Earlier this year, a paper published in the journal Science (and reported here on PS) claimed to explain the long-standing puzzle of the African ‘Fairy Circles’ — the strange circular patches of barren, reddish- earth ringed by tall grasses that range in size from a few to several yards in diameter. The circles, which appear and disappear from time to time, occupy a large area of the African landscape, from Angola to South Africa.
But as compelling as that March 2013 paper (Juergens et al) was, it did not put the mystery to rest.
In a new paper published on the PLoSONE website, ecologists Michael D. Cramer and Nichole N. Barger posit a new theory explaining the curious circular patches based upon their computer modeling of vegetation “spatial patterning” and competition.
The Juergens paper cites evidence of extensive termite tunneling several inches beneath the top soil of the inner portion of the circles. Termites feasting on the roots of any plants that would grow there tend over time to remove those plants, creating a moisture sink, and leaving a ring of grass around the circumference of the termite colony. The theory challenges or perhaps clarifies the claim by another researcher that the circles were “living” things. Locals believe that ‘faeries’ make them.
However, Juergens’ theory did not fully explain how the patches become mostly circular, nor why some seemed to ‘disappear’ while others suddenly appear out of nowhere. Still, both theories do agree on one thing: the circles are not the result of purely “mechanical”, self-organizing forces (as with sand dunes and “cloud streets”). Such formation require only positive feedbacks (due to a compiling of interacting forces). No, there is something decidedly biogenic about these fairy circles.
The Newest Theory of Fairy Circle Formation
In this newest study, the scientists focused on circles found in Namibia (a favorite region amongst fairy circle researchers), measuring the “edaphic properties of fairy circles and variation in fairy circle size, density and landscape occupancy (% land surface).” Additionally, they examined these “edaphic” (i.e., soil-related) properties and water availability on a local scale (<50 km) along with vegetation and climate details at a regional scale (greater than 50 km)
All this data was then fed into a computer model. Both the data, and model predictions based on it, suggested that competition over nutrient and water resources and “positive facilitative feedbacks” cause the circular spatial patterning. The circles are, quite simply, “resource reservoirs” (Juergens also notes this role, citing a different cause).
Fairy circles are hypothesized to result from competitive exclusion of grasses on the fairy circles and facilitative access of peripheral species to both fairy circle and matrix water/nutrient resources. Lack of vegetation results in increased soil moisture within the circles. Competition, particularly in surface soils, may compromise grass growth on the fairy circles restricting root development and resulting in death of grasses that do invade the fairy circles. Increased faunal activity due to higher circle soil moisture and lack of propagule establishment may also contribute to maintenance of the barren circles. Arrows show water flux and consequent nutrient mass-flow from the matrix and fairy circle “reservoir” towards deep peripheral grass roots. Nominal sizes of grasses, rooting depths (left axis) and soil moisture variation (blue line, right axis) with distance from fairy circle center are derived from data presented (Fig. 3).
doi:10.1371/journal.pone.0070876.g002
In a desert environment, different plant species compete for critical resources (soil nutrients and trapped moisture) producing winners and losers in the survival game. As the weaker plant species die, this stimulates the growth of neighboring/competing plants. This creates a vegetation “gap” which continues to expand until a critical size is achieved (limiting the competitive behavior). The researchers also note a moisture gradient present decreasing in depth from the interior to the periphery of the circles (See the inserted image to the left).
It is informative to note that the circles do not appear in regions further west where the climate is even drier. This suggests a certain ‘Goldilocks” factor operating in fairy circle growth: a just right zone of not too much but not too little rainfall.
Also, it is speculated that due to these just right conditions for fairy circle growth, local, transient changes in rainfall patterns (up or down) may be causing some circles to appear and others to disappear.
One other fairly circle fact that both this and the previous study show is that the rings of tall grass that outline the circles are not uniform in terms of types of grass; they exist in a “matrix of smaller grasses” which may help the taller grasses survive in the harsh (“resource-impoverished’), Namibian landscape that otherwise precludes their flourishing. Further, like tiny oases, the grass rings seem to be magnets for small fauna. And these animals too contribute to the maintenance of the circles.
The researchers theorize that Fairy Circles “may thus represent an emergent phenomenon in which peripheral grasses, possibly together with fauna, participate in the construction of their own resource-niche and a faunal habitat in an arid and nutrient-impoverished landscape.”
Nature provides for Itself in remarkable ways.
Will this latest theory settle the debate and mystery once and for all? Not likely…the only way to know more definitively is with more field research in which actual circles can be manipulated and controlled for various factors and the results observed in real-time.
The research was published under the title: ‘Are Namibian “Fairy Circles” the Consequence of Self-Organizing Spatial Vegetation Patterning?’ on PLoSONE
Some material for this post came from the Science Shot article ‘Fairy Circle Mystery Gets New Explanation’
Top Photo: Fairy circles close to “Jagkop” (−24.9770°, 15.8982°) on the NamibRand Nature reserve (June 2012). Site overview (A); a 9.5 m diameter fairy circle being measured (B; the persons in the photograph have given written informed consent, as outlined in the PLOS consent form, to publication of their photograph); grasses do invade the center of some fairy circles, but by this time in the dry season they were mostly dead (C); evidence of surface runoff across a fairy circle following rain (D; photograph by Ann Scott). source: doi:10.1371/journal.pone.0070876.g001
