A study by UP’s Faculty of Natural and Agricultural Sciences has detailed the mechanism behind the formation of circular patches in the Namib Desert known as fairy circles.

Commonly referred to as fairy circles, the hundreds of thousands of bare circular patches in the arid grasslands of the Namib Desert have puzzled the scientific community for decades. However, a new study by UP’s Faculty of Natural and Agricultural Sciences not only revealed that these circles also occur in South Africa’s Kalahari Desert, but also proposed the mechanism behind their formation. The results were published in the academic journal BMC Ecology.

In 2015, Prof Marion Meyer of the Department of Plant and Soil Sciences set out to study the effect of toxic Euphorbia species (milk bushes) on fairy circle soil chemistry and water repellency, their germination inhibition, and antimicrobial activity on beneficial root bacteria. He conducted the study mainly with three students, who were doing their BSc honours and MSc degrees with him: Christiaan Schutte (also supervised by Prof Greg Breetzke), JW Hurter and Nicole Galt. The researchers also wanted to determine if these plants could give rise to the fairy circle pattern, and so compared their current spatial patterning with that of fairy circles in four areas in Namibia.

“The fieldwork in the Namib Desert was always interesting, even exciting,” says Prof Meyer. “There were occasions when I – along with my students and co-authors, Petunia Degashu (PhD student) and my wife, Nicole Meyer – encountered desert lions, cheetah, elephants and even poisonous snakes.”

This multidisciplinary study provides soil chemical, phytochemical and spatial patterning evidence that the fairy circles of Namibia are caused by dead Euphorbia species. “The species colonised sandy plains when climatic conditions were more favourable in the past,” explains Prof Meyer. “Sandy soils have a low water-holding capacity, and when climatic conditions became less favourable, the lack of water and competition for nutrients would have resulted in increased competition between these plants, so many would have died.” It’s worth noting that the temperature increase in Namibia over the past two or three decades is about three times more than the global mean temperature increase reported for the 20th century.

The researchers further proposed that the decomposition of dead plants and their sticky latex altered the chemical properties of the sand, which made the soil water repellent (hydrophobic), and that various other toxic compounds also entered the soil from the decaying species. “Most of these compounds would probably have broken down in a relatively short time, but the milky latex can adhere to the sand, become hard and persist in soil for a long time,” says Prof Meyer. “These soil changes cause the formation of many fairy circles. The transition proceeds from a site with only plants, to a mixed site with plants and fairy circles, and then to a fairy circles-only site.”

Additionally, the researchers found that while seedlings emerge from fairy circles after good rainfall, they survive for only short periods. As the harsh desert conditions set in, they die due to the soil water that infiltrates to depths beyond the reach of the roots of seedlings. “In time, decades or even centuries, with occasional rain, the effect of Euphorbia species in the soil will slowly erode away; seedlings will survive for longer periods in older fairy circles until they eventually reach maturity; and the circles will no longer be visible.”

And finally, by integrating rainfall, altitude and landcover in a geographic information system (GIS)-based site suitability model, the researchers were able to predict where fairy circles should occur. The model largely agreed with the distribution of three Euphorbia species and resulted in the new discovery of fairy circles in the far southeast of Namibia and even in the Kalahari Desert of South Africa.

This study provided supporting evidence that the Euphorbia theory can explain the formation of fairy circles in the sandy, dry areas across southern Africa, and where these succulents co-occur now or previously with fairy circles. It was concluded that the allelopathic (inhibition of growth by substances released by another plant), adhesive, hydrophobic and toxic latex of Euphorbia species are the cause of fairy circles.

Further research in areas with many fairy circles where big populations of large succulent Euphorbia species are not currently present, like in the Namib Rand Nature Reserve and Marienfluss, is in progress and focuses on DNA analyses of soil and ancient samples of plant pollen.