Experiments Testing the Causes of Namibian Fairy Circles

Andrew C Singer,Walter R. Tschinkel

doi:10.1371/journal.pone.0140099

Abstract

The grasslands on the sandy soils of the eastern edge of the Namib Desert of Namibia are strikingly punctuated by millions of mostly regularly-spaced circular bare spots 2 to 10 m or more in diameter, generally with a margin of taller grasses. The causes of these so called fairy circles are unknown, but several hypotheses have been advanced. In October 2009, we set up experiments that specifically tested four hypothesized causes, and monitored these 5 times between 2009 and 2015. Grass exclusion in circles due to seepage of subterranean vapors or gases was tested by burying an impermeable barrier beneath fairy circles, but seedling density and growth did not differ from barrier-less controls. Plant germination and growth inhibition by allelochemicals or nutrient deficiencies in fairy circle soils were tested by transferring fairy circle soil to artificially cleared circles in the grassy matrix, and matrix soil to fairy circles (along with circle to circle and matrix to matrix controls). None of the transfers changed the seedling density and growth from the control reference conditions. Limitation of plant growth due to micronutrient depletion within fairy circles was tested by supplementing circles with a micronutrient mixture, but did not result in differences in plant seedling density and growth. Short-range vegetation competitive feedbacks were tested by creating artificially-cleared circles of 2 or 4 m diameter located 2 or 6 m from a natural fairy circle. The natural circles remained bare and the artificial circles revegetated. These four experiments provided evidence that fairy circles were not caused by subterranean vapors, that fairy circle soil per se did not inhibit plant growth, and that the circles were not caused by micronutrient deficiency. There was also no evidence that vegetative feedbacks affected fairy circles on a 2 to 10 m scale. Landscape-scale vegetative self-organization is discussed as a more likely cause of fairy circles.

Highlights

The eastern edge of the Namib Desert is home to a sparse grassland of Stipagrostis obtusa and S. uniplumis punctuated by millions of quasi-circular, mostly regularlyspaced bare areas from 2 to 12 or more meters in diameter, often somewhat concave, and with a perimeter of taller grass
In 2004, van Rooyen et al [1] evaluated all hypotheses that had been published at the time, and found that almost none were supported by the weight of available evidence
Causation by toxic gases or vapors emanating from subterranean hydrocarbon seeps or termite nests [3, 6, 7, 10] clearly falls short on this account, as was clearly explained by Getzin et al [28], since the hypothesis offers no explanation of the regularity, regional occurrence and size of the circles, nor their dynamic nature

Summary

Introduction

The eastern edge of the Namib Desert (the Pro-Namib) is home to a sparse grassland of Stipagrostis obtusa and S. uniplumis punctuated by millions of quasi-circular, mostly regularlyspaced bare areas from 2 to 12 or more meters in diameter, often somewhat concave, and with a perimeter of taller grass (usually S. ciliata and S. giessii). Cramer and Barger [2] determined the local conditions of rainfall, seasonal variation and soil associated with fairy circles at several sites, and showed that these factors successfully predicted the occurrence of fairy circles on a regional scale, identifying a narrow band where these conditions pertained They showed that the mean size of fairy circles increased with aridity, a finding in keeping with predictions based on models employing facilitation-competition interactions [18, 19]. Models in which plants draw some limiting resource to themselves thereby impoverishing more distant soil create a short-range positive feedback (i.e. facilitation) and a long-range negative (i.e. competitive) feedback Such self-organization models have been shown to result in a variety of vegetation spatial patterns whose specific geometry (bands, spots, stripes, gaps) depend on the relative scale of the positive and negative feedbacks [18,19,20,21,22,23,24,25,26,27,28]. These were (1) subterranean toxic vapors or gases; (2) micronutrient depletion; (3) plant inhibition by fairy circle soil; (4) fairy circle neighborhood interactions

Materials and Methods

Results

Discussion