Beyond the small mining town of Newman in Western Australia lie the first fairy circles scientists have described outside of Africa.
These patches of bare soil dot outback grasslands in almost regular polka-dot patterns, just like the puzzling circle landscapes known from Namibia, says ecologist Stephan Getzin of the Helmholtz Center for Environmental Research-UFZ in Leipzig, Germany. He and his colleagues publish the first scientific description of Australia’s fairy circles online March 14 in the Proceedings of National Academy of Sciences. The team proposes that the oddities arise from life-and-death struggles between plants.
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Explaining what causes fairy circles has become ecologists’ version of crop-circle mysteries (SN Online: 8/20/15). Until now, the debate has focused on scatterings of circles from an arid zone in Africa. Getzin heard about the Australian circles in 2014 when a burst of news stories about his latest fairy circle paper inspired Australian environmental scientist Bronwyn Bell of Perth to e-mail pictures of what she saw around Newman. “I was extremely surprised,” Getzin says.
He and colleagues examined the new site for themselves, finding hard-baked reddish soil in the gaps instead of the more permeable sand in Namibia’s circles. But otherwise, Australia’s arrays of bare spots about 4 meters wide had a familiar fairy circle look with each spot circled by roughly six more.
Using a computer simulation, Getzin’s team shows that the polka-dot landscape in Australia can arise from interacting feedback loops where there’s not quite enough rainfall for continuous vegetation. In a short-term positive feedback loop, plants on the edge of a bare spot get a bigger share of the bare spot’s rain, growing bigger themselves and thus capturing even more water. But there’s also long-term negative feedback for plant cover: As plants around the rim of the bare spot suck up more and more water, less water reaches plants farther away. Eventually, those more removed spots dry out so much that new barren spots appear. (Getzin sees it as an example of what’s called a Turing instability, named for the British computing pioneer Alan Turing.)
This scenario doesn’t satisfy vegetation ecologist Norbert Jürgens of the University of Hamburg. Among his objections are that soils in African circles retain some rainfall that plant roots don’t draw up. Fairy circles would be filled with plants, he says, if something weren’t killing them. That something, he contends, is sand termites that nip the roots of plants (SN Online: 3/28/13).
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Termites might matter in Australia, too, Jürgens speculates. Unlike in Africa, water doesn’t sink into the bare spots. Instead, a hard layer of clay sends rainfall flowing aboveground to thirsty plants at the circle’s rim. “Termites or other social insects might have caused the Newman circles by transporting clay and silt to their nest sites, again and again, over long periods of time,” he says.
Proponents of yet another possible cause for fairy circles — natural geochemical conditions such as plant-killing carbon monoxide seeping out of the earth — say they already knew about Newman. Analytical chemist Yvette Naudé of the University of Pretoria in South Africa says she and her colleagues got a tip about five years ago from a fairy circle enthusiast in Switzerland and realized that the site is visible via Google Earth. Without studying the Australian fairy circles in person, she declines to speculate about what’s causing them.
For any explanation of fairy circles, correlation is not causation, says Walter Tschinkel of Florida State University in Tallahassee. Final proof will require an experiment that uses the proposed cause to create fairy circles. How to do that for features that stretch over whole landscapes, he acknowledges, “remains to be seen.”