This study published in August 2020 (how did it not get on the national news?) hypothesised that the remains of dead succulent Euphorbias cause the fairy circles. They tested that hypothesis by making predictions that led to the discovery of new fairy circles.
The latex from the Euphorbia plants remains in the soil under the plant. When the plant dies the soil in the circle that was under the plant is still resistant to wetting because of the hydrophobic nature of the rubbery dried latex. Some of the water will run off to the edges of the circle where the grass there grows more abundantly. The water also follows the cavity left by the root system and goes too deep into the soil for the grass roots to reach. The plant probably benefited from this while alive as no weeds could establish under its canopy, with all the water directed down to the deep roots of the Euphorbia.
The Euphorbia latex may also be phytotoxic, directly inhibiting germination when water is in short supply. It also has antimicrobial properties, destroying the bacteria associated with the grass roots that help them survive the harsh conditions.
The termites seem to like the conditions in the fairy circles. There are some areas where there are no longer any Euphorbias that still have fairy circles. It is thought they died out due to changing climate conditions. Black rhino and some antelopes are able to eat the Euphorbia and depend on them, as they are immune to the Euphorbia toxins. Reduced availability of forage is a concern for the survival of these species.
The regular spacing of the circles is due to the living Euphorbia plants becoming spaced apart, avoiding competition for scarce water resources.
The known fairy circles were found in a strip inland from the coast running from south-western Angola through western Namibia and into north-western South Africa. The authors used their hypothesis to predict that other fairy circles would be found further east, in southern Namibia and the Kalahari Desert in South Africa. Using Google Earth, they found new fairy circles.
The authors also found a new type of fairy circle on rocky ground. Here the circles were characterised by a sand circle surrounded by rocky terrain. The Euphorbia bushes appeared to have trapped wind-blown sand and then stuck it together with the gluey latex. The same accumulation can be seen in some fairy circles on sandy soils.
Fairy circles marked at one site in 1978 were all found to be intact in 2016. The latex presumably inhibits erosion.
Euphorbia damarana was found to take more than 40 years to decompose in the Namibian conditions. One historical aerial image was found from 1966 that showed enough detail to determine the location of Euphorbia gummifera plants.
The image comparison revealed that of the test sample of 406 E. gummifera plants that were alive in 1966, 69 were dead (with remains still present) and 134 of them were replaced by FCs in 2011...
Most fairy circles are circular because the Euphorbia plants are circular. A few fairy circles are elliptical, caused by two plants growing together or by distortion caused by flowing downhill.
It is also suggested that similar fairy circles in Australia may be caused by spinifex grasses that produce resins.
Good summary, proper science journalism:
http://conservationnamibia.com/blog/b20 ... ircles.php
Full, open access journal article:
https://bmcecol.biomedcentral.com/artic ... 20-00313-7
I rather love this passage from a previous summary of what was known about fairy circles in 2008:
Because, at the time, the idea that the Himba god created the fairy circles had as much scientific support as any of the other theories.Theron (1979) suggested that an allelopathic substance released from Euphorbia damarana Leach upon the death of the plants while Moll (1994) reasoned that termites was the cause of the circles. According to Becker & Getzin (2000), the Himba people from Kaokoland in Namibia, believe that the fairy circles were made by their god and have been there ever since. This is but one of many beliefs existing today, regarding the origin of these mysterious rings. Explanations range from impact points of meteorites, rolling spots for zebras, possible locations of Ovahimba huts, unknown flying objects (UFOs) (Albrecht et al., 2001).
https://repository.up.ac.za/bitstream/h ... sequence=1