Origin and dynamics of Namibian fairy circles assessed in-situ and by high-resolution spatio-temporal analysis
Final Report Abstract
Fairy circles (FCs) have appeared mysterious for decades. What is the process that causes hundreds of thousands of circular grassland gaps in arid Namibia? Until 2019, there have been two main theories to explain these vegetation gaps of about 4-6 m in diameter. The termite theory argues that subterranean insect activity would cause the gaps via destructive root feeding of freshly germinating green grasses. The theory of plant self-organization states that the grasses compete for the sparse soil water, so that biomass-water feedbacks cause the FCs. In 2020, the four decades old Euphorbia hypothesis was “reactivated” by South African scientists, proposing that Namibia’s FCs would be caused by the death of these poisonous shrubs. In this research project, we investigated these three main hypotheses in 10 regions of the Namib across 1,000 km from north to south, covering the hotspot areas with FCs in Namibia. We employed a high-resolution spatio-temporal analysis using drone mapping of FCs, in-situ investigations on the cause of the grass death within FCs, as well as measurements on water infiltration and daily soil-water changes within and around FCs. Our data show that the Euphorbia hypothesis has to be rejected based on the fact that grass growth was not prevented under decaying shrubs and dead Euphorbias did not transform into FCs over a time span of 40 years. Moreover, the size of dead Euphorbias cannot explain the size of observed FCs, and the spatial distribution of predominantly clustered Euphorbias cannot cause the pattern signature of FCs, which are primarily regularly distributed. Also, we found no evidence that sand termites would kill the freshly germinating green grasses within FCs via foraging on the roots. This result is based on the systematic excavation of about 500 grasses in four regions of the Namib, showing that the grasses died immediately after rainfall without signs of root damage. While this grass death happened immediately within the first 10 days or few weeks after grass-triggering rainfall, termite-induced root damage became only noticeable at a later stage as detritus feeding, when grasses have been already dead for a long time. The dying grasses within FCs had roots that were as long or even longer as those of the vital matrix grasses outside of the FCs. This indicates that drought stress caused grasses to invest resources into roots to reach the percolating water. Finally, the drought-stress hypothesis was strongly supported. Our continuous soil-moisture measurements within and around FCs show that the vital matrix grasses outside of the circles strongly pulled the soil water from the interior of the FCs. The quick death of the germinating grasses inside the FCs is thus mainly due to plant water stress and associated biomass-water feedbacks. This is further supported by our repeated mapping of more than 2400 FCs at NamibRand Nature Reserve, where during the very wet rainfall year 2021 about 45% of all FCs closed with grass but only 15 new FCs appeared. Thus, water was the main driver of vegetation pattern formation in this arid system. In times of climate change and increasing droughts, our study highlights that plant self-organization and the ability of plants to redistribute scarce resources is key to understand the fairy circle phenomenon.
Publications
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Definition of “fairy circles” and how they differ from other common vegetation gaps and plant rings. Journal of Vegetation Science, 32(6).
Getzin, Stephan; Yizhaq, Hezi & Tschinkel, Walter R.
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Revisiting Theron’s hypothesis on the origin of fairy circles after four decades: Euphorbias are not the cause. BMC Ecology and Evolution, 21(1).
Getzin, Stephan; Nambwandja, Ailly; Holch, Sönke & Wiegand, Kerstin
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Timing in fieldwork on Namib fairy circles matters: grass dieback in circles happens immediately after rainfall, presented talk at the 50th Annual Meeting of the Ecological Society of Germany, Austria and Switzerland, 31.08.2021.
Getzin, S., Holch & S. & Wiegand, K.
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20 years of research on fairy circles in north-west Namibia: lessons from pattern analysis, grass excavations, and ecohydrological feedbacks, presented talk at the international virtual workshop Etosha-Kunene Conservation Conversations, 06.07.2022.
Getzin, S.
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Plant water stress, not termite herbivory, causes Namibia’s fairy circles. Perspectives in Plant Ecology, Evolution and Systematics, 57, 125698.
Getzin, Stephan; Holch, Sönke; Yizhaq, Hezi & Wiegand, Kerstin
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Plant water stress, not termite herbivory, causes Namibia’s fairy circles. presented talk at the Namibia Scientific Society, Windhoek, 09.02.2023.
Getzin, S.
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Solving the fairy circle mystery using scientific evidence
Conservation Namibia
