Abstract:
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.
