Barren circles dot the dry grasslands across about 1,500 miles of the Namib Desert stretching down the southwestern coast of Africa, emerging, growing, shrinking, and disappearing in lifetimes of 30 to 60 years. The empty patches are accentuated by a rim of particularly tall grasses that ring the circles, which range from 6 feet to 115 feet wide.
The fairy circles, as the strange bare soil spots are called, have long puzzled scientists. Although they look a bit like imprints left by massive raindrops, impacting meteors, or as legend would have it, the feet of gods, researchers suspect the pattern may form as a result of a more systematic natural process. But just what that process might be has been the subject of much debate.
Explaining this enigmatic pattern could help researchers figure out if there is some sort of universal principle behind these and other strange natural vegetation patterns.
Two dominant explanations have emerged. Some researchers blame regularly spaced social insect colonies, like sand termites, munching their way through the grasses until they reach some sort of barrier, perhaps a competing colony. By eating the roots of grasses, the termites create a sandy patch that is well-engineered to better capture the little rain that falls in the deserted region, say proponents of the theory.
Others argue that, instead, it's the plants that are competing with each other for water and other resources that create the barren circles. Grasses with deeper roots drain water from the patches, making it impossible for other plants to survive there. As a result, the plants self-organize into the regularly spaced circles.
Scientists in both camps have largely focused on each model separately. But new research suggests that both mechanisms might be involved.
According to a model described Wednesday in the journal Nature, both termites and vegetation, as well as the ways they interact, are needed to explain the fairy circles.
The researchers simulated what sort of vegetation patterns would arise from each of the two models both separately and together. Then, they compared their simulated pattern with images of the real fairy circles that dot the Namib Desert and, they say, it was a match: a large-scale hexagonal pattern.
Not only that, but the team spotted a smaller-scale pattern in the grasses around the fairy circles. Regularly spaced clumps of grass also created their own regular pattern. And, the researchers write, "that cannot be explained by either mechanism in isolation. These multi-scale patterns and other emergent properties, such as enhanced resistance to and recovery from drought, instead arise from dynamic interactions in our theoretical framework, which couples both mechanisms."
Norbert Jürgens, a botanist who championed the termite colony model with a paper published in 2013 and who was a peer reviewer of the new Nature paper, is "very pleased to see this publication because it's fully supporting the conceptual model that I proposed."
Dr. Jürgens, a professor at the University of Hamburg in Germany, says in a phone interview with The Christian Science Monitor that he agrees "both processes play a role, and both models can jointly explain nature." But, he says, the problem with the scale-dependent feedback model, as the plant-focused model is called, is that it assumes the vegetation can control patterning on such a vast scale. Sure, one plant can have a significant impact on its neighbor's access to water and nutrients, but he isn't so sure this neighborly relationship could create such a well-defined pattern across hundreds of miles.
Stephan Getzin, a theoretical ecologist at the Helmholtz Centre for Environmental Research - UFZ in Leipzig, Germany, whose own research suggests the scale-dependent feedback model is the right one, isn't so convinced by this new paper.
"An integrative model would allow for accounting for the presence of both, sand termites and self-organization," he writes in an email to the Monitor. "However, the authors use a model where the gap pattern of fairy circles solely emerges due to insect activity and colony establishment."
"Termites, especially the sand termite, are feeding preferentially on dead grass which suffered from drought," Dr. Getzin explains. So water depletion is a necessary prerequisite that could be influencing the pattern of where these fairy circles are popping up, he says. "We need to focus our research on those fairy circles that develop without the presence of termites because this will give us a clearer picture of the underlying processes without blurring or superimposing effects from termites."
Although the Namibian fairy circles are the most well-known, the same kind of strange pattern was spotted in Australia in 2016. And this could be a curveball for the insect model as Getzin, who studied the Australian fairy circles, told the Atlantic at the time that there was no sign of the termites there.
Walter Tschinkel, an entomologist and professor emeritus at Florida State University who was not part of this new research, warns against being too wedded to any of these theoretical or conceptual models without more experimental or field evidence. "If people are going to draw conclusions about the causes of fairy circles, then those conclusions can be drawn only from experimental work," he says in a phone interview with the Monitor.
"The termites that they propose are part of this mechanism that they modeled, are not termites at all. They're mathematical models," Dr. Tschinkel says. As such, he suggests manipulating the fairy circles to see just how sand termites might behave in that setting.
Nichole Barger, an aridlands ecologist at the University of Colorado whose research focuses on the vegetation self-organization theory, agrees that "there are still remaining questions with the social insect model and whether the Psammotermes termites [sand termites] actually behaves in nature the way it behaves in the model."
"The assumption of the model is the presence of a termite nest in every fairy circle," she writes in an email to the Monitor. "But no empirical evidence was presented of nest presence for the fairy circle sites in Namibia. Until the presence of the termite nests are confirmed with field data, it is speculation that fairy circles are formed by these termites."
Jürgens, however, would beg to differ. In his 2013 paper, Jürgens wrote that he did indeed investigate the soils in hundreds of fairy circles. And sure enough, he reported, sand termites (Psammotermes allocerus) were present in most of those sites.
But, he admitted in the paper, "Although these associations suggest a causal role for P. allocerus, it is possible that they may instead merely reflect the colonization of FCs [fairy circles] by the termites. However, sand termites were found even in the initial state of new FCs, that is, before the water accumulation has begun and the perennial grass belt has developed. Careful assessment of 24 newly formed FCs at Giribesvlakte in Namibia in March 2012 revealed the presence of P. allocerus in all of them. In these youngest FCs, the dying grass plants were damaged only at the roots, associated with underground galleries of P. allocerus ... No other organism has been observed foraging on the grass of young FCs."
The researchers behind the new, combined model didn't set out to explain only the fairy circles. Instead, they used them as a case study for other strange, self-organized regular vegetation patterns across the world. The idea was that there might be some sort of universal conditions that lead to the fairy circles, tiger bush, North American Mima mounds, Brazilian murundus, and South African heuweltjies, among others.
Dr. Barger agrees. "Fairy circles and their striking patterns are an entry into understanding the beauty of pattern formation in nature," she says. "There are a large number examples of vegetation patterns such as stripes, spots, and labyrinths from arid regions around the world, but it’s the circles that really capture people’s attention."
[Editor's note: An earlier version mischaracterized the backgrounds of the researchers in the study published in Nature.]