Nature
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Alan Turing's work formed the basis for much of modern computing and helped (temporarily) save the world from fascism, but in recent years one of his previously neglected theories has come to the fore. Turing believed certain biological phenomena could be explained by a “reaction-diffusion” system where two forces work against each other, dominating at different scales, to create remarkably complex patterns. In the years since his work was vindicated experimentally, numerous examples of Turing's process have been identified, with the latest explaining mysterious grass circles in the Australian desert.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Bare circular patches in Namibia and Angola 4 to 10 meters (13 to 33 feet) in diameter have been called “fairy circles," after the similarly shaped mushroom rings of European forests. Each circle has six neighbors, so they form hexagonal grids across the desert landscape. Once attributed to termites, the mystery deepened when similar shapes were found in one area of the Western Australian desert, with notable differences from termite-produced gaps elsewhere.
Dr Stephan Getzin of the University of Göttingen has studied the Western Australian circles, which are surrounded by Triodia grasses, including their re-establishment after fires. Besides confirming that Turing’s model is the cause of their shapes, Getzen and co-authors report in the Journal of Ecology on the role these circles play in local ecosystems.
Turing mechanisms occur where an activator produces positive feedback at shorter ranges and an inhibitor takes over further from the source. Such diffusion produces leopard spots, with the activator spreading the enzyme that turns the fur dark, while the inhibitor leaves surrounding areas yellow.
The part of Australia’s Pilbara east of the town of Newman is so dry, water availability is life’s chief limiting factor. The clay soil of the circles repels what little rain falls, producing run-off. Where grasses establish themselves, they form a barrier that captures this run-off and also cools the soil during the hottest parts of the day. This serves as a short-range activator for other grass seeds, building up communities around the circles’ edge. However, by capturing so much of the most precious resource, the grasses inhibit any growth within the circles.
If Turing was right, Getzen proposed, grasses at the edge of the fairy circles should be healthier and faster-growing compared to randomly distributed “low-vitality” grasses. Careful observations of five one-hectare plots confirmed the hypothesis.
"The intriguing thing is that the grasses are actively engineering their own environment by forming symmetrically spaced gap patterns,” Getzen said in a statement. “The vegetation benefits from the additional runoff water provided by the large fairy circles, and so keeps the arid ecosystem functional even in very harsh, dry conditions."
The shapes of the original fairy rings invited supernatural explanations before modern science, but are now attributed to fungi growing outwards as they exhaust the nutrients at their initial site.
