"We found that the size of the circle, the density and degree to which they occupy the landscape are all associated with the amount of resources available," Cramer said. Specifically, fairy circles are smaller if they have more resources, such as soil nitrogen and rainfall.
This makes sense, Cramer explained, because the taller grasses won't need a large reservoir of resources to get started and survive if water and nutrients are already available in the environment. On the other hand, the grasses require a large reservoir to sustain themselves if the soil is poor in water and nutrients.
The researchers also discovered that rainfall strongly determines the distribution of the fairy circles across Namibia, with circles only appearing in areas where there is just the right amount of rain (not too little, but not too much).
If there's too much rain, the bountiful resources would "relax" the competition for resources and the circles would close up; but if there's too little rain, the competition would become too severe and the circles would again disappear, Cramer said. Because the circles can only occur in this narrow moisture range, differences in rainfall from year to year may cause them to suddenly disappear and reappear in an area over time. With this information, they found that they could predict the distribution of the fairy circles with 95 percent accuracy.