25 Jul 2023
In the scientific debate about the cause of the mysterious bald circles in grasslands on the eastern edge of the Namib Desert, the termite theory remains the only sound explanation. This is the result of the article "Sand termite herbivory causes Namibia's fairy circles - A response to Getzin", which was published recently in the scientific journal "Perspectives in Plant Ecology, Evolution and Systematics" (PPEES), according to a media release of the University of Hamburg, published last week on the science news portal EurekAlert! of the American Association for the Advancement of Science (AAAS).
In their paper, biologist Norbert Jürgens and soil scientist Alexander Gröngröft refute central arguments of the alternative explanation attempt that fairy circles are caused by self-regulation of the grasses. This explanatory approach is advocated by ecosystem modeller Stephan Getzin.
Namibian.org News had already reported on this in February, after the article by Jürgens and Gröngröft had been published by PPEES in a preliminary version. Now, however, it is the final version, which has been reviewed by the PPEES editorial team and scientific reviewers and finally approved for publication. What makes this so piquant is that Getzin's article from October 2022 ("Plant water stress, not termite herbivory, causes Namibia's fairy circles") had also been published by PPEES.
Getzin has been claiming since 2015/16 that the bare patches of fairy circles in the grasslands on the eastern edge of the Namib are formed by self-organisation of the grasses when there is a lack of water. According to this, the grasses outside the circles create a pulling effect with their roots, which deprives the young grasses inside the circles of water. The ecosystem modeller bases his explanation on a mathematical model that can be used to reproduce the shape of the circle and the patterns of bare patches in the grass areas.
According to the University of Hamburg's media release, Jürgens counters that "the measurements of the hydrological properties of the desert sand carried out in the laboratory invalidate the crucial foundations of the assumption of self-regulation." More precisely: "The horizontal water transports over metres in a few days (...) are physically impossible according to current knowledge." Many fairy circles have a diameter of more than eight metres.
The readings of soil moisture in fairy circles taken by Jürgens at four depths every hour since 2008 prove that rainwater seeps downwards. According to his theory, the sand termite Psammotermes ensures that young grasses in the fairy circle die off so that they cannot draw moisture from the soil. The resulting water reservoir in the soil under the bare patches gives the termites a survival advantage on the edge of the Namib, where rainfall is scarce and irregular. Jürgens had already presented his theory and the moisture values in the renowned scientific journal Science in 2013.
The fact that ecosystem modeller Getzin neither referred to the state of research in soil science in his publications on the self-organisation of grasses nor mentioned Jürgens' soil moisture values is considered a fundamental scientific flaw. However, by doing so he would have admitted that the facts contradict his explanation.
This is not Getzin's first attempt to explain the fairy circles. In the article "Fairy Circles of Kaokoland" in 2000 he had claimed as co-author that the harvester termite Hodotermes mossambicus creates the circular places. There was no scientific evidence for this either.
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