The fairy circles (circular barren patches) of the Namib Desert - What do we know about their cause 50 years after their first description?

Abstract Since they were described for the first time 50 years ago, more than 30 peer-reviewed papers have been published on a number of extremely diverse theories as the cause of the formation of hundreds of thousands of nearly circular barren patches (fairy circles) found in southwestern Angola, Namibia and northwestern South Africa. However, scientists interested in finding an explanation for the cause and maintenance of these fairy circles in the driest parts of the world are still without agreement on the true reason(s) behind this intriguing phenomenon. In this review we firstly present the four main theories and explain their hypotheses on the formation and/or maintenance of fairy circles. Secondly, we summarise the previously described physical characteristics of the fairy circles, e.g. soil substrate, moisture content, shape, size, lifespan, dynamics and spatial patterning, and then present the supporting evidence as published by the proponents of the theories on vegetation self-arrangement, insects/ants, gas and Euphorbia allelopathy, as the causative drivers of this natural phenomenon. We also present the critical views of scientists on the theories they are not supporting.