Inside Africa

Namibia's fairy circles: Has one of nature's great mysteries been solved?

Thomas Page, CNNPublished 31st January 2017
(CNN) — For years fairy circles have been one of nature's great, enduring mysteries. A defining feature of the majestic Namib Desert, Namibia these dusty patches of earth, ringed with tall grass and dispersed evenly across 1,100 miles have eluded explanation and confounded scientists.
Like the Bermuda Triangle, or until recently the sailing stones of Death Valley, speculation has been rife. Explanations for the circles, which can be from 10 to 65 feet across, have ranged from alien invasion to poisonous gasses. A paper from 2001 says people have claimed they're the impact sites of meteorites, rolling spots for zebra or even localized radioactivity.
However a group of biologists and mathematicians in a Princeton-led study claim to have solved the curious case of the fairy circle -- and in the process, hope to unify the scientific community on the subject.
Before this month's study, published in the journal Nature on January 19, scientists were bitterly divided.
One camp, backed up by research from the 1990s and 2000s by Eugene Moll, Carl Albrecht and Norbert Juergens, argued for what's best summarized as "the termite theory": that the bare patches of earth were the result of hungry underground termites eating vegetation in the area surrounding their colony. This theory was originally derived from the proposal of ecologist Ken Tinley in 1971, who suggested fairy circles were fossilized termite mounds.
On the other side were proponents of self-organization: that vegetation naturally formed circles under the right conditions in order to make the most out of available moisture and soil nutrients. This theory was first applied to fairy circles in 2004 and expanded upon in 2008, explains Corina Tarnita from the Princeton team. In 2014 Stephan Getzin advocated strongly for self-organization, debunking the termite theory when speaking to CNN.
"The termite theory is very appealing to people, because it's relatively easy to understand," Walter Tschinkel, a biology professor at Florida State University, quipped at the time.
The optimal conditions for fairy circles are dry sandy desert grasslands in southwestern Africa that receive 50-100mm of rainfall in an average year.
The optimal conditions for fairy circles are dry sandy desert grasslands in southwestern Africa that receive 50-100mm of rainfall in an average year.
Courtesy NamibRand Nature Reserve
Now it's time for some truth and reconciliation -- because it turns out both sides may be correct.
Tarnita, Juan Bonachela and Robert Pringle bridged experimental fieldwork and mathematical theory, creating computer models while a team in Namibia watched them play out on the ground. What they witnessed has provided a unifying theory.
"Both ideas are an integral part of what we propose may be the mechanism behind fairy circles," argue Bonachela.
"Sand termites kill on-mound vegetation thus creating bare patches, which are regularly organized as a result of competition between colonies for space and resources; by doing so, termites facilitate the accumulation of water underneath the patch, which is essential for their survival in such an arid environment.
"Plants surrounding the bare patch use a plastic mechanism for root growth to take advantage of that increased moisture, thus creating the taller (perennial) vegetation ring that we call the fairy circle."
The team's computer models were able to map the fairy circles' hexagonal layout across the desert, representing each colonies territory and surrounding 'no-man's land'. The models also visualize a fairy circle's life cycle, which can span from "a few years to more than 200," says Bonachela. In the Namib Desert it takes approximately 20 years for a fairy circle to close up after the death of a termite colony, he adds.
Their theory, the team argues, can have wide-ranging implications.
"The fairy circles are just a single, beautiful case study for what we think is an extremely common and widespread set of processes," argues Pringle, citing hexagonal distributions of ant and termite colonies in Australia, Africa, Asia, Europe and South America.
"The way we see it, the value of our work lies not necessarily in providing a convincing explanation for fairy circles, although we're happy if it does that, but rather in bring together these two powerful ideas about ecological self-organization."
Ehud Meron, professor at the Blaustein Institutes for Desert Research & Physics Department, Ben-Gurion University, is critical of the study:
"The paper by Tarnita et al. makes a significant theoretical progress in substantiating the termite hypothesis, but does not resolve many difficulties this hypothesis still faces."
"The weakness of the termite hypothesis so far was that it did not account for the large-scale hexagonal fairy circle order," he says. "The model study presented in this paper fills up this gap."
But the termite theory remains weak, perhaps superfluous, in the eyes of Meron, a proponent of the self-organization theory, citing a lack of termites in certain locations and uneven dispersion of termites within fairy circles, suggesting vegetation should prosper in their center. "The alternative theory, on the other hand, is supported by an increasing number of empirical studies," he adds.
Perhaps the fairy circle community isn't as united after all.
"The controversy about the cause of fairy circle formation -- vegetation-self organization or termites -- has remained as strong as it was," Meron concludes.