Are these “mysterious circles” in the desert really the work of aliens?

In the desert grasslands of Namibia in southern Africa, there is a very strange natural phenomenon. On the surface of the arid grasslands, bare circular spots where plants cannot grow mysteriously appear . This phenomenon is called "fairy circles."

Image source: Reference [6]

Mysterious "Fairy Circles":

Strange vegetation distribution pattern

The Namib Desert has a unique landscape, desolate and yet magnificent. These circles are widely distributed in the arid grasslands along the Namib Desert, with as many as millions and diameters ranging from 2 to 12 meters. Each circle is bare sand and no vegetation can grow, while outside the circle is overgrown with weeds, and the grass on the edge of the circle is even more lush.

The "fairy circles" are evenly distributed throughout the vast arid grasslands of the Namib, forming a strange pattern. From a bird's eye view, they look like dense dots scattered on a piece of fabric.

In the local legends, these mysterious circles are "footprints of God". There are even stories that say there is a giant dragon lurking underground in the Namib Desert grasslands, and the dragon spews out poisonous gas underground, causing no grass to grow inside the strange circles. In addition, some people associate it with crop circles, believing that these circles are traces left by UFOs landing.

Overlooking the Namib Desert. Image source: Wikipedia

Endless controversy:

Opinions on “Fairy Circles”

For nearly half a century, scientists have been trying to crack the cause of "fairy circles", but there are many practical difficulties in reasonably explaining this mysterious natural phenomenon. The origin of "fairy circles" has been under constant debate, resulting in a variety of different theories.

There are two most mainstream views.

The first theory is that the death of plants in the circle is caused by termites gnawing on the roots of plants. In this view, animal activity was first hypothesized to be the cause of the formation of "fairy circles", especially the hypothesis of sand termites.

Sand termites. Image source: Wikipedia

German botanist Juergens and others believed that the sand termites living in the local grasslands were root-feeding, feeding on newly sprouted grass roots and thus killing the plants, resulting in the formation of bare areas without vegetation;

Or maybe termites eat grass leaves to prevent evaporation in order to prevent groundwater loss near their nests. This theory sounds simple, so it is easy for people to accept it.

But since then, many researchers have conducted extensive field excavations of fairy circles, and most of the findings have shown that there are no termites or their nests in the area, and that some fairy circles have no causal relationship with termites. There is no evidence that sand termites or other animals that live in fairy circles have caused root damage to plants in the circles.

The second theory is that the formation of fairy circles is related to the self-organizing behavior of plants. This theory holds that the formation of fairy circles is the result of plants competing for scarce water resources in Namibia. Plants extract water from the soil through their roots, and the competition leads to the death of plants in the circle due to drought. This theory has been further confirmed by the latest research results.

In the Namib Desert, the growth of annual grasses is an unstable, short-term response to rare precipitation events because the desert is extremely dry and precipitation is unpredictable. The formation of "fairy circles" is closely related to rainfall. After the rain, the newly grown grass only takes a few weeks to complete its life cycle. Therefore, if you want to solve the mystery of "fairy circles", you need to seize the opportunity in field surveys after rainfall.

Image source: Wikipedia

The fog is lifting:

A "race" for water resources among plants

German ecologist Getzin and others have been conducting tracking and investigations in the Namib Desert for many years in order to solve the mystery of "fairy circles".

In October 2022, they published their latest research results. The research team tracked and analyzed the rainfall along the coast of Namibia from 2020 to 2022, and evaluated the causes of grass death in the "fairy circles" at different time intervals after the rainfall that triggered the germination and growth of grass seeds, and also evaluated the impact of termites on plants in the circles.

Image source: Reference [1]

The research team used soil moisture sensors to measure soil moisture from the dry season to the rainy season to study how the newly emerged grass affects soil moisture content in space and time. The results showed that the cause of death of the grass in the "fairy circles" was not due to termite activity.

At the same time, the measurement and analysis results also found that there was no systematic difference in the water penetration rate between the fairy circles and the substrate. Therefore, it was concluded that the plant wilting was not caused by the faster penetration rate in the "fairy circles".

However, soil moisture measurement records show that after rainfall, the grass outside the "fairy circle" grew rapidly, seriously depleting the moisture in the upper soil of the "fairy circle".

It is precisely because of plant water stress that the grass in the "fairy circles" will die immediately after rainfall. In other words, the plants in the circle died due to lack of water because the plants outside the circle competed for water. This result shows that the "fairy circles" in Namibia are a self-organizing vegetation phenomenon caused by ecohydrological feedback.

In past studies, ecologists have always agreed that fairy circles can store groundwater. The soil moisture deep inside the fairy circles is several times higher than the soil moisture in the surrounding vegetation. However, higher soil moisture mainly occurs at a depth of 30 to 100 cm, and new plant seedlings can usually only germinate in the upper soil layer. The dry soil in the upper layer of the circle cannot support the survival of plants.

Image source: Wikipedia

According to the theory of vegetation self-organization , under drought conditions, the eco-hydrological feedback between the grasses around the "fairy circles" depletes the moisture in the upper soil layer. Therefore, the plants inside the gaps in the "fairy circles" will suffer from severe water stress and find it difficult to grow.

Without sufficient rainfall to provide enough water, vegetation recovery is almost impossible. The research team has witnessed bare soil turning back into dense grass after rare heavy rainfall.

In the Namib Desert, the formation of fairy circles is caused by high water uptake by plants and rapid lateral soil water diffusion relative to biomass expansion. The so-called "absorption-diffusion feedback" leads to an inverse spatial biomass-water distribution in the Namib sandy soils, resulting in the formation of vegetation gaps.

Based on previous field investigations by other research teams, it has been demonstrated that water can move horizontally more than 7 meters between fairy circles in a short period of time. This type of resource concentration and localized aggregation of biomass leads to the formation of self-organized patches on a global scale.

The gaps in the fairy circles and the periodic formation patterns of dryland-related vegetation stripes and spots reflect the lack of rainfall in the area to maintain the growth of a continuous layer of plants.

Namib Desert. Image source: Wikipedia

Overall, this study identified the eco-hydrological feedback effect and lateral soil moisture diffusion actively induced by the grasses around the fairy circles as the most likely mechanisms leading to water stress and ultimately rapid death of plants in the fairy circles.

Absorption-diffusion feedback destabilizes uniform vegetation, leading to so-called “Turing patterns”.

By the way, let me explain here that the famous "reaction-diffusion theory of morphogenesis" proposed by mathematician Alan Turing in 1951, so Turing's model is also called the reaction-diffusion mechanism, which has become the basic model of theoretical biology.

It describes how patterns in nature (such as dots, spots, stripes, and spirals) emerge naturally from a uniform state. These patterns are called Turing patterns.

Based on the Turing pattern theory, scientists can use mathematics to explain the stable structures and regular repetitive patterns formed in chemical and biological systems, such as the stripes of tigers and zebrafish, the spots on leopards, and so on.

Conclusion

In the Namib Desert steppe, grasses growing outside of fairy circles actively change soil moisture inside the circles, acting as "ecosystem engineers." They "benefit" from the extra water provided by the fairy circles by forming periodic vegetation gap patterns and preventing other grasses from growing inside them.

In the homogeneous sand of the Namib Desert, the extremely regular and spatially periodic aggregation of "fairy circles" creates high-density gaps between vegetation and provides water, the "source of life", to the grass.

This characteristic is crucial for plants to survive in environments with extremely scarce water resources, and it also leads to the emergence of strange vegetation gap patterns such as "fairy circles".

References

[1] Cramer MD and Barger NN. PLoS ONE, 2013, 8(8): e70876.

[2] Cramer MD, et al. Ecography, 2017, 40: 1210–1220.

[3] Getzin S, et al. PNAS, 2016, 113: 3551–3556.

[4] Getzin S, et al. PPEES, 2022, https://doi.org/10.1016/j.ppees.2022.125698

[5] Juergens N. Science, 2013, 339: 1618–1621.

[6] Kappel C, et al. Commun Biol. 2020, 3: 698.

[7] Tschinkel W R.PLoS One, 2015, 10: e0140099.

[8] Turing A M. Philos. Trans. R. Soc. B, 1952. 237: 37–72.

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Produced by | Science Popularization China

Author: Li Yin, School of Life Sciences, Sun Yat-sen University

Producer | China Science Expo

Editor: Yang Yaping

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