Otherworldly fairy circles reveal "optimistic signal for recovery"

The human eye is drawn to circular patterns in nature, from crop circles in the movie Signs to the ripples in a pond.

But there's another awe-inspiring, naturally-occurring, circular pattern that deserves far more recognition in the public consciousness, according to new research published in the journal Science Advances: fairy circles.

According to the researchers, one kind of fairy circle may be a symbol of greater ecosystem resilience in the face of environmental catastrophe, like climate change.

"Our new findings suggest that transient [fairy circle] patterns play an optimistic signal for recovery ecosystems, where they have high resilience," Quan-Xing Liu, corresponding author on the study and an environmental sciences professor at East China Normal University, tells Inverse.

Here's the background — Otherworldly fairy circles dot landscapes all around the world, from salt marshes along the Chinese coastline to more arid environments in Namibia.

"A fairy circle — as we observe them in Chinese emerging saltmarshes — is a circular patch of grass [that] is barren in the middle," Johan van de Koppel, also a corresponding author on the study and a senior researcher at Royal Netherlands Institute for Sea Research, tells Inverse.

Due to ecological processes affecting species formation, these fairy circles also have less vegetation in the middle than the outer portions, creating the appearance of a ring.

They can also form "rings within rings," or concentric rings, according to de Koppel, similar to the age-growth rings that form in trees.

Typically, research on fairy circles focuses on the more stable Turing-like fairy circles, named for the genius British mathematician, Alan Turing. Turing also popularized the self-organization theory, which focused on how certain chemical patterns form in nature.

But in this study, researchers wanted to study transient fairy rings, which form and behave differently than the more persistent, ecologically stable Turing-like circles.

"Comparing with well-known Turing patterns, the fairy circle is one of the spatially self-organized patterns, but it has not [received] widespread attention," Liu says.

The scientists wanted to know: are transient fairy rings resilient to ecosystem changes in the same way as Turing-like fairy circles?

How they did it — Between April 2018 and July 2019, the researchers conducted field experiments and captured drone images of salt marshes near Shanghai. They also compared their field research with statistical models and computer simulations.

Through their research, the scientists tested two possible mechanisms which affect fairy circle formation: sulphide toxin accumulation and nutrient restriction.

De Koppel explains the sulphide toxin mechanism as a process in which "oxygen levels in the soil can drop" due to the accumulation of carbon in the middle of the fairy circle patch.

The accumulation of carbon produces a toxic substance, sulfide, "that can kill the plants" in the middle, thereby producing a ring appearance.

The second process — nutrient depletion — creates fairy circles differently, according to De Koppel:

A second ring-forming process is that the plants in the middle have no access to nutrients, as they are filtered out of the water by the plants on the outside of the ring.

What's new — Surprisingly, the researchers confirmed both of these chemical processes in fairy circle formation.

According to Liu, the sulphide toxin method can explain how these fairy patterns transform from circles to rings, but it can't explain the formation of concentric fairy circles, which contain rings within rings.

Liu points out that "'ring' and 'concentric ring' types of 'fairy circles' often coexist in saltmarsh ecosystems in the same region," but form by different mechanisms.

Instead, the researchers found evidence for the formation of concentric rings in the nutrient depletion hypothesis. The researchers also fertilized the fairy circles with nitrogen to confirm the nutrition depletion method.

"Through a controlled experiment with the addition of nutrients, we further found that the nutrient depletion mechanism has a dominant contribution to the self-organizing patterns of such 'fairy circles,'" Liu says.

Taken together, these two mechanisms create a new theoretical model for the formation of transient fairy rings.

"More convincingly, the new theoretical model is different from the classic Turing paradigm of 'fairy circles' in that the self-organizing patterns produced by the new model are not mutually exclusive," Liu says.

Why it matters — Turing rings typically "self organize" or form by chemically repelling against each other, leading to the formation of distinct, separate rings.

But the fairy circles in this study instead merge with each other, contradicting the Turing phenomenon. The researchers suggest that this merging will lead to fairy circles that can ultimately return to a more plant-heavy, resilient state following ecological disaster.

"On the contrary, the adjacent vegetation patches will merge with each other due to expansion, and finally the spatial homogeneous vegetation state will become the final stable state of the whole world," Liu says.

By contrast, Turing-like fairy circles may suffer a "possible collapse to a degraded state once a tipping point is approached," according to the study.

One of the most well-known ecological tipping points: climate change.

What's next — Fairy circles can help scientists figure out which chemical processes are limiting plant formation in salt marshes, which can affect the long-term health of these ecosystems.

If the researcher's theoretical model of transient fairy circles is correct, then salt marshes might survive the climate crisis without irreversible ecosystem loss.

"Thus, such 'transient behavior' self-organizing patterns imply higher resilience of salt marsh ecosystems," Liu says.

Abstract: Spatial patterning is a fascinating theme in both theoretical and experimental ecology. It reveals resilience and stability to withstand external disturbances and environmental stresses. However, existing studies mainly focus on well-developed persistent patterns rather than transient patterns in self-organizing ecosystems. Here, combining models and experimental evidence, we show that transient fairy circle patterns in intertidal salt marshes can both infer the underlying ecological mechanisms and provide a measure of resilience. The models based on sulfide accumulation and nutrient depletion mechanisms reproduced the field-observed fairy circles, providing a generalized perspective on the emergence of transient patterns in salt marsh ecosystems. Field experiments showed that nitrogen fertilization mitigates depletion stress and shifts plant growth from negative to positive in the center of patches. Hence, nutrient depletion plays an overriding role, as only this process can explain the con-centric rings. Our findings imply that the emergence of transient patterns can identify the ecological processes underlying pattern formation and the factors determining the ecological resilience of salt marsh ecosystems.