Studies of herbivory and vegetation change in Karoo shrublands

Doctoral Thesis


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University of Cape Town

The broad objective of these studies was to view present landuse (extensive small stock ranching) in the Karoo against the background of relationships between indigenous plants and animals and to indicate how modification of herbivore regimes might affect this arid environment. Specific objectives were to determine how grazing by domestic livestock brings about vegetation change, why such changes are sometimes irreversible, and whether existing conceptual models of vegetation dynamics adequately explain the impact of domestic livestock on Karoo vegetation. Three interrelated aspects of plant-animal interactions were considered: the influence of herbivores on the evolution of Karoo plants, food selection by indigenotis and introduced herbivores and the effects of herbivory, competition and rainfall on plant reproduction and recruitment. The results of these investigations are presented as 14 papers. The first three papers interpret plant morphology and biogeography to provide information on past spatial and temporal use of the landscape by herbivores, and the next seven provide new information on food selection by invertebrates, indigenous vertebrates and by domestic sheep. Three papers examine the hypothesis that the reproductive output, survival and abundance of some Karoo plant species are influenced by herbivory. The possible consequences of various land management options on diversity and productivity of Karoo rangelands are discussed in the concluding paper. It was inferred, from biogeographic trends in the relative abundance of plants with thorns or propagules adapted for epizoochoric dispersal, that densities of large mammalian herbivores decreased from the north eastern to the southwestern Karoo. Within the most arid parts of the Karoo, mammalian herbivory appears to have been concentrated along drainage lines and in pans.

Includes bibliographical references.