Adopting a spatially explicit perspective to study the mysterious fairy circles of Namibia

Getzin, Stephan; Wiegand, Kerstin; Wiegand, Thorsten; Yizhaq, Hezi; Hardenberg, Jost von; Meron, Ehud

doi:10.1111/ecog.00911

Cited by 152 publications

(185 citation statements)

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“…Several competing hypotheses exist ranging from vegetation self-organization (13)(14)(15)(16)(17), termite-induced activity (18), and Euphorbia poisoning (19) to abiotic gas leakage (20). Among these, vegetation self-organization stands out as the most solid mechanism that explains the emergence of large-scale order (15,16) and the strong dependence of fairycircle distribution on mean annual precipitation (13). Furthermore, only self-organized biomass-water feedbacks can explain the shrinking in size and disappearance of fairy circles after aboveaverage rainfall years and the typical enlargement and increased appearance of FCs after below-average rainfall years (21).…”

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confidence: 99%

Discovery of fairy circles in Australia supports self-organization theory

Getzin

Yizhaq

Bell

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Vegetation gap patterns in arid grasslands, such as the "fairy circles" of Namibia, are one of nature's greatest mysteries and subject to a lively debate on their origin. They are characterized by small-scale hexagonal ordering of circular bare-soil gaps that persists uniformly in the landscape scale to form a homogeneous distribution. Pattern-formation theory predicts that such highly ordered gap patterns should be found also in other water-limited systems across the globe, even if the mechanisms of their formation are different. Here we report that so far unknown fairy circles with the same spatial structure exist 10,000 km away from Namibia in the remote outback of Australia. Combining fieldwork, remote sensing, spatial pattern analysis, and process-based mathematical modeling, we demonstrate that these patterns emerge by self-organization, with no correlation with termite activity; the driving mechanism is a positive biomasswater feedback associated with water runoff and biomass-dependent infiltration rates. The remarkable match between the patterns of Australian and Namibian fairy circles and model results indicate that both patterns emerge from a nonuniform stationary instability, supporting a central universality principle of pattern-formation theory. Applied to the context of dryland vegetation, this principle predicts that different systems that go through the same instability type will show similar vegetation patterns even if the feedback mechanisms and resulting soil-water distributions are different, as we indeed found by comparing the Australian and the Namibian fairy-circle ecosystems. These results suggest that biomass-water feedbacks and resultant vegetation gap patterns are likely more common in remote drylands than is currently known.drylands | spatial pattern | Triodia grass | Turing instability | vegetation gap P attern-formation theory (1) and the influence of Alan Turing's work on understanding biological morphogenesis (2) are increasingly recognized in environmental sciences (3). Vegetation patterns resulting from self-organization occur frequently in waterlimited ecosystems and, similar to Turing patterns, show pattern morphologies that change from gaps to stripes (labyrinths) to spots with decreasing plant-available moisture (4-6). The patterns may emerge on completely flat and homogeneous substrate and are induced by positive feedbacks between local vegetation growth and water transport toward the growth location.

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confidence: 99%

Discovery of fairy circles in Australia supports self-organization theory

Getzin

Yizhaq

Bell

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

167

215

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“…While many hypotheses have been proposed in attempts to explain the origin of Namib Desert fairy circles (2,4,8,10,11,(13)(14)(15), none take into account the presence of fairy circles in both the gravel plain and dune environments and the differences between them. Considering that almost all FC studies have focused exclusively on dune FCs, we compared microbial communities in dune and gravel plain FCs.…”

Section: Discussionmentioning

confidence: 99%

“…Fauna-based hypotheses suggest that FCs result from the harvesting activities of termites, ants, or rodents (1,5,(10)(11)(12). The hypothesis of spatial self-organization of plants argues that FCs are the product of extreme aridity (13,14). During times of drought, competition between plants for nutrients and water resources leads to the formation of barren patches (14), which in turn act as water traps, utilized by the peripheral grasses through extensive root systems and soil-water diffusion (13).…”

Section: Importancementioning

confidence: 99%

“…The hypothesis of spatial self-organization of plants argues that FCs are the product of extreme aridity (13,14). During times of drought, competition between plants for nutrients and water resources leads to the formation of barren patches (14), which in turn act as water traps, utilized by the peripheral grasses through extensive root systems and soil-water diffusion (13). Other published hypotheses implicate localized radioactivity hot spots (2), natural gas seepages (8), and the release of allelopathic compounds by dead Euphorbia damarana plants (5,15).…”

Section: Importancementioning

confidence: 99%

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Unique Microbial Phylotypes in Namib Desert Dune and Gravel Plain Fairy Circle Soils

Walt

Johnson

Cowan

et al. 2016

Appl Environ Microbiol

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Fairy circles (FCs) are barren circular patches of soil surrounded by grass species. Their origin is poorly understood. FCs feature in both the gravel plains and the dune fields of the Namib Desert. While a substantial number of hypotheses to explain the origin and/or maintenance of fairy circles have been presented, none are completely consistent with either their properties or their distribution. In this study, we investigated the hypothesis that FC formation in dunes and gravel plains is due to microbial phytopathogenesis. Surface soils from five gravel plain and five dune FCs, together with control soil samples, were analyzed using high-throughput sequencing of bacterial/archaeal (16S rRNA gene) and fungal (internal transcribed spacer [ITS] region) phylogenetic markers. Our analyses showed that gravel plain and dune FC microbial communities are phylogenetically distinct and that FC communities differ from those of adjacent vegetated soils. Furthermore, various soil physicochemical properties, particularly the pH, the Ca, P, Na, and SO 4 contents, the soil particle size, and the percentage of carbon, significantly influenced the compositions of dune and gravel plain FC microbial communities, but none were found to segregate FC and vegetated soil communities. Nevertheless, 9 bacterial, 1 archaeal, and 57 fungal phylotypes were identified as FC specific, since they were present within the gravel plain and dune FC soils only, not in the vegetated soils. Some of these FC-specific phylotypes were assigned to taxa known to harbor phytopathogenic microorganisms. This suggests that these FC-specific microbial taxa may be involved in the formation and/or maintenance of Namib Desert FCs. IMPORTANCEFairy circles (FCs) are mysterious barren circular patches of soil found within a grass matrix in the dune fields and gravel plains of the Namib Desert. Various hypotheses attempting to explain this phenomenon have been proposed. To date, however, none have been successful in fully explaining the etiology of FCs, particularly since gravel plain FCs have been largely ignored. In this study, we investigated the hypothesis that microorganisms could be involved in the FC phenomenon through phytopathogenesis. We show that the microbial communities in FC and control vegetated soil samples were significantly different. Furthermore, we detected 67 FC-specific microbial phylotypes, i.e., phylotypes present solely in both gravel plain and dune FC soils, some of which were closely related to known phytopathogens. Our results, therefore, demonstrate that microorganisms may play a role in the formation and/or maintenance of Namib Desert FCs, possibly via phytopathogenic activities. (1, 2). Common in the Namib Desert, and recently observed in the Australian outback (3), FCs are limited to arid areas that receive between 50 and 100 mm precipitation per annum. FCs are nonpermanent; they typically appear, enlarge, and disappear, with an estimated life span ranging from 22 to 60 years (4, 5).Although FCs were first reported in the 1970...

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“…Bare-soil gaps in arid grasslands often appear in conjunction with termites or ants (1,2), but should this common correlation (3) automatically infer a causal relationship? To the unfamiliar, it may be difficult to realize that plant populations can organize themselves into geometric order so as to optimize their access to scarce soil water (4)(5)(6).…”

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confidence: 99%