The bizarre circular patches of bare land called "fairy circles" in the grasslands of Africa's Namib Desert have defied explanation, with hypotheses ranging from ants to termites to grass-killing gas that seeps out of the soil. But the patches may be the natural result of the subsurface competition for resources among plants, new research suggests.
Grasslands in the Namib Desert start off homogenous, but sparse rainfall and nutrient-poor soil spark intense competition between the grasses, according to the new theory. Strong grasses sap all of the water and nutrients from the soil, causing their weaker neighbors to die and a barren gap to form in the landscape.
The vegetation gap expands as the competition ensues, and the grass-free zone becomes a reservoir for nutrients and water. With the additional resources, larger grass species are then able to take root at the periphery of the gap, and a stable fairy circle develops. (See Photos of Mysterious Fairy Circles of the Namib Desert)
"It's a really good theory because it accounts for all the characteristics of fairy circles," including the presence of tall grass species, Florida State University biologist Walter Tschinkel, who was not involved in the study, told LiveScience. "No other proposed cause for fairy circles has ever done that."
Fairy circles have been a mystery to scientists for decades. Last year, Tschinkel discovered that small fairy circles last for an average of 24 years, whereas larger circles can stick around for up to 75 years. However, his research didn't determine why the circles form in the first place, or why they disappear.
Earlier this year, University of Hamburg biologist Norbert Juergens claimed to have found evidence for a termite theory of fairy circles. Essentially, he discovered colonies of the sand termite, Psammotermes allocerus, were nearly always found in the centers of fairy circles, where he also found increased soil moisture. He reasoned that the termites feed on the grasses' roots, killing the plants, which usually use up the soil's water, and then slurp up the water in the resulting circular patches to survive during the dry season.
But Tschinkel is critical of the work, stressing that Juergens confused correlation with causation.
Michael Cramer, a biologist at the University of Cape Town in South Africa and lead researcher of the current study, which was published recently in the journal PLOS ONE, also thinks the termite theory falls short.
"I think the major hurdle that explanations have to overcome is explaining the regular spacing of the circles, their approximate circularity and their size," Cramer told LiveScience. "There's no real reason why termites would produce such large circles that are so evenly spaced."
Scientists have also previously proposed that fairy circles are an example of a "self-organizing vegetation pattern," which arises from plant interactions. In 2008, researchers developed a mathematical model showing the vegetation patterning of fairy circles could depend on water availability.