Circles of barren land, ranging from one to several feet in diameter, appear and disappear spontaneously in Namibian grasslands. The origins of these ‘fairy circles’ remain obscure, and have been attributed to causes ranging from the fantastic (the poisonous breath of a subterranean dragon) to those backed by more evidence, such as the work of a soil termite. A recent PLOS ONE paper suggests another possibility: Patterns that emerge during normal plant growth. Author Michael Cramer elaborates on the results of this study:
How did you become interested in studying the Namibian fairy circles, and are similar circles seen elsewhere?
It would be hard not to be intrigued by these mysterious barren circles on the edge of the spectacular Namibian sand sea! These circles are also reminiscent of soil mounds in other places, for example mima mounds in the US, “heuweltjies” in South Africa and “campos de murundus” in South America that have primarily been ascribed to faunal activity. Like fairy circles, these mounds may, however, represent a distinct product of patterns formed by vegetation. My co-author, Nichole Barger, became intrigued by both these phenomena while I was on sabbatical in her lab.
Many other scientific ideas have been proposed to explain the occurrence of these circles. What’s missing from these explanations?
Any explanation of fairy circles has to provide a plausible mechanism for regular spacing of these relatively large circles in the landscape. The most common explanation to date has been that termites cause the circles. While it is undoubtedly true that ants, termites and other fauna do occur in the circles and may play a role in maintenance of the circles, we suggest that inter-plant competition is the primary cause that drives circle formation. This places plant competition in focus as a possible mechanism for determining the shape, size and distribution of the circles.
What made you think the patterns could be formed by plant growth patterns themselves?
We stood on the shoulders of giants! Previous studies have alluded to vegetation patterning as a possible cause. Other researchers have also produced computer models to predict fairy circle occurrence and found plant growth may play a role. More generally, understanding of spatial patterns formed by plants and the realization that this emergent phenomenon is common in arid landscapes has increased recently. Several groups have produced mathematical models that explain the production of vegetation patterns (gaps, bands and spots) and show that increasing aridity can result in transition from one pattern to another.
We adopted two approaches. We used Google Earth to obtain images of sites across Namibia, analyzed these to determine circle morphological characteristics, and then combined the images with environmental data to predict the distribution of fairy circles. We performed ground surveys to measure circle morphology and collect soil samples. Soils were sampled at various depths and regular intervals inside and outside the circles and analyzed for water and nutrient contents.
What did you find?
We found that we could predict, with 95% accuracy, the distribution of fairy circles based on just three variables. Rainfall strongly determined their distribution, and differences in rainfall from year to year may thus explain why circles dynamically appear and disappear in this landscape. The patterns of moisture depletion across the circles are also consistent with plant roots foraging for water in the circle-soil. The size and density of the circles is inversely related to resource availability, indicating that bigger circles occur in drier areas and where soil nitrogen is lower.
Do the data in this study strengthen previous results or disprove any older explanations for the circles?
Our results corroborate previous results and extend them, but we have interpreted the results in a novel manner. Since our study was correlative, i.e: we correlated the occurrence of fairy circles with certain environmental conditions, it does not disprove existing hypotheses. Direct experiments that result in fairy circles being created or closing up are perhaps the only way to prove or disprove any of these ideas.
Circular grass rings do occur in many contexts. For example, Stipagrostis ciliata in the Negev and Muhlenbergia torreyi (ring muhly) in the US (e.g. New Mexico, Utah) form rings. The distinction is that these are much smaller (ca. < 1 – 2 m diameter) and less regularly spaced than fairy circles. Nevertheless, their origins may have some commonalities with fairy circles. The special circumstance that results in the spectacular Namibian fairy circles may be the fact that the soils are very sandy and homogenous.
More generally, the fairy circles represent an example of how patterns formed by growing plants can create heterogenous spaces in otherwise homogenous grassland. Differences in soil moisture or composition across the span of a fairy circle can provide habitat for both grasses and fauna that would otherwise not thrive in this arid environment.
Citation: Cramer MD, Barger NN (2013) Are Namibian “Fairy Circles” the Consequence of Self-Organizing Spatial Vegetation Patterning? PLoS ONE 8(8): e70876. doi:10.1371/journal.pone.0070876
Images: fairy circles by Vernon Swanepoel (top); images below from 10.1371/journal.pone.0070876