For more than ten years, researchers have been talking about how the many circular bare patches in the middle of African grasslands—the so-called fairy circles—appear.
In their current study “Sand termite herbivory causes Namibia’s fairy circles—A response to Getzin,” biologist Prof Dr. Norbert Jürgens and earth scientist Dr. Alexander Gröngröft from Universität Hamburg confirmed that termites are the cause of fairy circles. At the same time, they refute the central arguments of the explanation presented by the ecosystem modelers that the circles are caused by the self-regulation of the grasses. The study was published in the journal Perspectives on Plant Ecology, Evolution and Systematics.
In 2013, Hamburg botanist Norbert Jürgens published that subterranean termites of the genus Psammotermes cause hollow patches and, by eliminating plants in sandy soil, can create long which stores water after frequent rains. This explanation, published in Scienceconfirmed in the following years by entomologists from southern Africa (Prof Mike Picker, Dr. Joh Henschel, Dr. Kelly Vlieghe).
Other researchers have also investigated the mysterious phenomenon, for example at the University of Göttingen using modeling methods. The researchers published that the hollow patches are caused by the self-organization of the grass plants, which take the water unevenly among themselves with their roots and through the extensive spreading of the sandy areas. soil, thereby causing the death of weeds in bare patches.
In addition, by measuring the soil moisture under the circular fairy at a depth of 20 cm, they found drying, which they interpreted as the cause of the rapid horizontal absorption of water by the surrounding grasses.
Norbert Jürgens and Alexander Gröngröft now refute the central arguments of the models from Göttingen in the article published by PPEES: In their study, Jürgens and Gröngröft show the presence of sand termites in 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 Jürgens’ soil moisture measurements in 2013. However, the interpretations differ: While the modelers measure the topsoil and interpret its drying as water withdrawal in surrounding weeds, Jürgens showed in 2013 by simultaneously measuring four different depths of up to 90 cm that the fairy circles in the subsoil store water for a long time.
“More importantly, the analysis of my colleague Gröngröft and the measurements of the hydrological properties of the desert sand made in the laboratory do not validate the important foundations of the claim of self-regulation,” said Jürgens. “The water conductivity of the coarse-grained sand in the fairy circles, where the termites live, is actually very high when there is a lot of water during heavy rains, which can quickly seep into the large holes.”
“However, the situation is completely different when the sand releases readily transferable water into the depths and dries to less than eight percent of the volume of the soil. Then the water is stored only at the points of contact between in the grains of sand, a continuous film of water is lost and the ability of the soil to flow water is reduced to a very low level. This means that at the level of moisture found below the fairy circles (≤5% of volume), very little liquid water transport can occur over short distances.” The formation of dry sand layers on the surface of the soil directly above the wet subsoil shows this physical phenomenon.
“Horizontal water carrying meters in a few days that the representatives of self-regulation believe is physically impossible according to current knowledge. said Jürgens.
“The measurements of the soil moisture in the fairy circles and the soil hydraulic properties of the sand found in the laboratory thus reject the self-regulation hypothesis as an explanation for the fairy circles. The reason for the formation of fairy circles are very clear— these are termites in the sand that ensure a substantial survival benefit by storing moisture in the soil.”
Norbert Jürgens et al, Sand termite herbivory causes Namibia’s fairy circles—A response to, Perspectives on Plant Ecology, Evolution and Systematics (2023). DOI: 10.1016/j.ppees.2023.125745
Provided by the MIN-Faculty Universität Hamburg
Citation: Termites as cause of fairy circles in Namib Desert confirmed (2023, July 18) retrieved 19 July 2023 from https://phys.org/news/2023-07-termites-fairy-circles-namib.html
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