Fairy Circles | Namibia | Grass | Desert
Fairy Circles in Namibia – Interesting Information about the Mystery
Fairy circles are round, bare patches in the dry grassland that are characterized by an extremely ordered distribution in the landscape. Most fairy circles have diameters of 4-8 m, but they can be up to more than 20 m in diameter. They exist along the Namib Desert from northern South Africa to southern Angola and have recently been discovered by us also in Australia. In the following I would like to give you a short overview of my research activities on fairy circles. They are all based on cooperation with various colleagues.
First research activities in the year 2000
When I explored the fairy circles in early 2000 in the north-west of Namibia, nearly nothing was known about these mysterious circles. No extensive data basis existed by that time and just very few hypotheses about the origin of fairy circles were mentioned in either Afrikaans language (Theron 1979, Eicker et al. 1982) or in a conference proceeding (Moll 1994). Our publication in the journal Basic and Applied Ecology (Becker & Getzin 2000) was the first scientific paper on the “fairy circles” topic that was listed in the Web of Science. This Web of Science is a subscription-based scientific citation indexing service originally produced by the Institute for Scientific Information which includes publications in peer-reviewed journal. The two competing hypotheses that existed until that time were 1) the allelopathic interaction between poisonous Euphorbia damarana plants and grass vegetation where the decomposed Euphorbia remains are thought to kill the grass (Theron 1979) and 2) harvester termites of the species Hodotermes mossambicus are clipping grass and cause a circular bare-soil patch (Moll 1994). While the Euphorbia hypothesis cannot explain the recurrent formation of new fairy circles in areas that are completely lacking Euphorbia bushes, on our excursion in 2000 we did indeed find harvester termites that were clipping off grass stems and likely caused a vegetation-free patch. We also recorded how heat-tolerant carnivorous Ocymyrmex ants were preying on the harvester termites that suffered from thermal distress at temperatures above 40°C, leading to the hypothesis that this heat sensitivity of the termites and predation by ants could explain the limited size of fairy circles.
A new, holistic research approach
However, these early observations and hypotheses in support of the termite theory have one crucial shortcoming – that is the field recordings were done on a very local scale of evaluating potential processes just for the individual fairy circle. But later work has demonstrated that many fairy circles in Namibia show no sign of termite activity – neither past nor present. Also, these arid grasslands along the Namib Desert are largely lacking ecological resistance, hence multiple disturbing agents can disrupt the grass layer and a circular bare-soil patch may form. More than ten years later and having previously unavailable new data and methods accessible, I had to reconsider that any valid hypothesis about the origin of fairy circles must be able to explain also the detailed characteristics of the spatial patterning at small scales and large landscape scales, as well as the fact that fairy circles do only occur within a very narrow climatic envelope. That is, the core area of their occurrence is found between 70-120 mm mean annual precipitation, which is only a very narrow strip of a few dozen kilometers from west to east along the Pro-Namib. One research discipline that made tremendous progress in the first ten years of the new Millennium is called Spatial Ecology where plant and animal distributions are precisely analyzed with spatial statistics and null models that correspond to null hypotheses or put simply, to ecological questions. These methods which are primarily based on point-pattern analyses allow spatially-explicit hypothesis testing for adequate pattern-process inference. That means at first the “spatial fingerprint” of the inherent structure of a pattern has to be identified and then the question is which known process is most likely – or on the contrary unlikely – to cause that particular pattern characteristics. The change of pattern characteristics with scale from a few meters to a few hundred meters and beyond is thereby of central interest. During my Ph.D. between 2003 and 2006 when I specialized into this still quite novel type of hypothesis testing, these methods became more and more powerful and sophisticated due to better computers and more precise and extensive spatial data based either on high-resolution aerial imagery or modern GPS recordings.
Vegetation self-organization as a new explanation
In 2009, after a visit of the fairy circles in the Kunene, I acquired aerial imagery of fairy circles from the Marienfluss Valley and the Giribes Plains. During that time fairy circle research was still relatively “inactive” when compared with the tremendously busy research efforts nowadays. After the year 2000, only five more articles on the topic “fairy circles” appeared in the Web of Science until 2011, with on average one new paper being published every second or third year. In 2012 and 2013, each year two articles appeared and since 2015 we do have a real “boom” with up to ten new papers appearing every single year. Meanwhile, three main hypotheses crystallized between 2011 and 2015: that is 1) the hypothesis on toxic, abiotic gas-leakage from deep-lying underground reservoirs that would cause plant death where gas “chimneys” reach the soil surface, 2) the well-known insect hypothesis that was added with new observations or correlations on ants and sand termites as potential causal agent, and 3) the vegetation self-organization hypothesis claiming that fairy circles are natural patterns that appear just by plant competition for the scarce soil water. The latter hypothesis argues that the vegetation-gap pattern, hence the bare soil of the circles, is an expression of water shortage along the Pro-Namib because there is not enough rainfall to sustain a continuous layer with grass vegetation. The so-called pattern-formation theory from physics predicts that in that case, strongly ordered gap patterns will emerge. Based on the purchased aerial images from 2009 and spatially-explicit analyses of the fairy-circle distributions, we evaluated these three main hypotheses on the origin of fairy circles in a new article that was published in May 2014 in Ecography. We searched through all available publications on gas and termite distribution patterns and found that there is no evidence that would show the ability of these two agents to cause the extremely ordered and at the same time large-scale homogeneously distributed pattern that is such a unique characteristic of the fairy circles. Instead, we support the hypothesis of vegetation self-organization because not only our process-based model was able to reproduce the exceptional ordering of fairy circles but also all knowledge on plant self-organization does support this hypothesis. We are following a new line of thinking where it is appreciated that plants may be a lot more “intelligent” than was previously thought. Notably, the general theory about vegetation-pattern formation along a moisture gradient such as the growth of spots, stripes or gaps is relatively novel in ecological science because this theory was mainly rooted in physics and mathematics and thus it was published in journals that are usually not red by ecologists.
CNN: Nature’s greatest mystery?
By systematically reviewing the knowledge about the ability of various agents to cause such patterns and by refuting the termite hypothesis as unlikely, we received attention in the public media. In June 2014, CCN took up our work and wrote an article with the exemplifying title Namibia’s ‘fairy circles’: Nature’s greatest mystery? In September 2014 the BBC joined in with a title Mystery fairy circles defy explanation. All the controversy and the public attention was and is good for science. In February 2015, the First Fairy Circle Symposium took place at Wolwedans where various active fairy circle researchers met to discuss the different hypotheses. In 2015, we also published a new article in Ecological Entomology which was again in support of the vegetation self-organization hypothesis and which explained more in detail the pattern characteristics.
Discovery of fairy circles in Australia
Our explanation of fairy circles as a universal expression of self-organized plant patterns got considerable momentum when I received in 2014 an e-mail with an aerial image of an apparently similar phenomenon being found 10,000 km away from Namibia in the remote outback of Western Australia. Up to that time genuine fairy circles, which are defined by their exceptionally rare spatial pattern, were unknown from any other place except the Namib Desert. Soon, spatial analysis of those “new” fairy circles verified that the Australian gap pattern must be indeed true fairy circles and not just common, less ordered insect-induced gap patterns that are often known from other parts of world. In December 2014 we flew to Newman in the remote Australian outback and investigated those fairy circles for the first time. It turned out that these Australian fairy circles are even in stronger support for the self-organization hypothesis because at one and the same place the grasses there may form circular gaps and stripe-like patterns in response to water stress. In contrast, termites as primary causal agent could be excluded after digging at typical fairy circles sites. These study results were published in March 2016 in the journal Proceedings of the National Academy of Sciences. In short, we postulated in this article that identical spatial patterns may globally emerge if they are triggered by the same type of natural instability. The gaps in Namibia and Australia both function as an important extra source of water for the surrounding grass vegetation in this arid environment which was well illustrated in a graph published in New Scientist. Other reports followed in the BBC, The New York Times, The Washington Post or The Namibian and they testify to the general interest in unresolved nature puzzles.
During your 12-day tour through Namibia, I will gladly share my knowledge about the fairy circles with you. I hope you will enjoy the beauty of the fairy circles, as well as the scientific debate which they have triggered.
Welcome to NAMIBIA-ECO-TOURS. Due to my studies in Namibia and interest in dryland ecology, I have known this exceptionally beautiful country for more than 20 years now. I invite you to an exclusive tour through Namibia.