Fairy circles are extremely ordered round patches of bare soil within arid grasslands. They are known from southwestern Africa around the Namib Desert and from Western Australia. Up to now, scientists offered several theories to explain the honeycomb-like pattern formed by these circles. Some believed termites or ants nibble away at the roots of the grasses and kill them. Others suspected toxic carbon monoxide gas may rise from soil under the circles and kill the vegetation. And a third camp thought that the barren areas simply arise of their own accord under certain conditions. An international team of researchers from Germany, Australia and Israel has now got to the bottom of their cause with soil investigations and drones. Their results suggest that the fairy circles in Australia were caused by processes such as the weathering of the soil by heavy rainfall, extreme heat and evaporation. The extensive data collected by the team argued against a causal relationship to underground termite structures.
Fairy circles seen from the air. Image credit: Stephan Getzin.
The landscape scale vegetation patterns of fairy circles are characterized by extremely ordered grassland gaps that look like polka dots from a bird’s eye view.
In northwest Western Australia, the typical diameters of the circles are about 13 feet (4 m), and in Namibia, the diameters increase with aridity from 13 feet in the south to 20 feet (6 m), and even 33 (10 m), further to the northwest.
While the origin of Namibia’s fairy circles has been puzzled over since the early 1970s, the Australian fairy circles were only discovered in 2014.
University of Göttingen’s Dr. Stephan Getzin and colleagues have now dug a total of 154 holes in 48 fairy circles east of the Australian town of Newman over a length of 7.5 miles (12 km) in order to assess the possible influence of termites objectively and systematically.
With the help of drones, the scientists mapped areas of 1,640 by 1,640 feet (500 by 500 m) to compare typical vegetation gaps — such as those caused by harvester termites in large parts of Australia — with typical fairy circle gaps.
In addition, they investigated the soil conditions in the area of the fairy circles and in adjacent reference areas, where no grass grows over large areas.
“The vegetation gaps caused by harvester termites are only about half the size of the fairy circles and much less ordered,” said Dr. Getzin, first author of a paper published in the journal Ecosphere.
“And in most cases, we didn’t even find any hard subterranean termitaria that elsewhere in Australia prevent the growth of grasses.”
However, the high soil compaction and clay content in the investigated fairy circles and vegetation-free reference areas are indications that fairy circles are formed by abiotic processes such as mechanical weathering of the soil by heavy rain in cyclones, extreme heat and evaporation.
“Overall, our study shows that termite constructions can occur in the area of the fairy circles, but the partial local correlation between termites and fairy circles has no causal relationship,” Dr. Getzin explained.
“So no destructive mechanisms, such as those from termites, are necessary for the formation of the distinct fairy circle patterns; hydrological plant-soil interactions alone are sufficient.”
In Namibia, research has so far concentrated on typical fairy circles in homogeneous landscapes.
In a pilot study, published in the Journal of Arid Environments, Dr. Getzin and co-authors focused on extraordinary fairy circles in atypical environments in order to better understand the actual threshold conditions for circle formation.
Using Google Earth imagery, they found unusual circles: huge ones more than 66 feet (20 m) in diameter, chain-like oval ones more than 98 feet (30 m) long in drainage lines, circles in car tracks and circles in particularly dry, disturbed or Euphorbia plant-rich areas.
“Here our soil moisture studies showed that under such varied conditions the fairy circles function less as water reservoirs than under typical homogeneous conditions where they are extremely well ordered,” Dr. Getzin said.
“With this work, we intend to open up a new thematic field, as we hope to gain further insights into the formation and persistence of the fairy circles from studying ‘extraordinary’ fairy circles.”
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Stephan Getzin et al. A multi-scale study of Australian fairy circles using soil excavations and drone-based image analysis. Ecosphere, published online February 20, 2019; doi: 10.1002/ecs2.2620
Stephan Getzin & Hezi Yizhaq. Unusual Namibian fairy circle patterns in heterogeneous and atypical environments. Journal of Arid Environments, published online February 18, 2019; doi: 10.1016/j.jaridenv.2019.01.017
