The origins of the numerous round exposed patches on African grasslands known as fairy circles have been the subject of discussion among researchers for more than a decade. In a recent study, biologist Prof. Norbert Jürgens and soil scientist Dr. Alexander Gröngröft from the University of Hamburg confirmed that termites are the cause of fairy circles. At the same time, they refute the central argument put forward by ecosystem modelers that fairy circles are caused by the self-regulation of grasses.
Scientists confirmed that sand termites create African fairy circles and overturned the theory that grasslands self-regulate by demonstrating that these fairy circles store water over time. Pictured above is a fairy circle in the Namib Desert. Image source: UHH/MIN/Juergens
As early as 2013, Hamburg botanist Norbert Jürgens published an article pointing out that pure subterranean termites of the genus Psammotermes create exposed patches and achieve long-term water storage despite infrequent rainfall by eliminating plants in the sand. This explanation was published in the journal Science and confirmed in subsequent years by southern African entomologists (Professor Mike Peake, Dr. John Henschel, Dr. Kelly Flig).
This mysterious phenomenon has also been studied by other researchers, such as those at the University of Göttingen using modeling methods. Researchers published articles (Getzin et al., 2015, 2022) stating that bare patches are caused by the self-organization of grass plants. Grass plants absorb water unevenly through their root systems and through extensive diffusion in sandy soil, resulting in the death of grass plants in bare patches.
In addition, by measuring soil moisture at a depth of 20 centimeters beneath the fairy circles, they discovered drying phenomena and explained them to the surrounding grass rapidly absorbing water horizontally.
Writing in PPEES, Norbert Jürgens and Alexander Gröngröft refute the central argument of the Göttingen modellers: In their study, Jürgens and Alexander Gröngröft demonstrated the presence of sand termites on more than 1,700 fairy circles in Namibia, Angola and South Africa.
The soil moisture measurements cited by Getzin et al. (2022) as evidence for the self-organization hypothesis coincide with those of Jürgens 2013. However, the two explanations differ: the modelers took measurements at the topsoil and interpreted the drying of the topsoil as the surrounding grass pulling away moisture, while Jürgens showed in 2013 by simultaneously measuring soil moisture at four different depths (up to 90 cm deep) that fairy circles in the subsoil can store water for long periods of time.
"What's more, my colleague Glenn Groft's analysis and laboratory measurements of the hydrological properties of desert sand invalidate an important basis for the self-regulation hypothesis," Jürgens said. "The hydraulic conductivity of the coarse-grained Fairy Circle sand where termites live is indeed high during heavy rains, and large amounts of water can seep away quickly through large pores. But it's a completely different story when the sand releases easily mobile water deep enough to dry out to less than 8 percent of the soil's volume."
At this time, water can only be stored at the contact points between sand grains. Without a continuous water film, the water conductivity of the soil drops to a very low level. This means that at humidity levels below the fairy circle (volume ≤ 5%), the short-distance transport capacity of liquid water is "very low". This physical phenomenon is evidenced by the formation of a dry sand layer directly on the soil surface above the moist subsoil.
"According to current knowledge, the situation of horizontal water transport of several meters over several days postulated by representatives of self-regulation is physically impossible. Therefore, the debate about opposing explanations of biological phenomena turns out to be determined by physics, in this case soil physics," says Jürgens. "Soil moisture measurements of fairy circles and the hydraulic properties of sandy soil found in the laboratory rule out the self-regulation hypothesis as an explanation for fairy circles. Therefore, the reason for the formation of fairy circles is obvious - the sand termites gain a considerable survival advantage through soil moisture storage."