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,
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, 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,
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, 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,
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, 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,
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, water doesn’t sink into the bare spots. Instead,
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, 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,
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, again and again, over long periods of time,
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,” he says.
Proponents of yet another possible

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