Mysterious rings: In Australia’s arid landscape, grasses grow naturally in circular patterns. In these “fairy circles”, a ring of grass surrounds a bald spot. How and why these structures come about has now been unraveled by researchers. Accordingly, interlinked feedbacks between rain, soil conditions and plants ensure that these circles are self-organized and are maintained.
The so-called fairy circles occur mainly in dry grasslands in southern Africa such as Namibia or in northwest Australia. These are circular bald spots, but they are surrounded by a ring of strong, grown grass. These edges of the rings act like oases in their dry locations. With a diameter of up to ten meters, the conspicuous circles are arranged at regular intervals in a grid pattern.
What is the cause of the fairy circles?
In order to explain the rare phenomenon of fairy rings, researchers have already looked for toxins in the ground, suspected digging ants as the cause or certain geochemical conditions. Others assumed that the grasses form these circular patterns by themselves through a process of self-organization and feedback. Mathematical models also explained the origin of the ring pattern. In order to get to the bottom of these hypotheses in practice, Stephan Getzin from the University of Göttingen and the Helmholtz Center for Environmental Research in Leipzig and an international research team examined the growth of grasses in a natural location in Newman, Australia.
Preliminary analyzes of the Triodia grasses, which are only native to Australia, have already shown that significantly less rainwater seeps into the bald spots inside the rings than outside the circles. Based on more than 150 excavations in the ground, it has also been proven that this effect and the grass rings also occur where no ants are present. Following on from this, Getzin’s team now used a drone and multispectral camera to investigate how strong and well the Australian grasses are growing. For this purpose, the area was divided into five plots, each one hectare in size, and the grasses were classified according to high and low vitality. The team also continuously recorded data from a weather station.
Marked by positive feedback
The result: the hardened soil inside the fairy rings favors the growth of the plants on the edge of the circular pattern. “The highest plant cover was formed directly at the edges,” said the scientists. “This is a strong indication that the grasses near the fairy rings must benefit from the large gaps.” And Getzin and his colleagues were also able to explain this observation: “The vegetation benefits from the additional runoff water caused by the large fairy circles is provided, and thus keeps the dry ecosystem functional even under very inhospitable, dry conditions. “
At the same time, the bald spots themselves ensure that they stay dry and bare: during one of the rare but heavier downpours, the rainwater does not seep into the encrusted bald spots in the middle of the rings. “The soil moisture after rainfall was lowest within the fairy rings with their weathered surface,” report Getzin and his team. Instead, the water drains sideways and flows into the grassy edges of the circles. Over time, this leads to a self-reinforcing feedback, which creates the regular pattern of the fairy rings: the raindrops pound the hard ground firmly, thereby increasing the encrustation. The water drainage inside the circle permanently prevents plant growth there. At the edge, on the other hand, the increased runoff promotes plant growth.
Plants create favorable conditions for themselves
The researchers also found that the protective grass cover also lowers the surface temperature of the soil in these areas: during the hottest part of the day, the subsoil can reach up to 75 degrees Celsius – but the plants reduce the temperature by around 25 degrees. This makes it easier for new grasses to germinate and grow where there are already plants. The tufts of grass also provide more shade and allow more water to penetrate the nearby roots – this is also a self-reinforcing process.
“The key is that the grasses actively shape their own environment by creating symmetrically arranged gap patterns,” says Getzin, summarizing the results. The rings of the Australian fairy circles result from the interactions of the grasses, the soil and the rainwater. In doing so, the researchers confirm that the plants secure a favorable environment for themselves to a certain extent. “Without the self-organization of the grasses, this area would likely become a desert dominated by bare ground,” adds the expert. The fact that the grasses redistribute their own water resources and thus change their physical environment makes them “ecosystem engineers”.
Fairy circles as a Turing pattern
And the work of the researchers brought another insight: For the first time, the mathematical theory of the “Turing pattern” could be tested in such an ecosystem. The model of the scientist Alan Turing, for example, already explained the origin of the dot pattern in Dalmatians or the stripes of zebras. In this he justifies the different distribution of the pigments of the animals with a “reaction-diffusion-mechanism” in which two actors or substances interact with each other in such a way that self-organized boundaries between their areas of influence form and a pattern is created.
Earlier modeling of the Australian fairy circles had already indicated that this theory could also apply to the fascinating vegetation patterns. The findings of Getzin and his team now confirm this. This not only facilitates the understanding of ecosystems that shape themselves under extreme conditions – such as the heat in Australia – but also enables realistic modeling for the first time.
Source: Georg-August-Universität Göttingen, Article: Journal of Ecology, doi: 10.1111 / 1365-2745.13493