Background: In this multidisciplinary study we present soil chemical, phytochemical and GIS spatial patterning evidence that fairy circles studied in three separate locations of Namibia may be caused by Euphorbia species. Results: We show that matrix sand coated with E. damarana latex resulted in faster water-infiltration rates. GC-MS analyses revealed that soil from fairy circles and from under decomposing E. damarana plants are very similar in phytochemistry. E. damarana and E. gummifera extracts have a detrimental effect on bacteria isolated from the rhizosphere of Stipagrostis uniplumis and inhibit grass seed germination. Several compounds previously identified with antimicrobial and phytotoxic activity were also identified in E. gummifera. GIS analyses showed that perimeter sizes and spatial characteristics (Voronoi tessellations, distance to nearest neighbour ratio, pair correlation function and L-function) of fairy circles are similar to those of fairy circles co-occurring with E. damarana (northern Namibia), and with E. gummifera (southern Namibia). Historical aerial imagery showed that in a population of 406 E. gummifera plants, 134 were replaced by fairy circles over a 50-year period. And finally, by integrating rainfall, altitude and landcover in a GIS-based site suitability model, we predict where fairy circles should occur. The model largely agreed with the distribution of three Euphorbia species and resulted in the discovery of new locations of fairy circles, in the far southeast of Namibia and part of the Kalahari Desert of South Africa. Conclusions: It is proposed that the allelopathic, adhesive, hydrophobic and toxic latex of E. damarana, E. gummifera, and possibly other species like E. gregaria, is the cause of the fairy circles of Namibia in the areas investigated and possibly in all other areas as well.

a b s t r a c tThe cause and maintenance of the fairy circle phenomenon of the Namibian pro-desert have not yet been satisfactorily explained. Several diverse hypotheses have been put forward, but none have universally been accepted. In the current study a possible link between the barren patches (fairy circles) of the Garub region (southern Namibian pro-desert) and Euphorbia gummifera was investigated, by determining if a characteristic compound(s) from this poisonous plant is present inside the fairy circles. With the use of GCMS trace analysis it was possible to identify euphol, a characteristic triterpenoid of the Euphorbia genus, in substantial quantities in 19 out of 20 soil samples from inside fairy circles in the Garub region. Only trace amounts of euphol could be found in 3 of 20 samples from outside the fairy circles. This implies that E. gummifera was present where there are now fairy circles. A comparison of the size of the fairy circles and E. gummifera found within the same area (Garub) additionally linked the barren patches to E. gummifera. The circles in this area have an average diameter of 3.53 m and the plants 3.44 m. This study partially supports the first published hypothesis by Theron in 1979, that the fairy circles are caused by Euphorbia spp. or their remains.

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