Stefan Getzen
The so-called “fairy circles” are bare, reddish circular patches found particularly in the grasslands of Namibia and northwestern Australia. Scientists have long debated whether these unusual patterns are caused by termites or to an ecological version of the self-regulating Turing mechanism. A few years ago, Stefan Getzen 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 paper published in the journal Perspectives in Plant Ecology, Evolution and Systematics.
“We can now reject the termite hypothesis permanently, because termites are not a prerequisite for the formation of new imaginary circles,” Getzen told Ars. This applies to both Australian and Namibian fictional circles.
As previously reported, the Himba Bushmen in the Namibian grasslands have littered legends about mysterious imaginary circuits in the region. It can reach several feet in diameter. Dubbed the “footprints of the gods,” they are often said to be the work of the Himba deity Mukuru, or an underground dragon whose poisonous breath kills anything growing within those circles.
Scientists have their own ideas, and over the years, two different hypotheses have emerged about how circles form. One theory attributed the phenomenon to the special species of termites (Custom Psammmotermes), whose digging damages plant roots, causing excess rainwater to seep into sandy soil before plants can absorb it—giving termites a handy water trap as a resource. As a result, plants die again in a circle from the site of the insect’s nest. The circles expand in diameter during dry spells because termites must venture away from food.
Another hypothesis—the one espoused by Getzen—is that circles are a type of self-regulating spatial growth pattern (the Turing pattern) that arises when plants compete for scarce water and soil nutrients. In his 1952 paper Seminal Alan Turing was trying to understand how non-random natural patterns (such as zebra stripes) appear, and focused on chemicals known as morphogens. He devised a mechanism that involves the interaction between an activating chemical and an inhibitory chemical that propagates throughout the system, much like gas atoms do in a closed box.
It is akin to injecting a drop of black ink into a beaker of water. Usually this leads to the stability of the system: the water gradually turns into a uniform gray color. But if the inhibitor spreads at a faster rate than the activator, the process is disrupted. This mechanism will produce a Turing pattern file: patches, stripes, or, when applied to an ecosystem, groups of ant nests or imaginary circles.
https://www.youtube.com/watch؟v=dC6WZvrYrNY
A researcher investigates the death of weeds within fairy circles on a plot of land near Cambridge, Namibian. The recording was made about a week after it rained in March 2020.
In 2019, Getzin’s team conducted a study of fictional circuits in northwest Australia, near an ancient mining town called Newman. The team dug more than 150 holes in nearly 50 fairy circles in the area 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 usually caused by termites in the area, with the imaginary circles that sometimes form.
The vegetation gaps caused by the harvester termites were about half the size of the imaginary circles and much less arranged, so they did not find any tough subterranean plants that inhibited the growth of weeds. But they found the high soil compaction and clay content in the circles, evidence of the contribution of heavy precipitation, intense heat, and evaporation to their formation. “Termite constructions can occur in the region of fairy circles, but the partial local association between termites and fairy circles has no causal relationship,” Getzen said at the time. “Therefore no destructive mechanisms, such as those found in termites, are necessary for the formation of characteristic fairy circle patterns; Hydrological interactions between plant and soil are sufficient alone.”
Having effectively disproved the Australian termite origin hypothesis, Getzen turned his attention to testing the termite hypothesis specifically in Namibia, using a similar methodology. While his previous work on Namibian fictional circuits has not specifically addressed investigations of plant roots, this new study shows that plant roots are not affected by herbivores.
https://www.youtube.com/watch؟v=ekkWX3rBayo
Investigating a fairy tale in Brandberg, Namibia after 35 days of rain in March 2021.
“For the first time, we went right after the rain to the fairy circles and checked the new grasses for herbivorous termites,” Getzen told Ars. “Our excavations show that termites certainly did not cause the weeds to die. If you are too late in the imaginary episodes, the weeds are long dead and perhaps destructive animals like termites have already fed on the woody grass. But they didn’t kill the grass. We show unequivocally that weeds die by and completely independent of any termite action.”
So what’s next for Getzin? He believes that more research is needed on the intelligence of swarm plants, likening plants to beavers in the sense that they can act as “ecosystem engineers” who modify their environment. “Most people can’t believe this or are unwilling to believe it, because plants don’t have brains,” Getzen said. “But plants behave similarly like beavers as ecosystem engineers because their only way to survive is to form strict geometric patterns” — in other words, Turing patterns.
DOI: Perspectives in Plant Ecology, Evolution, and Systematics, 2022. 10.1016/j.ppees.2022.125698 (about DOIs).