Mysterious ‘fairy circles’ in the African desert get a new explanation
The eerie circular patches of bare land called “fairy rings” in the grasslands of Africa’s Namib Desert have defied explanation, with theories ranging from ants to termites to grass-killing gas oozing from the ground. But the patches may be the natural result of below-ground competition for resources between plants, new research suggests.
According to the new theory, the grasslands of the Namib Desert are initially homogeneous, but the sparse rainfall and nutrient-poor soil trigger intense competition between the grasses. Strong grasses sap all water and nutrients from the soil, causing their weaker neighbors to die and a barren void to form in the landscape.
The vegetation gap widens as competition ensues, and the grassless area becomes a reservoir of nutrients and water. With the additional resources, larger grass species can then take root on the periphery of the gap, and a stable fairy circle develops. [See Photos of Mysterious Fairy Circles of the Namib Desert]
“It’s a very good theory because it takes into account all the characteristics of fairy circles,” including the presence of tall grass species, Florida State University biologist Walter Tschinkel, who tells LiveScience. did not participate in the study. “No other proposed cause for fairy circles has ever done this.”
A lingering mystery
Fairy circles have been a mystery to scientists for decades. Last year, Tschinkel discovered that small fairy circles last an average of 24 years, while large circles can last up to 75 years. However, his research has not determined 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 fairy circle termite theory. Basically, he discovered colonies of sand termites, Psammoterms allocerate, were almost always in the center of the fairy circles, where he also saw an increase in soil moisture. He reasoned that the termites fed on the roots of grasses, killing the plants, which typically consume water from the soil, and then sucked the water into the resulting circular patches to survive through the dry season.
But Tschinkel criticizes the work, pointing out that Juergens confused correlation with causation.
Michael Cramer, a biologist at the University of Cape Town in South Africa and principal investigator of the current study, which was recently published in the journal PLOS ONE, also thinks the termite theory is insufficient.
“I think the main hurdle explanations have to overcome is explaining the even spacing of the circles, their approximate circularity, and their size,” Cramer told LiveScience. “There’s really no reason why termites should produce such large, evenly spaced circles.”
Scientists have also previously proposed that fairy rings are an example of a “self-organizing pattern of vegetation”, which arises from plant interactions. In 2008, researchers developed a mathematical model showing that the vegetation pattern of fairy rings could depend on the availability of water.
To test this theory, Cramer and his colleague Nichole Barger of the University of Colorado at Boulder first measured the size, density and landscape occupancy of fairy ring sites across Namibia, using both Google Earth and ground surveys. They then took soil samples from different depths inside and outside the circles and analyzed them for water and nutrient content. Finally, they incorporated the information, along with climate data such as rainfall and seasonal temperatures, into their computer models. [Images: The 10 Strangest Sights on Google Earth]
“We found that circle size, density, and the degree to which they occupy the landscape are all associated with the amount of resources available,” Cramer said. Specifically, fairy circles are smaller if they have more resources, such as soil nitrogen and precipitation.
This makes sense, Cramer explained, because taller grasses won’t need a large reservoir of resources to get started and survive if water and nutrients are already available in the environment. On the other hand, grasses need a large reservoir to sustain themselves if the soil is poor in water and nutrients.
The researchers also found that rainfall strongly determines the distribution of fairy circles across Namibia, with the circles only appearing in areas where there is just the right amount of rain (not too little, but not too much). If there was too much rain, the abundant resources would “loosen” the competition for resources and the circles would close; but if there is too little rain, the competition will become too severe and the circles will disappear again, Cramer said. Since circles can only occur in this narrow humidity range, differences in precipitation from year to year can cause them to disappear and suddenly reappear in an area over time. With this information, they found they could predict the distribution of fairy circles with 95% accuracy.
Additionally, the even spacing between fairy circles may be the result of competition between the circles, with grasses in each circle “battle” with other grasses in the circle for resources, Cramer said.
Cramer notes that termites may still be involved in fairy circles. “What sets the circles is competition between plants,” he said. “Termites are a secondary phenomenon, and their role is to maintain the circles by killing the grasses that grow in the center of the circles.”
Yvette Naudé, a chemist at the University of Pretoria, South Africa, who was not involved in the study, thinks it’s refreshing to see a non-insect hypothesis for fairy circles, although she has expressed some doubts as to its validity.
“It is unclear how competition from peripheral herbaceous resources could induce such abrupt and synchronized plant mortality across an entire plot,” Naudé, who has previously studied fairy circles, told LiveScience in an email. (Cramer actually thinks that plant mortality starts small and the plot grows larger as competition continues.) “The answer to the riddle [of fairy circles] stay elsewhere.”
To examine whether the theory is correct, Cramer plans to conduct experimental tests, as his study only provides correlative evidence for the concurrency theory.
“If fairy circles really grow from a shortage of water and nutrients, then simply watering and fertilizing the circles should cause them to close in with vegetation,” Tschinkel said.