For decades, people have been mystified by thousands of bare, circular soil patches that dot arid western African landscapes with inexplicable geometric precision.

They're known as fairy circles – and the fairies, says ecologist Norbert Juergens of Germany's University of Hamburg, are termites engaged in an extraordinary, landscape-scale act of ecological engineering that sustains not only themselves but a rich web of grassland life.

Other proposed explanations have involved toxins deposited by poisonous plants, ant species raising colonies of plant-eating insects, and 'self-organizing' vegetation dynamics that elsewhere produce mosaic vegetation patterns. Yet while each hypothesis is plausible, convincing evidence hasn't yet been found.

"The origin and the ecosystem function of fairy circles are still a much-debated mystery," wrote Juergens in the March 28 Science paper describing his own termite hypothesis.

After counting insect species at fairy circles in Angola, Namibia and South Africa, Juergens found that Psammotermes allocerus, a desert-dwelling termite species, lived most frequently near them, and could often be found burrowing beneath the circles.

Levels of sand termite activity around and within fairy circles. Image: Norbert Juergens/Science

He then measured soil water content and the composition of vegetation around the fairy circles. The soil was surprisingly water-rich, even after months without rain.

Juergens speculates that the termites, which feed on plant matter, clear fairy circles by feeding on the roots of grass seedlings.

Water then collects beneath the barren soil, in turn nourishing concentric circles of perennial and short-lived grasses that often rim fairy circles.

Termites feed on these, too, Juergens thinks, and many other animals feed on the termites.

"Fairy circles, like oases in the desert, increase biodiversity," he wrote. Just as beaver ponds nourish nature in temperate regions, P. allocerus "turn wide desert regions of predominantly ephemeral life into landscapes dominated by species-rich perennial grassland, supporting uninterrupted perennial life even during dry seasons and drought years."

Other researchers, however, remain unconvinced. Biologist Walter Tschinkel of Florida State University, who has studied the long-term dynamics of fairy circles, said Juergens confuses correlation with causation. Just because termites are there doesn't mean they're responsible.

Insect taxonomist Vivienne Uys of South Africa's Agricultural Research Council said termites are biologically capable of producing the circles, but noted that Tschinkel and others have not observed the high levels of termite activity that Juergens did.

Mike Cramer, a plant ecologist at the University of Cape Town, said termites are rare at the fairy circles he's studied. He still thinks some self-organizing forces are at work.

"The only way for this question to be properly answered," said Cramer, "is with more thorough investigations and focused experiments." For now, the fairy circles remain a much-debated mystery.

Citation: "The Biological Underpinnings of Namib Desert Fairy Circles." Science, Vol. 339, 29 March 2013.