Photo credit: Dr. Stephan Getzin/ Helmholtz Centre for Environmental Research
An African Mystery
by Justine Alford
Mysterious bare patches of ground that speckle a vast landscape in Namibia have puzzled scientists for years. These vegetation-free disks of varying sizes, coined “fairy circles” are found in the millions across an extensive area of arid grassland stretching across southern Angola, western Namibia and northern South Africa. But what causes them?
Researchers have sought an answer this question for around 30 years and although a few different theories have been thrown out in the past, there has been some speculation over whether they are convincing enough to satisfactorily solve the mystery. Explaining them has proved an arduous task because no one has been able to directly observe their formation.
One popular hypothesis proposed to explain the origin of fairy circles suggested that insects such as termites or ants were responsible by chewing away at the roots of the grass. In support of this, field studies demonstrated a correlation between the circles and populations of ants or termites; however, no one had ever actually observed these critters nibbling holes into the grasslands. Another theory was that leakage of gases such as methane and butane from hydrocarbon deposits displaced the soil atmosphere near the surface, causing a depletion of oxygen around the roots of the plants and ultimately death.
Now, a recent investigation that set out to shed light on these curious circles has thrown a new theory into the mix. Lead author of the study Dr Stephan Getzin from the Helmholtz Centre for Environmental Research and colleagues have been studying the phenomena for around 15 years. Their novel hypothesis proposed that the fairy circles are actually a result of competition for local resources between plants. They suggest that some plants might be more efficient at taking up water from the soil than others, resulting in patches of bare land encircled with lush vegetation. This could make sense since these circles form in areas with limited water supplies.
The researchers adopted a novel methodology to study the fairy circles. They gathered aerial images of representative land from northwest Namibia and analyzed the spatial location and distribution of the fairy circles. From this information, the researchers hoped to be able to discern whether these circles appeared random and distributed purely by chance, or whether patterns were evident. Perhaps, for example, there existed a certain minimum distance between the circles.
The results from statistical tests carried out by the team revealed that the fairy circles were not distributed randomly. In fact, it turns out that they are very regular and homogenous. “The occurrence of such patterning in nature is rather unusual,” said Getzin in a news-release. “There must be particularly strong regulating forces at work.” According to the team, such a distribution could plausibly be explained by competition for resources between the vegetation.
If this new local resource-competition theory is correct, it stands to discredit the termite theory which was actually supported by a study published in Science back in 2013. “There is, up to now, not one single piece of evidence demonstrating that social insects are capable of creating homogenously distributed structures, on such a large scale,” added Getzin. The team also believe that such a regular distribution also refutes the gas leakage theory.
To take the study further, the researchers simulated water competition beneath the ground with computer models to see what effect this would have on the distribution of vegetation. In support of their new theory, the pattern that emerged was strikingly similar to the actual fairy circle distribution in the field.
According to the researchers, the results from this computer modeling married with the distribution data represents a convincing argument that fairy circles are the result of spatially self-organizing grass growth caused by resource competition.