A multi‐scale study of Australian fairy circles using soil excavations and drone‐based image analysis

Abstract

Fairy circles (FCs) are extremely ordered round patches of bare soil within arid grasslands of southwestern Africa and northwestern Australia. Their origin is disputed because biotic factors such as insects or abiotic factors such as edaphic and eco‐hydrological feedback mechanisms have been suggested to be causal. In this research, we used a multi‐scale approach to shed light on the origin of Australian FCs. At a local scale, we investigated the potential cause of FCs using analyses of soil compaction and texture within FCs, the surrounding matrix vegetation, and in nearby large bare‐soil areas. We found that soil hardness and clay content were similarly higher inside the FCs and in the large bare‐soil areas. When compared to the matrix soils with protective grass cover, the 2.6–2.8 times higher clay content in FCs and large bare‐soil areas is likely sourced via multiple abiotic weathering processes. Intense rainfall events, particle dispersion, surface heat, evaporation, and mechanical crust building inhibit plant growth in both areas. At the landscape scale, a systematic survey of 154 soil excavations within FCs was undertaken to evaluate the presence of pavement termitaria that could inhibit plant growth. We show that in up to 100% and most of the excavations per plot, no hard pavement termitaria were present in the FCs. This fact is substantiated by the observation that small, newly forming FCs are initiated on soft sand without evidence of termite activity. At the regional scale, we investigated the spatial properties of FCs and common termite‐created gaps in Western Australia, using spatially explicit statistics. We mapped three 25‐ha FC plots with a drone and compared them with three aerial images of typical vegetation gaps created by harvester and spinifex termites. We demonstrate that the small diameters, the lower ordering, and the heterogeneous patterns of these common termite gaps strongly differ from the unique spatial signature of FCs. Our multi‐scale approach emphasizes that FCs are not trivial termite gaps and that partial correlation with termites at some sites does not imply causation. Instead, we highlight the need to study the edaphic and eco‐hydrological drivers of vegetation‐pattern formation in water‐limited environments.

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

“…From Australia, we analyzed four FC plots in aerial images (FC Australia C1, C2, L2, 1) and a process‐based model output (FC Australia Mod) in which the pattern‐forming process was solely based on biomass‐water feedbacks and resultant vegetation self‐organization (Getzin et al, ). The three plots “FC Australia C2, L2, 1” were mapped with a quadcopter Microdrone md4‐1000 (http://www.microdrones.com) and a 24‐megapixel photo camera Sony NEX‐7 in July 2017 (Getzin et al, ) and subsequently orthorectified and georeferenced with OneButton software (http://www.icaros.us). To make these high‐resolution aerial images (3 cm/pixel) directly comparable with published image analyses of FC patterns (Getzin et al, , , ), we used a threshold value of 2 m for the smallest identified gap diameters.…”

“…These structures may result from long‐term development, making experimental manipulation of pattern formation difficult to achieve (Tschinkel, ). As a consequence, several authors have resorted to numerical modeling techniques to uncover the detailed structure of the patterning (Getzin et al, , , ) and to relate the patterning to mechanistic causes (Grabovsky, ; Zelnik et al, ). Spatial pattern analysis can be used to contrast alternative theories for causal agent behavior and thus for understanding the drivers of self‐organized biological systems (Grimm et al, ; McIntire & Fajardo, ).…”

“…FCs probably represent the most spectacular example of regular ecological patterning and serve as an important benchmark example of the generation of regular pattern (Figure a, b). FCs are roughly circular barren patches occurring in a narrow band approximately 100 km inland of the southwest African coast spanning the Namib Desert (van Rooyen et al, ) and also in Western Australia (Getzin et al, , ). In Namibia, these FCs generally have conspicuously taller peripheral grasses in a shorter grass matrix.…”

“…The origin of these FCs in water‐limited drylands has puzzled scientists for decades (Sahagian, ; van Rooyen et al, ). The hypotheses include (a) allelopathic influences of a plant that no longer occurs in FCs (Meyer et al, ; Theron, ); (b) seepage of carbon monoxide, hydrocarbons, or other volatiles from below ground that kills grass in regular patterns (Albrecht et al, ; Jankowitz et al, ; Naude et al, ); (c) an unspecified microbial effect (Eicker et al, ; Ramond et al, ; van der Walt et al, ); (d) faunal removal of grasses by termites (Becker & Getzin, ; Juergens, ; Moll, ; Tarnita et al, ; Vlieghe et al, ) or detrimental influence on grasses by ants (Picker et al, ); and (e) self‐organized emergent vegetation patterning arising from competitive and facilitative interactions between grasses (Cramer et al, ; Cramer & Barger, ; Getzin et al, , , , ; Ravi et al, ; Tschinkel, ; van Rooyen et al, ).…”

“…While there are multiple hypotheses for FCs origins, the two that enjoy current support are that dependent on faunal termite causation and on emergent vegetation patterning (Sahagian, ). Several studies have demonstrated that there are no direct observational data of past or present termite activities that support their potential role in the genesis of FCs in the central or southern Namib (Ravi et al, ; Tschinkel, , ) or in Australia (Getzin et al, ). There has also been no empirical verification of the role of either termites or vegetation patterning in the formation of FCs.…”