Why do circular bare patches form in fertile grasslands?

MIT Technology Review : Randomly distributed circular areas devoid of vegetation, otherwise known as fairy circles, have long been noted in the arid grasslands of southern Africa and elsewhere. And the drier it gets, the bigger the circles grow. With the use of computer simulations, Cristian Fernandez-Oto of the Université libre de Bruxelles in Belgium and colleagues explain that the phenomenon occurs in areas where both barren and fertile conditions exist simultaneously. They propose that there exists a “front” where the two regions meet. As the fertile area grows, the barren area shrinks—and the plants along the front begin to grow closer together. To maximize the efficiency of packing closer together, they end up forming a circle around the barren area. The size of the circle grows and shrinks directly in correlation to the availability of water. The researchers also show that their model can apply to any extreme system in which units compete for a limited resource in a region where they either flourish or die.