The Life Cycle and Life Span of Namibian Fairy Circles

Walter R Tschinkel,Kimberly Patraw Van Niel

doi:10.1371/journal.pone.0038056

Abstract

In Namibia of southwestern Africa, the sparse grasslands that develop on deep sandy soils under rainfall between 50 and 100 mm per annum are punctuated by thousands of quasi-circular bare spots, usually surrounded by a ring of taller grass. The causes of these so-called “fairy circles” are unknown, although a number of hypotheses have been proposed. This paper provides a more complete description of the variation in size, density and attributes of fairy circles in a range of soil types and situations. Circles are not permanent; their vegetative and physical attributes allow them to be arranged into a life history sequence in which circles appear (birth), develop (mature) and become revegetated (die). Occasionally, they also enlarge. The appearance and disappearance of circles was confirmed from satellite images taken 4 years apart (2004, 2008). The frequency of births and deaths as a fraction of the total population of circles allowed the calculation of an approximate turnover rate, and from this, an estimate of circle lifespan. Lifespan appeared to vary with circle size, with small circles averaging about 24 years, and larger ones 43–75 years. Overall lifespan averaged about 41 yr. A second, independent estimate of lifespan was made by revisiting circles 2 to 9 years after their clear status had been confirmed. This resulted in a lifespan estimate of about 60 years. Any causal explanation of fairy circles must include their birth, development and death, their mean lifespan and the variation of their features under different conditions.

Highlights

The eastern edge of the Namib Desert of southwestern Africa is home to a mysterious phenomenon called ‘‘fairy circles’’–nearly circular barren patches within a sparse matrix of small short-lived grass species (e.g. Stipagrostis uniplumus (Licht.) De Winter)
The phases of the Life Cycle Of Fairy Circles–Birth, Maturation, Enlargement and Death There was a great deal of variation in details of structure and condition of fairy circles, allowing circles to be categorized into a sequence from first appearance as a bare spot in the grassy matrix, acquisition of a rim of tall bushman grass, and revegetation of the bare area within the grass rim, and the disappearance of the grass circle, leaving an area indistinguishable from the matrix
Newly formed circles were visible as a bright, reddish area in 2008, but this same spot was indistinguishable from the matrix in 2004 (Fig. 4A–I)

Summary

Introduction

The eastern edge of the Namib Desert of southwestern Africa (the pro-Namib) is home to a mysterious phenomenon called ‘‘fairy circles’’–nearly circular barren patches within a sparse matrix of small short-lived grass species (e.g. Stipagrostis uniplumus (Licht.) De Winter). The patches are often surrounded by a halo of taller grass (Stipagrostis giessii Kers or S. hochstetteriana (Beck ex Hack.)) and range from about 2 to 12 m in diameter. They are conspicuous in aerial or satellite photographs as well as on the ground, and occur by the hundreds of thousands in quasi-regular spacing [1] from southern Angola to northern South Africa wherever the soil is sandy and the rainfall is between 50 and 100 mm per annum [1,2]. Tschinkel [10] found no association between the nests or underground foraging tunnels of the endemic termite Baucaliotermes hainseii and fairy circles, nor have other termite species been found to be associated with fairy circles [2,9]

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Conclusion