Abstract
BackgroundThe Euphorbia hypothesis on the origin of fairy circles (FCs) in Namibia dates back to 1979. It proposes that the remains of decaying shrubs would induce an allelopathic interaction with the grasses and thereby cause bare-soil FCs. Here, we investigated this hypothesis based on revisiting marked Euphorbias after four decades, comparing the typical size distribution of dead Euphorbia damarana and FCs, and analyzing the spatial patterns of Euphorbias and FCs within the same drone-mapped study plots in three regions of Namibia.ResultsWe found four dead Euphorbias in the southern Giribes that were marked by G.K. Theron about 40 years ago. Those locations did not develop into FCs over this time span. However, for the four dead Euphorbias, we provide photographic evidence that grass tufts were growing at the metal pins of those decaying shrubs, agreeing with previous research findings that the soil taken from beneath dead E. damarana shrubs was stimulating rather than inhibiting the growth of grasses. In the Giribes, there were very large FCs that ranged in diameter from 13.0 to 19.1 m. By contrast, the measured dead E. damarana, including the largest plants that we could find, ranged in size only between 4.2 and 11.7 m. At Brandberg, we found particularly small FCs with diameters between 2.4 and 2.7 m but the dead E. damarana, including the smallest dead shrubs in the area, ranged in size between 4.1 and 7.2 m. Hence given these size mismatches, the decaying Euphorbias cannot induce such observed FCs in the two regions. Spatial patterns of E. damarana and FCs in the two regions Giribes and Brandberg, as well as of E. gummifera and FCs near Garub, showed a strong mismatch within the same habitat: in four out of five plots the patterns differed significantly. FCs were regularly distributed while Euphorbias were predominantly clustered.ConclusionsWe reject the Euphorbia hypothesis based on the fact that grass growth was not prevented under decaying shrubs, the size of dead Euphorbias cannot explain the size of observed FCs and the spatial distribution of Euphorbias cannot cause the specific pattern signature of FCs.
Highlights
The Euphorbia hypothesis on the origin of fairy circles (FCs) in Namibia dates back to 1979
Revisiting marked Euphorbia damarana after more than 40 years In total, we found nine metal pins at the site “Gir-1” in the southern Giribes that were placed by Theron [6] more than 40 years ago (Additional file 1: Fig. S2)
We identified them as four marked Euphorbia damarana, two fairy circles and three control points in the matrix (Figs. 2, 3)
Summary
The Euphorbia hypothesis on the origin of fairy circles (FCs) in Namibia dates back to 1979. Fairy circles (FCs) are round grassland gaps that can be found along the Namib Desert in south-western Africa. At this transition from arid grassland to desert, FCs are found on nutrient-deficient sandy soils where just one or two grass species of the genus Stipagrostis predominate. 50 and 100 mm mean annual precipitation [1] In these arid grasslands along the Namib, FCs are characterized by having the unique ability to form spatially periodic patterns where each circle has on average six nearest neighbors, but importantly the distances to the six neighbors are approximately equal [2]. The higher soil moisture is restricted to deep aeolian sands but can be found in the gravel plains of the Central Namib [5]
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