses are growing, the plants keep the surface cooler and the soil looser so water seeps in easily. This allows other plants to colonise the area and the conditions again improve slightly.

"In Namibia, the sandy soils of the fairy circles are much more permeable and precipitation can drain away with ease," says Getzin. Water reservoirs form under the barren areas, supplying the surrounding grass with moisture. The mechanism is different in Australia and Namibia but the same vegetation pattern is formed as both systems of gaps are triggered by the same instability, he adds.

Insect activity was random and not specific to the circles. "There we found in the majority of cases no nests in the circles and unlike in Namibia, cryptic sand termites do not exist in Australia," reports Stephan Getzin. "And the ones we did find have a completely different distribution pattern to the fairy circles."

Dr Getzin notes the results support current thinking in dryland ecological research where such distinct vegetation patterns are recognized as a consequence of competition for scarce water. The dominant grasses of the Triodia genus found in the direct vicinity of the fairy circles have also been seen to form elsewhere other typical drought patterns such as stripes, labyrinths or spot patterns with individual plants surrounded by bare earth.

FAIRY2
A large fairy circle with a hardened top-soil layer that prevents the growth of grass. Australian fairy circles have mean diameters of 4 meters but some may exceed 7 meters. Dr. Stephan Getzin