People have tried to explain Namibia’s fairy circles for centuries. Locals call these patches of bare ground surrounded by rings of tall grass “god’s footprints” or the mark of a fire-breathing, underground dragon. Over the past 16 years, scientists have also gotten in on the game, attributing the rings to intense competition between termite colonies or even water-thirsty plants. Now, mathematical ecologists say that both termites and plants might be behind the rings—thanks to shortages of food and moisture.
The edge of the Namib Desert, where the rings form, is a dry, sandy place that sees less than 100 millimeters of rainfall each year. That’s likely enough moisture to support the growth of some grass, but not enough to make possible a continuous grassland. The rings are also found in the outback of western Australia. But scientists still don’t know the exact conditions that lead to the circles.
Corina Tarnita didn’t start out to resolve this debate. A theoretical biologist at Princeton University, she wanted to understand how nature creates patterns on different spatial scales—from tigers’ stripes to landscape-level phenomena such as fairy circles. To do so, she creates mathematical models that predict what patterns will look like given the different factors involved. She and her husband, Princeton ecologist Rob Pringle, thought that fairy circles might be a good way to test the model.
With fellow theoretical ecologist Juan A. Bonachela from the University of Strathclyde in Glasgow, U.K., they first predicted how termites might transform the landscape, given available vegetation. In the simulation, they found that over decades colonies would appear and disappear (much like observations of actual fairy circles) and that they would eventually self-organize into regularly spaced honeycomb patterns, with each colony surrounded by six others. To find out how plants might contribute, the researchers turned to already published work, which suggests that plants also self-organize based on available moisture. “There is not enough water to sustain a complete coverage with continuous grass vegetation, which leads to the gap pattern,” explains Stephan Getzin, an ecologist at the Helmholtz Centre for Environmental Research in Leipzig, Germany, who was not involved with the new work.
Combining these two models, Bonachela and Tarnita came to a mixed solution: Termites may create the large-scale pattern, whereas plants may establish a small-scale pattern, they report today in Nature. With their mound building, the termites reshape the water flow that leads to barren patches surrounded by a ring of tall grasses. But the model also predicted there would be regularly spaced clumps of grass between the fairy circles that should create a pattern within a pattern. When the team photographed fairy circles in Namibia, it found these clumps of grass, suggesting the model was on target. Others have overlooked these clumps, Tarnita says.
The conclusion makes sense to some. “To me the paper shows that multiscale patterns in Namibia can be explained by a coupled termite-vegetation model,” says M. G. “Max” Rietkerk, an ecologist at Utrecht University in the Netherlands. He says this new study could help explain patterns found in other systems and fields.
But for others, the answer is already clear. “There is absolutely no doubt that fairy circles are a product of ecosystem engineering achieved by soil-living termites,” says Norbert Jürgens, an ecologist at the University of Hamburg in Germany, who first proposed in 2013 termites were responsible and thinks this paper fully supports that idea. On the other side, physicist Cristián Fernández-Oto, soon to be at Ben Gurion University of the Negev in Beer Sheva, Israel, says the paper does not change his thinking that plants are the “main ingredient.”
So the jury is still out: Scientists simply don’t have enough observations of real termites to match them to the model. “We need experiments to establish causes,” says Walter Tschinkel, an emeritus entomologist at Florida State University in Tallahassee who has studied fairy circles, but not with this group. “[We also need] a whole lot more knowledge of the biology of the termites and … of the grasses involved.” Tarnita’s group agrees that experiments altering termite mounds, soil moisture, and local plants there still need to be done. Who will do this work is up in the air.
Until then, says Rietkerk, “the ecological riddle still remains.”