Browsing by Author "Currie, Gwen"

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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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    Soil drying cycles in the succulent Karoo : different rooting systems and water uptake
    (2003) Currie, Gwen; Carrick, Peter J; February, Edmund C
    In the semi-arid Succulent Karoo water is one of the most limiting resources, and plants that survive in the area require competitive water-use strategies suited to long periods of drought. Over the years much emphasis has been put on mechanisms that allow for the competitive success of water-use strategies, and root structure features as an important attribute. This study investigated soil drying cycles in the semiarid Succulent Karoo. It measured the rates at which soil water was depleted at two different depths in five different microhabitats, and investigated whether these are related to different rooting systems. Three prevalent plant species in the region were used, namely Leipoldtia schultzei of the Mesembryanthemaceae, Tripteris sinuata of the Asteraceae, and Galenia africana of the Aizoaceae, all of which have their roots in the upper 20cm of soil. However, Leipoldtia's roots are concentrated in the top 5cm of soil, while the other two species root mainly at depths between 10 and 20cm. This vertical separation of roots provided the opportunity to relate soil water depletion at two different depths (0-10cm and 10-20cm) to the different rooting strategies, and explore what this means in te1ms of compatibility between close neighbours and seedling establishment under adult plants. The shallow-rooting Leipoldtia showed the highest rates of soil moisture depletion in both the shallow and deeper soil zones, and is therefore a strong competitor for water and probably an undesirable neighbour. Soils at both depths below Tripteris plants maintained the highest moisture content over the 2-week period of sampling, suggesting it to be favourable site for seedling establishment. Galenia showed intermediate soil-drying effects in the upper 0-10cm, but high rates of moisture depletion at 1 0-20cm below the surface, suggesting that it may be a suitable neighbour for shallow-rooting species but not for other deeper rooting species. Additionally, the two sampling methods - electronic probe readings and gravimetric analysis - were examined. The two methods correlated fairly well to each other (R2 = 0.759 ± 0.002), although the probe readings were consistently higher and showed greater variation (probe SD = 2.8 as opposed to gravimetric SD = 1.2)