The barren, perfectly circular patches of dirt — often outlined by a thin ring of tall grass — seem to be somehow “alive”; they have been known to last years, and then simply vanish. The mysterious ‘fairy circles’, as they are popularly known, dot a 2000 kilometer stretch of desert that ranges from Angola to South Africa. The origin of these circles has defied explanation by scientists for decades.
But now, ecologist Norbert Juergens of the University of Hamburg, Germany, claims he has solved themystery of the fairy circle’s formation. His evidence, based upon soil analysis, though not definitive, is quite compelling.
Scientific interest in solving the puzzle peaked just last year, when a 4-year satellite image analysis of the circles in Namibia’s NamibRand Nature Reserve, by biologist Walter Tschinkel at Florida State University, Tallahassee, revealed that some circles grew and others vanished over that 4-year period. Based on the rate of growth, Tschinkel calculated an average “lifespan” of a circle to be 41 years. However, he was not able to detemine their cause. He had heard theories floating around that somehow underground termites were to blame, but he found no evidence for this nor any abnormality in the soil itself.
Observing Nature, Digging for Clues
Juergens first became intrigued with the circles back in 2006 and had similarly noticed that they seemed to appear and disappear from the desert lndscape. So, Juergens decided to do what ecologists and naturalists do best: observe and record. Visiting over a thousand fairy circles in the Angolan desert, he recorded every sign of animal life that he found and even dug trenches from the circles’ centers to their perimeters in order to expose any subterranean lifeforms.
Over the span of 40 field trips and recordings of activity in and around 1200 circles, Juergens noticed a pattern emerging — one species was almost always present at every fairy circle: the sand termite.
The “extremely clandestined” insects, which emerge only sporadically at night, do not construct elaborate underground tunnel networks or above ground nests. They seem to “swim” through the sand leaving behind only very fragile, thin tunnels that can be easily missed or destroyed if one is digging too deep or with too much force.
Juergens concentrated his investigation primarily on the soil layer a few centimaters to several tens of centimeters down, where he found sand termites, or their tiny tunnels, in nearly every circle he studied, he reported yeterday in the on-line journal Science.
The Mystery and the Theory
Juergens theorizes that the tunneling of the sand termite (Psammotermes allocerus) damages the roots of whatever plants were able to grow there originally. The insects also feed on the roots as they tunnel. Of the 24 newly-formed circles he investigated in Namibia, Juergens found sand termites in all of them.
As to why some fairly circles seem to vanish, or “die”, Juergens speculates that competition or predation by other insects (like ants) — or both — may be responsible.
The desert region receives about 100 mm of rain per year. The clearing of plants from these circles allows that small amount of moisture to seep into the porous ground where it remains for some time, due to a lack of evapotranspiration, and provides the minimal moisture the termites need to survive. Juergens thinks that the sand termites build and tend to these circles “on purpose” — keeping them vegetation free (as plants would quickly suck up all the water).
To verify this, he inserted humidity probes at varying depths in the circles’ centers to measure their moisture levels. Results seem to confirmed his theory.
“These bare patches are water traps. Over the years, I didn’t measure 1 hour with less than 5% water at 60 centimeters, which is certainly wet enough to support termite life.” explained Juergens
Juergens thinks that these water sinks are also responsible for the belts of tall grass that ring the edges of the fairy circles. It is a form of symbiosis: the tall grass does not have to compete for water with other plants (that may have grown, at one time, in what is the inside of the circles). During the brief rainy season, the sand termites move into the grass belts to feed, and, during drought, they also consume some of the grass for food. In so doing, they slowly expand the fairy circles’ diameter.
Questions, Doubt and Other Explanations
But not everyone is convinced by Juergens evidence. And some scientists think that more evidence is needed before sand termites can be declared, definitively, the “pixie” culprits.
“The link between foraging activity of the termite resulting in the formation of a perfect circle of bare soil is unclear,” stated Vivienne Uys, a termite taxonomist at the Agricultural Research Council in Pretoria.
And Tschinkel also questions his colleague’s conclusions, asserting the oft-used scientific maxim: correlation is not causation. “If Juergens claims termites are killing the grass, he’s got to show that they’re actually attacking living plants. That’s not easy to do, and he didn’t do it.”
Other scientists with competing thoeries also challenge their Juergens’ findings. One such theory asserts that termites have nothing to do with the formation of the circles, and that they are natural vegetation patterns resulting from competition over very finite resources (water and soil nutrients). These scientific dissenters are calling for more focused experiments.
A Marvel of Ecosytem Engineering
But Juergens is standing firm on his fairy circle findings. He marvels at the insects’ ability to make the circles (by whatever automatic program or emergent impetus) and observes, importantly, that these fairy circles serve as mini oases for a diverse collection of desert creatures. Juergens recorded a range of species — including jackals, springbok, moles, foxes, aardvarks and other insects — all spending time at the circles either foraging on the termites or the tall grass, or preying on the animals that came to forage there. He also notes that bodiveristy is 10 to 20 times greater at these fairy circles than in the surrounding desert landscape.
Despite successive years of drought, the insect-engineered circles pesist — often lasting for decades.
“These tiny termites have managed to turn rainfall as little as 50 millimeters per year into a continuous, permanently livable ecosystem,” Juergen says in admiration. “Identification of this termite as opposed to other candidates behind fairy circles is part of the story, but the more interesting story is that this insect evolved to be a masterpiece of ecosystem engineering.”
Source material, image and quotes for this blog came from the Science NOW article: ‘Fairy Circle’ Mystery Solved? by Rachel Nuwer
Top photo: Fairy circles freckle the landscape in NamibRand, Namibia. Are sand termites (inset) to blame? Image Credit: Norbert Juergens