For centuries people have struggled to explain the existence of so-called "fairy circles," saucers of naked soil that punctuate the Namib Desert in southern Africa.
The Himba bushmen who live in the region often describe the rings as creations of their mythical ancestor, Mukuru, or as footprints of the gods; another story attributes them to an underground dragon whose noxious breath kills vegetation on the desert's surface. Still others accredit the circles to aliens.
Now, the dispute has spread to the scientific community, where researchers recently published dueling editorials in the journal Ecography.
One side says evidence points to the mysterious discs arise from plant competition, as the configuration allows for maximum usage of limited water supplies in the desert. The other attributes the rings to grass-harvesting termites.
The fairy circles are coins of deep red earth, ranging from roughly 10 to 65 feet in diameter, and often circumscribed by a fringe of tall grass. They look like perfect circles, spaced evenly across the landscape. They appear and disappear over time, with life span estimates ranging from 40 to hundreds of years. If you were to take an aerial time-lapse over a few centuries, you'd see the barren craters open and close like millions of gaping mouths.
Plants. Termites. Dragons. Which is it?
The scientific debate that recently came to a boil began in 2013, when Norbert Juergens, a biologist at the University of Hamburg in Germany, authored a paper in Science that implicated termites.
Juergens observed that a specific species of sand termite, Psammotermes allocerus, appeared at nearly every fairy circle he encountered over more than 40 field expeditions.
Headlines screamed, "fairy circles mystery solved: it's termites!"
He also found that, without grasses to suck up water from the soil, fairy circles retained water down to three feet all year round—even during prolonged droughts. Juergens hypothesized that termites intentionally create fairy circles to maintain islands of moisture that they can inhabit. Headlines screamed, "fairy circles mystery solved: it's termites!"
A year later, a team of scientists proposed another explanation in Ecography, claiming that the regular distribution of fairy circles arises from an equilibrium between plants competing for scarce water.
The researchers, led by Stephan Getzin, an ecologist at the Helmholtz Centre for Environmental Research in Leipzig, Germany, created a mathematical model based on localized feedback. As grasses deplete water from the surrounding soil, their roots grow longer and drink in more water.
Headlines announced, "fairy circles not caused by termites after all!"
Getzin's model successfully replicated fairy circle patterns. Retracting claims from the year before, headlines announced, "fairy circles not caused by termites after all!"
Then this past December, Juergens published an editorial disputing several claims made by Getzin's group. Among other arguments, Juergens stated that his own observations of grass root lengths were much shorter than the foot-long roots characterized by Getzin's team. He also questioned why plants don't organize themselves in circular patterns according to the Getzin's mechanism in other ecosystems.
Last week, Getzin published a retort with Thorsten Wiegand, his colleague at the Helmholtz Centre for Environmental Research and a co-author on the Ecography paper from last year. Getzin and Wiegand explained that, even with short roots, grasses would draw in water through hydraulic conduction, which causes water to flow from spots of high concentration to low concentration. They also gave Juergens a dose of his own medicine, asking why sand termites don't also create fairy circles in neighboring ecosystems.
Other researchers joined in on the conversation. In a comment on Getzin and Wiegand's editorial, Florida State's Walter Tschinkel pointed out that scientists don't know enough about the biology of sand termites. "In this context, it seems almost random to invoke termite territoriality to explain the dispersed patterns of fairy circles," he wrote.
"The termite thing is a prime example of confusing correlation and causation," said Tschinkel, who researches social insects and has studied the fairy circles. "Just because termites live in fairy circles doesn't mean they are constructing them."
Meanwhile Yvette Naudé, a chemist at the University of Pretoria in South Africa, said she is "not convinced by the self-organization of plants hypothesis." Through her own field research Naudé favors a third popular hypothesis: that fairy rings are created by the seepage of natural gases from the earth, which can deplete oxygen around plant roots.
Though Tschinkel prefers Getzin's self-organization theory, he explained that the mystery is still very much alive. "Self-organization is a hypothesis based on mathematical models and aerial images of the landscape," he said. "We still need experimental proof."
He's actually leaving for Namibia today with some other prominent fairy circle researchers to test his hypothesis that plant self-organization primarily catalyzes fairy circle formation, and then termites maybe secondarily reinforce them.
He also acknowledges that scientists may come up with another viable model altogether. "But, we always work with what we have," he said. "Unless you believe in aliens of course."