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University of KwaZulu-Natal (2001)

Grasshopper ecology and conservation in the Nama-Karoo

Bekele, Solomon Gebeyehu

Titre : Grasshopper ecology and conservation in the Nama-Karoo

Auteur : Bekele, Solomon Gebeyehu

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy PhD, Zoology, 2001

Résumé
This study was undertaken in the Karoo, a semi-arid grazing land in South Africa, to elucidate the interaction between grasshopper assemblages and various aspects of the Karoo landscape. It falls into four sections, the first of which was a three-year study which was undertaken on and around a prominent South African mesa to determine its role as an elevational conservation refugium for grasshoppers in a sea of grazed flatlands (Chapter 2). The number of grasshopper species and individuals on the summit, slopes and flatlands varied significantly, in relation to measured environmental variables. The summit, through inaccessibility to livestock grazing, was effectively a conservation refugium for one grasshopper species, Orthochtha dasycnemis. There was no significant difference in species richness between years of sampling, although there were significant variations in grasshopper abundance between years. The difference in rainfall between years was significant and appeared to be the key factor influencing grasshopper population dynamics. This clearly shows that a mesa can act as a conservation island and refugium supporting an insect assemblage that would be otherwise altered by heavy livestock grazing on the surrounding flatlands. This summit assemblage is strongly linked with that on the slopes and below, and is determined not so much by an island effect per se, but by low grazing intensity and associated soil and vegetation structure.

The second part of the study focussed on the interaction between grasshopper assemblage response and three hill sizes at a regional scale (Chapter 3). Small hills contained a significantly higher grasshopper species richness and abundance than medium and large hills. There were significantly higher numbers of small-sized grasshopper species and individuals than medium and large-sized ones. Flatlands surrounding small hills had significantly higher grasshopper species richness and abundance than those surrounding medium and large hills. The slopes of the three hill sizes did not show significant difference in species richness and abundance. There was no significant variation between the summits of the three hill sizes in species richness but they varied in grasshopper abundance. The summits of small hills had significantly higher grasshopper abundance than the summits of medium and large hills.

Detrended Correspondence Analysis showed two clear grouping of sampling site and grasshopper species. While the flatlands of small hills formed a separate assemblage of several grasshopper species, slopes and summits of all hills formed another clump of few grasshopper species. Canonical Correspondence Analysis revealed that flaltands surrounding small hills occurred along increasing gradients of shrub cover whereas those surrounding medium and large hills occurred along increasing gradients of grass cover, vegetation density and greenness of grasses. Slopes and summits of all hill sizes occurred along increasing gradients of rock cover, cragginess, grass height and soil temperature. Patterns of grasshopper dominance were markedly variable among sites. There were low dominance patterns on flatlands of small hills where most species were rare. The distributional patterns varied of higher taxonomic groups varied among the three hill sizes. Small hills contained species from four families and nine subfamilies, but medium and large hills had only members of Acrididae in five subfamilies. About 50 % of the total grasshopper abundance was associated with small hills. The study revealed the patterns of grasshopper assemblages at regional scale, and showed that variability in hill sizes across the Karoo has marked role in grasshopper conservation, and that grasshoppers interact differentially with variable hill sizes across the Karoo.

The third part of the study was undertaken at twelve grassland sites in the Mountain Zebra National Park (MZNP) and the surrounding farms to assess changes in grasshopper assemblages to grazing by indigenous mammals inside the park in comparison with grazing by domestic cattle outside (Chapter 4). The MZNP has been restored from cattle-grazed farmland to indigenous mammal parkland for 62 years. The number of grasshopper species and families inside the park was not significantly different from outside the park, but the number of individuals inside the park was significantly higher.

Multivariate statistics did not reveal any strong site groupings based on simple inside/outside comparisons, but there were clear groupings of sites based on vegetation characteristics and other environmental variables. The park boundary, therefore, does not significantly determine grasshopper assemblages, although intensity of grazing does. The indigenous mammals inside the park had the same effect as the domestic cattle outside, and it was the level of defoliation and trampling that was important rather than type of mammal. Very intensive livestock grazing and trampling leads to bush encroachment and reduction in cover and/or disappearance of several grass species. In response to this pressure, grasshopper populations dropped, with localized extirpation of some species. Vegetation composition and structure showed a significant influence on grasshopper assemblages, particularly grass height and percentage cover. The MZNP is thus a localized area of elevated grasshopper abundance in comparison with the surrounding farm landscape, and presumably represents a situation prior to the current, intensive farming activities. Such elevated grasshopper abundances are important for maintaining soil quality and hence ecological integrity of this landscape which is poor in organics and nitrogen.

The significantly lower population of grasshoppers on the surrounding farms, with local extirpation of some species also suggests that the MZNP could be viewed as a local centre to which species with higher capacity for mobility may seek refugia from anthropogenic pressures. Hence the MZNP serves as a reference showing the difference between restored-through-natural-succession and anthropogenically-disturbed habitats, and compares desirable with undesirable ecosystem changes for herbivorous invertebrates such as grasshoppers.

The fourth part of the study was on grasshopper assemblage response to seasonal grazing (including summer, winter, spring and autumn grazing), rotational grazing, continuous resting and continuous grazing at a long-term experimental site (Chapter 5). Rotationally-grazed sites supported the highest number of grasshopper species and abundance, while continuously-grazed sites had the lowest. Cluster analysis revealed that spring-grazed and winter-grazed sites were the most similar, with continuously-rested sites being the next most similar to these. Rotationally-grazed sites showed the lowest similarity to the rest of the sites. DCA showed clear groupings of sites and grasshopper species, with most species associated with rotationally grazed sites. Continuously-grazed sites had a different grasshopper assemblage. CCA showed that the assemblages followed definite gradients of measured environmental variables. Rotationally-grazed sites occurred along gradients of increasing bare ground, while continuously-grazed and summer-grazed sites occurred along increasing gradients of shrub cover and soil temperature. Spring-grazed, autumn-grazed, winter-grazed and rotationally-grazed sites were characterized by high vegetation density. Grasshopper dominance patterns were very different at different sites. Summer-grazed sites had the highest percent dominance (40%) by Picnodictyaflavipes, while winter-grazed sites showed higher percent dominance (32%) by Pseudogmothela sp. The significance of variable grazing management systems for maintaining floral and grasshopper diversity is discussed. It is recommended that rotational grazing in this arid system is most suited to maintaining plant and insect diversity.

These four parts in this study all clearly showed that grasshoppers interact with the landscape in a way that their assemblage patterns are dictated by patch as well as by regional dynamics. Topography in particular contributes significantly to biodiversity patterns at the spatial scale of landscape. But these patterns are also strongly determined by differential grazing pressures from domestic livestock which in turn interact with the various topographical features. These findings enable recommendations to be made on optimal grazing regimes relative to the hilly features of the landscape. The results also show that restoration which incorporates low-pressure grazing regimes and which takes cognizance of topographical features can maintain grasshopper abundance and diversity in the long term.

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