An explanation for the longstanding mystery of how Namibian fairy circles are formed is presented in a paper published in Nature this week.
Self-organized regular vegetation patterns are widespread in nature, but the mechanisms behind their existence remain disputed. In particular, there is controversy regarding the origins of overdispersed (evenly spaced) patterns, such as Namibian fairy circles: circular patches of bare soil that are 2-35 m wide, surrounded by rings of tall grass, that punctuate grasslands in parts of the Namib Desert. One hypothesis suggests that they are created by scale-dependent feedbacks, whereby plants help their neighbours but compete with distant individuals. The alternative hypothesis attributes the fairy circles to subterranean ecosystem engineers, such as termites, ants or rodents.
Corina Tarnita and colleagues reconcile these competing perspectives by integrating them both into model simulations, which they then validate with field data from four continents. They show that, instead of one or the other, a combination of competition between subterranean social-insect colonies of the same species and scale-dependent feedbacks between plants can explain the self-organized regular vegetation patterns. Using the Namibian fairy circles as a case study, they show that interactions between sand termite colonies, and between the sand termites and the grass, are jointly responsible for the large-scale hexagonal vegetation patterns.
The authors conclude that multiple mechanisms of ecological self-organization should be considered when trying to explain such regularly spaced landscape features.