Enlarge / Drone image of a car driving through the NamibRand Nature Reserve, one of the fairy tale regions in Namibia.
Stephen Getzin
So-called “fairy circles” are bald, reddish-colored circular patches found primarily in the Namibian grasslands and north-western Australia. Scientists have long debated whether these unusual patterns are due to termites or an ecological version of a self-organizing Turing mechanism. A few years ago, Stephan Getzin of the University of Göttingen found strong evidence for the latter hypothesis in Australia. And now his team has found similar evidence in Namibia, according to a new article published in the journal Perspectives in Plant Ecology, Evolution and Systematics.
“We can now finally dismiss the termite hypothesis because the termites are not a requirement for new fairy circles to form,” Getzin told Ars. This applies to both Australian and Namibian fairy circles.
As we have previously reported, Himba Bushmen in the Namibian grasslands have passed down legends about the region’s mysterious fairy covens. They can be several meters in diameter. Dubbed the “footprints of the gods,” they are often said to be the work of the Himba deity Mukuru, or a subterranean dragon whose poisonous breath kills anything that grows within these circles.
Scientists have their own ideas, and over the years two different hypotheses have emerged about how the circles form. One theory attributed the phenomenon to a specific species of termite (Psammmotermes allocerus) whose burrowing damages plant roots, causing extra rainwater to seep into the sandy soil before the plants can soak it up – providing the termites with a handy watertrap resource . As a result, the plants die in a circle from the place of an insect nest. The circles expand in diameter during drought because the termites have to venture further out to find food.
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The other hypothesis – the one put forward by Getzin – suggests that the circles are a type of self-organized spatial growth pattern (a Turing pattern) that arises when plants compete for scarce water and soil nutrients. In his seminal 1952 work, Alan Turing attempted to understand how natural non-random patterns arise (like a zebra’s stripes) and he focused on chemicals known as morphogens. He devised a mechanism involving the interaction between an activator chemical and an inhibitor chemical propagating through a system, much like gas atoms in a closed box.
It’s like squirting a drop of black ink into a cup of water. Normally this would stabilize a system: the water would gradually turn a uniform grey. However, if the inhibitor diffuses faster than the activator, the process will be destabilized. This mechanism creates a Turing pattern: spots, stripes, or when applied to an ecosystem, clusters of ant nests or fairy circles.
A researcher studies the dieback of grasses within fairy circles on a property near Kamberg in the Namib. The picture was taken about a week after rainfall in March 2020.
In 2019, Getzin’s team conducted a study of fairy circles in northwestern Australia near an old mining town called Newman. The team dug more than 150 holes in nearly 50 fairy circles in the region to collect and analyze soil samples, specifically to test the termite hypothesis. They also used drones to map larger areas of the continent to compare the gaps in vegetation typically caused by harvester termites in the region with the fairy circles that sometimes form.
The gaps in vegetation created by harvester termites were only about half the size of the fairy circles and much less ordered, so they found no hard underground termites that would prevent grasses from growing. But they found high soil compaction and clay content in the circles, evidence of the contribution of heavy rainfall, extreme heat and evaporation to their formation. “Termite constructions can occur in the area of fairy circles, but the partial local correlation between termites and fairy circles has no causal relationship,” Getzin said at the time. “Therefore, no destructive mechanisms, such as those of termites, are necessary for the formation of the distinctive fairy circle pattern, but solely hydrological plant-soil interactions.”
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Having effectively disproved the Australian termite origin hypothesis, Getzin turned his attention to testing the termite hypothesis specifically for Namibia, using a similar methodology. While his previous work on Namibian fairy circles did not specifically address the study of plant roots, this new study shows that plant roots are not touched by insectivores.
Investigating a fairy circle in Brandberg, Namibia 35 days after rainfall in March 2021.
“For the first time, we went to the fairy circles right after the rain and checked the new grasses for termite damage,” Getzin told Ars. “Our excavations show that termites certainly didn’t cause the grasses to die off. If you’re late to the fairy circles, the grasses will be long dead and detritivores such as termites may have already fed on the woody grass. But they didn’t kill the grasses.” Grass. We clearly show that the grasses die off before and completely independently of termite action.”
What’s next for Getzin? He believes more research is needed into the swarm intelligence of plants, and likens plants to beavers in the sense that they can act as “ecosystem engineers” modifying their environment. “Most people can’t believe that or don’t want to believe that because plants don’t have a brain,” Getzin said. “But plants behave like the beaver as ecosystem engineers because their only way of surviving is by forming optimal, strictly geometric patterns” – in other words, Turing patterns.
DOI: Perspectives in Plant Ecology, Evolution and Systematics, 2022. 10.1016/j.ppees.2022.125698 (About DOIs).
