A fierce competition
To test this theory, Cramer and his colleague Nichole Barger from the University of Colorado at Boulder first measured the size, density and landscape occupancy of fairy circle sites across Namibia, using both Google Earth and ground surveys. They then collected soil samples at various depths from inside and outside the circles, and analyzed them for water and nutrient content.
Finally, they plugged the information, along with climate data such as seasonal precipitation and temperatures, into their computer models. (Images: The 10 Strangest Sights on Google Earth)
"We found that the size of the circle, the density and 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 rainfall.
This makes sense, Cramer explained, because the 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, the grasses require a large reservoir to sustain themselves if the soil is poor in water and nutrients.
The researchers also discovered that rainfall strongly determines the distribution of the fairy circles across Namibia, with circles only appearing in areas where there is just the right amount of rain (not too little, but not too much).
If there's too much rain, the bountiful resources would "relax" the competition for resources and the circles would close up; but if there's too little rain, the competition would become too severe and the circles would again disappear, Cramer said. Because the circles can only occur in this narrow moisture range, differences in rainfall from year to year may cause them to suddenly disappear and reappear in an area over time. With this information, they found that they could predict the distribution of the fairy circles with 95 percent accuracy.
Additionally, the regular spacing between fairy circles may be the result of inter-circle competition, with grasses from each circle "battling" with other circle grasses for resources, Cramer said.
Cramer notes that termites may still be involved in fairy circles. "What sets up the circles is the competition between plants," he said. "Termites are a secondary phenomenon, and their role is to serve as a maintenance for the circles by killing off the grasses that spring up 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, though she expressed some doubts about its validity.
"It is unclear how peripheral grass resource-competition could induce such abrupt and synchronized plant mortality over an entire patch," Naudé, who has previously studied fairy circles, told LiveScience in an email. (Cramer actually thinks the plant mortality starts off small, and the patch grows as the competition continues.) "The answer to the enigma (of fairy circles) remains elsewhere."
To examine whether the theory is correct, Cramer plans to conduct experimental tests, as his study only provides correlative evidence for the competition theory.
"If fairy circles really do develop from a shortage of water and nutrients, then simply watering and fertilizing the circles should cause them to close up with vegetation," Tschinkel said.
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