Unraveling Africa’s Fairy Circle Mystery Scientists Identify True Source

For over a decade, the origin of the numerous circular bare patches in the middle of the African grasslands, known as fairy circles, has been a topic of discussion among scientists. In a recent study, biologist Prof. Dr. Norbert Jrgens and soil scientist Dr. Alexander Grngrft from the University of Hamburg that termites are the cause of the fairy circles.

At the same time, they refute the central arguments for the explanation put forward by ecosystem modelers that the circles are caused by the grasses’ self-regulation.

As early as 2013, the Hamburg botanist Norbert Jrgens published that purely subterranean sand termites of the genusPsammotherms cause the bare spots and, by removing the plants in the sandy soil, enable long-term storage of water after rare rains. This explanation, published in Sciencewas confirmed in the years that followed by entomologists from southern Africa (Prof Mike Picker, Dr Joh Henschel, Dr Kelly Vlieghe).

Other researchers also investigated the mysterious phenomenon, e.g. at the University of Göttingen using modeling methods. The researchers published (Getzin et al. 2015, 2022) that the bare patches are caused by the self-organization of the grass plants, which draw water unevenly with their roots and through extensive diffusion in the sandy soils, thus causing the death of grasses in the bare spots.

By measuring the soil moisture under the adventure circle at a depth of 20 cm, they also found drying, which they interpreted as caused by rapid horizontal absorption of water by the grass in the surrounding area.

Norbert Jrgens and Alexander Grngrft now refute the central arguments of modelers from Gttingen in the article published by PPEES: In their study, Jrgens and Grngrft 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 coincides with Jrgens’ soil moisture measurements in 2013. However, the interpretations are different: while modelers measure in the topsoil and interpret its drying out as withdrawal of water by the surrounding grasses, Jrgens showed in 2013 by simultaneous measurement at four different depths of up to 90 cm, that the adventure circles in the underground store the water for a long time.

Of even greater importance is that the analysis by my colleague Grngrft and the measurements of the hydrological properties of the desert sand carried out in the laboratory invalidate the decisive basis for the assumption of self-regulation, says Jrgens. The water conductivity of the coarse-grained sand in the fe-circles in which the termites live is very high when there is a lot of water present during a heavy rain event, which can so quickly seep away in the large pores. The situation is quite different, however, when the sand has released the easily mobile water into the depths and has dried out to less than eight percent of the soil’s volume.

After that, water is stored only in the contact points between the sand grains, a continuous film of water is missing, and the soil’s ability to conduct water drops to very low levels. This means that at the moisture levels found below adventure circles (5% by volume), very little liquid water transport can take place over short distances. The formation of dry sand layers on the soil surface directly over moist subsoil demonstrates this physical phenomenon.

The horizontal water transports over meters in a few days assumed by the self-regulation representatives are physically impossible according to current knowledge. The debate about opposing interpretations of a biological phenomenon is thus surprisingly decided by physics, in this case earth physics, says Jrgens. The soil moisture measurements on the fae circles and the soil hydraulic properties of the sand found in the laboratory thus exclude the self-regulation hypothesis as an explanation for the fae circles. The reason for the formation of the fairy circles is thus clear that it is the sand termites that ensure a significant survival advantage through the storage of soil moisture.

Reference: Herbivorous sand termite causes adventure in Namibia A reply to Getzin et al. (2022) by Norbert Jrgens and Alexander Grngrft, 3 June 2023, Perspectives in plant ecology, evolution and systematics.
DOI: 10.1016/j.ppees.2023.125745