The ecology of Namibian fairy circles and the potential role of sand termites (Psammotermes allocerus Silvestri) in their origin
Doctoral Thesis
2016
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University of Cape Town
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Regularly-dispersed patterning in the landscape occurs globally, both in the form of vegetated patches or gaps of bare earth within a vegetated matrix. Most theories link these periodic patterns to various biotic factors including selective grazing, allelopathy, nests of social insects, and competitive interactions between plants ('self-organisation'). Fairy circles are perhaps the most archetypal of these patterns, taking the form of evenly-spaced, circular to elliptical barren patches 2-12m in diameter. They occur in dense fields within sandy, species-poor grasslands in the Pro-Namib Desert. Fairy circles often display a well-vegetated peripheral ring of grasses and have been shown to retain higher levels of soil moisture compared to the surrounding matrix soils. They can persist in the landscape for decades, and show evidence of birth and senescence, when the bare disc becomes overgrown by grasses and fades back into the matrix. Since the 1970's several hypotheses have been forwarded for their origin including; herbivory by the termite Hodotermes mossambicus (or the release of volatile chemicals from their nests), the excavation of grass roots by a widespread and common ant species, an allelopathic chemical released by Euphorbia damarana plants, geochemical gas seeps, plant self-organisation, and nest building activities of the Sand termite Psammotermes allocerus. A consensus has yet to be reached regarding the origin and nature of fairy circles, and the two theories currently receiving the most attention and debate are the Sand termite hypothesis and plant competition hypothesis. The latter proposes that short-range facilitation of plant growth occurs within the matrix and on the periphery of fairy circles, while long-range competition for resources (primarily water) by Stipagrostis roots inhibits plant growth on the bare disc thus generating the regular bare patches. The Sand termite hypothesis states that the termite P. allocerus creates and maintains a bare patch around their polycalic nests primarily through central-based foraging on grass roots and culms. This thesis aims to test the Sand termite hypothesis for fairy circle formation as well as expand on the ecology of fairy circles. Novel features relating to their ecology which are examined include: (1) various potential mechanisms for maintaining the circle's bare appearance (specifically examining seed banks and excavation of seedlings by ants), (2) changes in circle density and size in relation to environmental variables at a local scale (including soil properties and vegetation cover/type), (3) their ontogeny, lifespan and survival, (4) the high degree of spatial ordering seen in fairy circles, (5) soil properties on fairy circles compared to the surrounding matrix (including particle size, temperature, moisture, pH and electrical conductivity) and (6) the influence of fairy circles on nearby plant and insect communities. Fieldwork for the above was carried out in NamibRand Nature Reserve, Southwest Namibia.
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Vlieghe, K. 2016. The ecology of Namibian fairy circles and the potential role of sand termites (Psammotermes allocerus Silvestri) in their origin. University of Cape Town.