02 April 2013

"Fairy circles" in the Namibian desert - updated


As reported in PLoS One -
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...

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)... Overall lifespan averaged about 41 yr.
Several hypotheses of the causes of these circles were reviewed and evaluated by van Rooyen et al. but to date, none are well supported and the formation of the circles remains a mystery. Although soil taken from the barren center did not inhibit germination of grass seed, it supported plant growth poorly, whereas that from the edge and matrix allowed growth, but these effects were not consistent for all areas or sampled years. More recently, Jankowitz et al. found that grass growth in pots was poor inside the circles but not the matrix when the pot was open below, but not when it was closed, and suggested a semi-volatile toxic factor. Neither macronutrient nor soil microbiota differences between circle and matrix soil could account for fairy circles...
There's lots more information (and photos) at the source link, including the one at the right, which shows a newly-formed circle, demonstrating that the vegetation in the center dies rather than being scraped away by physical processes.

Had I been asked to guess, I would have thought there must be some fungal hyphae spreading underground, as in the conventional fairy rings,* but the authors report no differences in microbiota (presumably including fungi) and they favor a "landscape scale, self-organizing process" resulting from vegetative competition (see their discussion).

Very interesting.

Via the New York Times, and Neatorama.

Update:
When I posted this last year, there was speculation in the Comments that the circles were the product of the activities of termite colonies.   This week both the BBC and New Scientist are reporting on a Science article supporting the termite hypothesis.
When Norbert Jürgens of the University of Hamburg in Germany examined hundreds of fairy circles in over 2000 kilometres of the Namib desert in southern Africa, he found a species of sand termite, Psammotermes allocerus, was the only organism consistently present.

Jürgen observed the insects eating grass roots, killing plants before they could sprout through the soil. With no foliage for water to transpire from, the water is retained in the ground beneath the patches, helping the termites survive the dry conditions. Their burrows make the soil more porous, allowing rainwater to percolate down to the cooler, deeper sand, again reducing the amount of water lost from the surface.
With a tip of the blogging hat to reader William D. Richards for helping me track down this information.

*coincidentally I ran across this photo of a traditional "fairy ring" today at The Soul is Bone:


Clouded agaric(Clitocybe nebularis) - Fairy Ring.  Photo credit: Keith Wilson

9 comments:

  1. Strange... They do have a distinct 'petri-plate' look to them, kind of like phages spread on a lawn of bacteria. Do they appear suddenly, or do the patches 'grow'?

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    1. The answer to your question is in the link...

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  2. I have seen similar evenly-spaced bare patches in the deserts of eastern Oregon. The ones here are obviously caused by ants. I doubt they missed something so obvious.

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  3. Common known as 'heuveltjies' here in southern Africa, they're a common feature of the arid western regions, almost certainly caused by termite activity. I had a look at the PLOS article, and although it's interesting and detailed work, I'm not sure why the author thinks the "causes of these so-called “fairy circles” are unknown". Actually, reading deeper in the comments, I notice he says " I did not delve deeply into causes because my purpose was to describe the variation of circles, to sequence the life cycle and to estimate the life span." Ah, the benefits of peer review :)

    The image at the top is a great one, though, and the science of heveltjies is fascinating. For those wanting to read deeper, there are a lot of published journal articles:
    http://www.sciencedirect.com/science/article/pii/S0140196312001632#
    http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.2006.00189.x/pdf
    http://web.uct.ac.za/depts/geolsci/downloads/Potts_etal_09.pdf

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    1. He does comment on the termite hypothesis as follows:

      Several authors [1], [3], [4], [7], [8], [9] proposed some version of causation by termites either through direct action, residual effect or emission of a toxic agent. However, 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].

      and -

      The termite hypothesis of Albrecht et al. [1] would require that the termitaria deep underground be spaced in patterns like those of the circles above them. In principle, termite colonies have the capacity to be overdispersed, but unfortunately even the existence of such termitaria, let alone their spatial distribution is unknown. Moreover, Tschinkel [10] found no association between termite nests or foraging tunnels and fairy circles.

      Albrecht's hypothesis apparently involved harvesting water:

      Albrecht et al. [1] (2001) report that 12 days after rains, circle soil contained more water than matrix soil (but this was unreplicated, as well as a single sample point in time) and suggested that the circles act as “water traps” to benefit subterranean termites.

      I looked up "heuweltjies", and they seem to be described as "mounds" both in Wikipedia -

      Heuweltjies are fossil termite mounds occurring widely in the south-western Cape of South Africa. Heuweltjies can sometimes be recognised as large mounds above or near the surface of the landscape. There are two conflicting interpretations as to the origins of heuweltjies, the one view maintaining that heuweltjies were built by the harvester termite Microhodotermes viator while other researchers maintain that heuweltjies were built by a now possibly extinct termite species.[1] The earliest radiocarbon dates on heuweltjies date to about 30,000 B.P.

      - and in your first link:

      Heuweltjies are large circular earth mounds created by termites and are important features of the landscape in the Succulent Karoo biome in South Africa

      And also in the Quaternary Research article. So perhaps those are different entities from the fairy circles discussed here?

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    2. Although... while searching for the etymology of the interesting word "heuweltjie," I have seen some photos of barren spots, so perhaps the word has various meanings or applications.

      (Still haven't found the etymology. Probably Dutch.)

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  4. This article is at New Scientist. http://www.newscientist.com/article/dn23331-termites-are-the-fairies-behind-weird-desert-rings.html

    It postulates that a particular species of termite is associated with these circles, including photographic evidence showing these termites being found living among the roots of the grass they eat.

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    1. I saw that story via the BBC this week -

      http://www.bbc.co.uk/news/science-environment-21970408

      - but then to my frustration couldn't find this old post via the search engine. Thanks for your note; now I can post an update.

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  5. The third photo kind of reminds me of the ice circles that I saw in Lake Michigan this winter.

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