Browsing by Author "Picker, Michael D"

Now showing 1 - 4 of 4
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    The ecology of Namibian fairy circles and the potential role of sand termites (Psammotermes allocerus Silvestri) in their origin
    (2016) Vlieghe, Kelly E P; Picker, Michael D; Erni, Birgit
    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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    The impact of megaherbivore grazers on grasshopper communities via grassland conversion in a savannah ecosystem
    (2003) Currie, Gwen; Picker, Michael D; Bond, William J
    Grasshoppers are sensitive indicators of the state of disturbance of grassland ecosystems. This study examined the grasshopper communities inside a game reserve, comparing those found on frequently grazed areas with communities inside plots that exclude megaherbivores. The vegetation inside the protected plots was found to differ from the openly grazed areas in terms of grass height and aerial cover, but not in % greenness or richness of forb species. Grass species varied with locality rather than grazing impact. Total numbers of grasshoppers did not differ significantly between the two contrasting areas (100.2 in vs 93.5 out), however grasshopper species richness did, with the outside, short-grass plots having on average 17 different species, and the inside tall-grass plots a higher mean of 24.5. Grasshopper communities responded primarily to grass height and vegetation cover, but not to grass species or greenness of vegetation.
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    Long-term population trends and habitat preferences of waders at Strandfontein wastewater treatment works
    (2016) Essig, Erica Carla; Picker, Michael D; Underhill, Leslie G; Whitelaw, Dave
    The aims of the dissertation are to describe long term migrant and residential wader population trends at Strandfontein, and to identify the potential factors which influence local wader abundance. Once identified, these factors are integrated into a management plan that would create ideal wader habitat, and attract larger numbers of waders to Strandfontein.
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    Neglected pollinator syndromes in seasonally inundated Renosterveld
    (1996) Goldberg, Karen; Picker, Michael D; Cowling, Richard M
    A range of fairly common plants were investigated in Darling in the Western Cape, to determine their pollinator syndromes, and to evaluate the relative importance of monkey beetles (Scarabaeidae: Rutelinae: Hopliini) and relatively short-tongued horseflies (Tabanidae) in their pollination. Detailed observations showed that all the plants investigated are visited by more than one insect species. Visitation rates and pollen loads of all insects found on the plants were used to assess their pollination efficiency. For all plants investigated, only a subset of the visitor-suite was found to contribute to the plant's reproductive success. Geissorhiza radians (Thunb.) Goldblatt and Wachendorfia paniculata L. seem to have specialized pollinator systems, both relying on tabanids for their pollination, while Heliophila coronipifolia, L. Monsonia speciosa, L.f. Ornothogalum thyrsoides, Jacq. Romulea hirsuta (Klatt) Baker and Ursinia anthemoides (L.) Poir. appear to have more generalized pollinator syndromes. Monkey beetles were the predominant and generally the most efficient pollinators for all these species. It is therefore concluded that these two insect groups are important pollinators of the plants investigated and probably play a part in the pollination of several other plants in the community. An evaluation of the larval requirements of pollinators revealed that although some species show clear patterns in terms of what types of soil conditions they prefer, successful emergence of insect species is generally not limited by a shortage of suitable habitats. Environmental variability may therefore play the largest role in determining the emergence and abundance of pollinators. This has implications for plants reliant on insects for their pollination, especially for species with specialized pollinator syndromes. Fluctuations in the environment may be a partial explanation for the prevalence of the generalized pollination syndromes observed.