So-called “fairy circles” are bare, reddish-hued circular patches notably found in the Namibian grasslands and northwestern Australia. Scientists have long debated whether these unusual patterns are due to termites or to 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 paper published in the journal Perspectives in Plant Ecology, Evolution and Systematics.
“We can now definitively dismiss the termite hypothesis, as the termites are not prerequisite to form new fairy circles,” Getzin told Ars. This holds both for Australian and Namibian fairy circles.
As we’ve reported previously, Himba bushmen in the Namibian grasslands have passed down legends about the region’s mysterious fairy circles. They can be as large as several feet in diameter. Dubbed “footprints of the gods,” it’s often said they are the work of the Himba deity Mukuru, or an underground dragon whose poisonous breath kills anything growing inside those circles.
Scientists have their own ideas, and over the years, two different hypotheses emerged about how the circles form. One theory attributed the phenomenon to a particular species of termite (Psammmotermes allocerus), whose burrowing damages plant roots, resulting in extra rainwater seeping into the sandy soil before the plants can suck it up—giving the termites a handy water trap as a resource. As a result, the plants die back in a circle from the site of an insect nest. The circles expand in diameter during droughts because the termites must venture farther out for food.
The other hypothesis—the one espoused by Getzin—holds that the circles are a kind of self-organized spatial growth pattern (a Turing pattern) that arise as plants compete for scarce water and soil nutrients. In his seminal 1952 paper, Alan Turing was attempting to understand how natural, non-random patterns emerge (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 that diffuse throughout a system, much like gas atoms will do in an enclosed box.
It’s akin to injecting a drop of black ink into a beaker of water. Normally this would stabilize a system: the water would gradually turn a uniform gray. But if the inhibitor diffuses at a faster rate than the activator, the process is destabilized. That mechanism will produce a Turing pattern: spots, stripes, or, when applied to an ecological system, clusters of ant nests or fairy circles.
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 almost 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 vegetation gaps caused by harvester termites were only about half the size of the fairy circles and much less ordered, so they didn’t find any hard subterranean termitaria that would prevent the growth of grasses. But they did find high soil compaction and clay content in the circles, evidence for the contribution of heavy rainfall, extreme heat, and evaporation to their formation. “Termite constructions can occur in the area of the fairy circles, but the partial local correlation between termites and fairy circles has no causal relationship,” Getzin said at the time. “So no destructive mechanisms, such as those from termites, are necessary for the formation of the distinct fairy circle patterns; hydrological plant-soil interactions alone are sufficient.”
Having effectively disproven the Australian termite origin hypothesis, Getzin turned his attention to specifically testing the termite hypothesis for Namibia, using a similar methodology. While his earlier work on Namibian fairy circles did not specifically address the investigations of plant roots, this new study shows that plant roots are not touched by insect herbivores.
“For the first time, we went right after rainfall to the fairy circles and checked the new grasses for termite herbivory,” Getzin told Ars. “Our excavations demonstrate that termites did certainly not cause the death of the grasses. If you come too late to the fairy circles, the grasses are long dead and detritivores like termites may have already fed on the lignified grass. But they did not kill the grass. We are showing unambiguously that the grasses die before and completely independent of any termite action.”
So what’s next for Getzin? He believes more research is needed on the swarm intelligence of plants, likening plants to beavers in the sense that they can act as “ecosystem engineers” that modify their environment. “Most people cannot believe this or are unwilling to believe that, because plants have no brains,” said Getzin. “But plants act similarly like the beaver as ecosystem engineers because their only way to survive is 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).