The community ecology of herbaceous vegetation in a semi-arid sodic savanna

Abstract

Ecological conservation of savanna ecosystems is dependent upon interactive mechanisms involving bottom-up drivers such as nutrient availability, and top-down controls relating to fire, herbivory and water availability at various spatial scales. Alterations in diverse savanna herbivore communities, suppression of natural fire regimes and increased rainfall variability alter the functioning of these mechanisms. Protected areas, such as the Kruger National Park (KNP), South Africa, provide valuable natural experimental settings where vegetation structure and function are being maintained by similar drivers under which they have evolved. Since plant communities form the structural and functional basis for most terrestrial ecosystems, functional understanding of species is becoming progressively important. Despite increased awareness of understanding resilience in complex systems, there is limited information available on the underlying functions of herbaceous life forms. This is particularly true for the forb component within the herbaceous layer, which is generally overlooked in ecological studies. Research presented in this thesis primarily aimed to evaluate how the species–and functional composition of the herbaceous layer of a semi-arid sodic savanna responds to changes in herbivory, fire and rainfall. Since the observed patterns in savanna community ecology is driven by underlying effects of herbivory, fire and rainfall variability, this study furthermore aimed to evaluate how interactions within the herbaceous component relate to the drivers they are exposed to, or released from. Specific objectives were therefore to test interactive effects of long-term exposure and/or exclusion of: (1) herbivory and fire on forb and grass diversity and abundance patterns of various functional groups, and how these effects interact with rainfall variability in a nutrient-rich semi-arid savanna ecosystem without elephants, (2) elephants (partial herbivore loss) versus all large mammalian herbivores (LMH) (total herbivore loss) on forb and grass diversity patterns and differences in forb and grass abundances of various functional groups, and how these effects interact with rainfall variability in a fire-excluded nutrient-rich semi-arid savanna system, (3) herbivory and fire during an episodic drought on system function by evaluating patterns in herbaceous species composition, trait diversity and functional group assemblages in a system without elephants, and (4) herbivory during an episodic drought on system function by evaluating patterns in herbaceous species composition, trait diversity and functional group assemblages in a system without fire. Results obtained from this study suggested that semi-arid savanna herbaceous community dynamics are largely dependent on variable life-form (i.e., grass and forb) responses to common savanna drivers at both species and functional level. These findings add to current understanding of the community ecology of savanna herbaceous layers by acknowledging the important ecological role of a previously neglected herbaceous life-form, the herbaceous forb component. However, further research on forbs within sites with different soil conditions and geographical aspects is necessary to improve the understanding of savanna herbaceous communities and hence the management of herbivore forage security when considering complex environmental changes.