Fairy circle research: Status, controversies and the way forward

Abstract

• 1)
  Fairy circles are regularly spaced barren circular patches in arid landscapes, typically encircled by a ring of taller grasses. They occur in South-western Africa and have also been suggested to occur in Australia, North Africa, Middle East and Madagascar. The enigmatic origins of fairy circles in arid landscapes have intrigued ecologists and sparked heated debate about the two main competing hypotheses: the termite origin (TO) and vegetation self-organization (VSO) hypotheses. In this review we attempt to untangle the claims and counter-claims regarding the two hypotheses in a dispassionate manner.
• 2)
  The TO hypothesis posits that fairy circles form due to activities of Psammotermes allocerus termites, which through their foraging and nest-building behaviour create barren patches in arid grasslands, leading to the characteristic circular formations whose spacing results from competitive interactions between P. allocerus colonies.
• 3)
  The VSO hypothesis posits that fairy circles in arid landscapes are the product of self-organizing behaviour of plants competing for limited water resources. Competition between neighbouring plants leads to the formation of barren circular patches with grassy peripheries. Water and nutrient mobility in coarse aeolian sands play a critical role in the shape and spacing between fairy circles.
• 4)
  Problems with the TO hypothesis include the inconsistent central location of termite colonies within fairy circles, the difficulty of aligning the long-term persistence and stability of fairy circles with termite population dynamics, and the lack of evidence for aggressive termite interactions at the scale of the fairy circle pattern.
• 5)
  The main challenge for the VSO hypothesis is a lack of direct empirical evidence, especially concerning complex underground water and nutrient fluxes. The precise mechanisms behind VSO remain unclear, making it difficult to fully validate this hypothesis as the sole explanation for fairy circles.
• 6)
  Synthesis: This analysis underscores the VSO hypothesis as a coherent explanation for fairy circle formation. Progress will require manipulative experiments with environmental factors (e.g., termite presence, soil nutrients, water availability) that test the hypotheses directly. Long-term monitoring to observe fairy circle development and changes under varying conditions is also required.