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Mysterious Desert 'Fairy Circles' Explained by Math
The evenly spaced barren patches surrounded by rings of grasses in the Namib Desert have long puzzled scientists trying to tease out how the fairy circles formed.
Credit: Tyler Coverdale, Princeton University; Jen Guyton, Princeton University

Scientists have been divided over the mysterious pattern of evenly spaced bald patches known as "fairy circles" in Africa's Namib Desert. Some say the spots are the scars of termite mounds; others think thirsty, self-organizing plants create the pattern in their fight for scarce water.

Now, it seems both camps could be partially right — at least according to a new explanation that harmonizes the competing theories.

Led by Corina Tarnita, an assistant professor of ecology and evolutionary biology at Princeton University, the researchers looked at satellite images of insect nests from across the globe. The team found that the nests are often evenly spaced, each with an average of six neighbors. [Image Gallery: Amazing 'Fairy Circles' of the Namib Desert]

Then, the researchers ran computer simulations of territorial battles between neighboring termite colonies, and found that such skirmishes could produce the large-scale hexagonal, or honeycomb, distribution of nests seen in Namibia.

"Many social insects tend to be territorial, and colonies often fight to [the] death," Tarnita said in a statement. "When any incipient mound appears in an existing territory, the established termites eventually find it and destroy it. Over time, large colonies obliterate the smaller ones. But large colonies end up coexisting in a perpetual border war, with neither gaining any ground."

This standstill between colonies — at least in a homogeneous landscape like the Namib Desert amcan produce a pattern of six-sided territories, Tarnita and her colleagues found.

Meanwhile, on a smaller scale, each termite colony strips the vegetation from its mound, creating a bald spot where water pools. Plants can take advantage of this arrangement.

Vegetation is known to self-organize according to "scale-dependent feedbacks," the researchers said. This means that evenly spaced clumps of grass might form in a desert, for example, to balance the pros (increased shade and moisture concentration) and cons (competition for resources) of sticking together in a harsh environment. By these principles, tall grass grows in a ring around termite colonies to take full advantage of the moisture and nutrients stored in the mounds, the researchers said.

The study, which was published Wednesday (Jan. 18) in the journal Nature, suggests that the self-organization of plants and the presence of insects are not mutually exclusive mechanisms when it comes to fairy circles.

"Our findings harmonize both theories and find a possible explanation for regular vegetation patterns observed around the globe,” study researcher Juan Bonachela, a theoretical ecologist at the University of Strathclyde in Scotland, said in a statement.

"This behavior affects the whole ecosystem, allowing it to survive harsher conditions and recover from droughts much more quickly than if there were no termites,” Bonachela added. "The fairy circles remind us of the delicate balance of interactions necessary to sustain ecosystems."

Original article on Live Science.