Weaknesses in the plant competition hypothesis for fairy circle formation and evidence supporting the sand termite hypothesis

Jürgens, Norbert; Vlieghe, Kelly; Bohn, Claerin; Erni, Birgit; Gunter, Felicitas; Oldeland, Jens; Rudolph, Barbara; Picker, Mike D.

doi:10.1111/een.12266

Cited by 26 publications

(41 citation statements)

References 33 publications

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“…The sand termite ( Psammotermes allocerus ) has been suggested to cause Namibian FCs (Juergens, ), but there has been no spatial analysis of the nest pattern of this termite species because the nests are rare and cryptic and termite colonies possibly migrate between ephemeral nests (Vlieghe et al, ). As a consequence of this, the Namibian FCs have instead been compared to grassland gaps of harvester ants in North America (Juergens et al, ; Picker et al, ) and other remote termite patterns (Tarnita et al, ). While individual harvester ant grassland gaps look similar to individual FCs, the spatial patterns of gaps created by harvester ants are not spatially periodic as FCs and are often heterogeneous at larger scales.…”

Section: Discussionmentioning

confidence: 99%

“…In order to infer potential causality, Namibian FCs are sometimes directly compared with grassland gaps created by North American harvester ants (e.g., Picker et al, ) or with Macrotermes termite mounds in Namibia (e.g., Juergens et al, ). For this reason, we reanalyzed the original data for harvester ants from Arizona (A Arizona 1, 2) of Tarnita et al ().…”

Section: Methodsmentioning

confidence: 99%

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Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

et al. 2019

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Numerical analysis of spatial pattern is widely used in ecology to describe the characteristics of floral and faunal distributions. These methods allow attribution of pattern to causal mechanisms by uncovering the specific signatures of patterns and causal agents. For example, grassland-gap patterns called fairy circles (FCs) in Namibia and Australia are characterized by highly regular and homogenous distributions across landscapes that show spatially periodic ordering. These FCs have been suggested to be caused by both social insects and competitive plant interactions. We compared eight Namibian and Australian FC patterns and also modeled FCs to 16 patterns of social insect nests in Africa, Australia, and America that include the most regular termite mound patterns known. For pattern-process inference, we used spatial statistics based on both nearest-neighbor analysis and neighborhood-density functions. None of the analyzed insect-nest distributions attain the spatially periodic ordering that is typical of FCs. The inherently more variable patterns of termite and ant nests are commonly attributable to well documented aspects of the faunal life-history. Our quantitative evidence from drylands shows that the more variable insect-nest distributions in water-limited environments cannot explain the characteristic spatial signature of FCs. The analysis demonstrates the interpretation of scale-dependent neighborhood-density functions and that it is the identification of unique spatial signatures in regular patterns that need to be linked to process. While our results cannot verify a specific hypothesis, they support the hypothesis that FCs in these drylands are more likely an emergent vegetation pattern caused by strong plant competition for water.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

et al. 2019

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“…self-organization hypothesis argue "if the model of self-organization were correct, it should be generic and circular bare patches should occur globally" (25).…”

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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.

167

215

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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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“…The ecological mechanisms for the origin of fairy circles are currently contentious (Getzin et al , , Juergens et al ). An alternate hypothesis for the origin of fairy circles to the vegetation‐patterning hypothesis is that termites generate the pattern by feeding on grasses (Moll , Juergens , , Juergens et al , Vlieghe et al ). Based on the vegetation‐patterning hypothesis, however, we hypothesized (Supplementary material Appendix 1, Fig.…”

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confidence: 99%

Edaphic properties enable facilitative and competitive interactions resulting in fairy circle formation

2016

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Millions of generally regularly spaced, roughly circular barren patches called fairy circles occur in a narrow band ca 100 km inland of the south‐west African coast. These generally have conspicuously taller peripheral grasses in a shorter grass matrix. The origins of these fairy circles are controversial, but one possibility is that they are self‐organizing emergent vegetation patterns that are the consequence of interplay between positive (facilitative) and negative (competitive) interactions between grasses. We hypothesized that the coarse textured sand on which fairy circles occur creates a hydraulically and nutritionally connected landscape, in which neighbouring fairy circles competitively influence each other over several metres, while providing opportunity for focusing of resources around the peripheral grasses. To test our hypotheses we conducted three main groups of analyses: 1) we measured grass biomass to assess facilitative and competitive effects of the component grasses; 2) across a region with fairy circles we measured the size and density of fairy circles and correlated that with water infiltration rates into soil; 3) we measured the capacity of soil to conduct water pulses and 15N tracers. We found evidence of facilitative interactions in the periphery of the fairy circles and competitive suppression of the matrix grass proximal to the periphery. Across the region, fairy circle size was positively correlated with soil infiltration rates and negatively with precipitation. This suggests that fairy circles emerge in soils with high capacity for water flux that enables landscape hydraulic connectivity. Water‐ and 15N‐pulse experiments showed that edaphic resources were highly mobile, moving up to 7.5 m over a period of 1–3 weeks. We concluded that the evidence is consistent with an emergent vegetation pattern explanation for the origins of fairy circles and that the circles are more closely associated with a highly connective edaphic environment, rather than with particular biota.

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Weaknesses in the plant competition hypothesis for fairy circle formation and evidence supporting the sand termite hypothesis

Cited by 26 publications

References 33 publications

Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

Discovery of fairy circles in Australia supports self-organization theory

Edaphic properties enable facilitative and competitive interactions resulting in fairy circle formation

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