Discovery of fairy circles in Australia supports self-organization theory

Abstract

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.

Reply to Walsh et al.: Hexagonal patterns of Australian fairy circles develop without correlation to termitaria

“…This pattern has been shown for Drepanotermes harvester termites in the study of Noble et al (9). Similarly, our mapping of termite nests found aggregated and nonordered distributions at the gap edges that are fundamentally different from the extremely regular and homogeneous pattern of FCs (8). In figure 1B of ref.…”

“…We verified in the field that mechanical crusts resulting from soil weathering were easily distinguishable from crusts potentially resulting from pavement mounds (8).…”

“…Most importantly, Walsh et al (7) have not examined our field areas L1 and L2 (8), where actively forming, irregularly shaped gaps can be witnessed. Especially in area L2, large gaps are developing on loose sand with relatively young, and hence very thin, crust layers.…”

“…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). Based on the preliminary data obtained by excavating two gaps from the fairy circle (FC) area near Newman, Walsh et al (7) claim that subterranean termitaria are the sole cause of the Australian FCs (8). This claim is unjustified for several reasons.…”

“…9). However, when termite (and ant) mounds were present in the FC area near Newman, then they occurred primarily at the edges of the large gaps, indicating a general insect preference for these favorable microenvironments (8). Thus, mound erosion can hardly explain the circular shape, and especially not the fact that significantly higher clay contents were found in the gap center compared with the gap edges (8).…”