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Local and global controllers of grassland ecosystem stability during global change.

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Human impacts on grasslands dramatically affect grassland biodiversity which impacts the ability of ecosystems to sustainably provide ecosystem services. As the extents of these anthropogenic impacts increase (due to agricultural intensification, for instance) solutions to this problem are becoming increasingly important. The ecosystem stability concept provides a framework to investigate how biological systems such as grasslands respond to disturbances. However, there is uncertainty relating to the ecosystem components which influence the various facets of ecosystem stability. Therefore, the aim of this dissertation is to 1) outline the current academic consensus pertaining to the drivers of grassland ecosystem stability, 2) contribute to underrepresented research areas identified in the literature review, and 3) investigate whether there are general environmental conditions which predispose to grassland destabilisations following anthropogenic disturbance. Academic consensus was assessed using a systematic map of review articles discussing grassland ecosystem stability concepts. This review highlighted the many complex interactions that exist in grassland ecosystems. There was also a strong consensus that diversity mediates ecosystem functioning and stability. Other ecosystem processes such as fire, herbivory, woody encroachment, and plant invasions were also well represented and discussed in these review publications, however, climatic impacts on grasslands were identified as an important knowledge gap. To address this, nutrient enriched grassland stability responses to temperature variability were studied using a long-term nutrient addition experiment. Surprisingly, nutrient enriched grassland productivity was more stable than control grasslands in response to temperature variability. Finally, environmental drivers of grassland stability changes following nutrient addition were assessed using a globally replicated experiment. This investigation showed that grasslands with a history of intensive anthropogenic management are positively affected by nutrient addition whilst stability in more naturally assembled grasslands is greatly reduced following nutrient addition. Stability changes were also associated with changes in nutrient availability and soil macronutrient (specifically Ca and K, but not micronutrient) status. Sward structure changes (such as increased compositional dissimilarity, greater dominance, and reduced asynchrony) were associated with stability reductions following nutrient addition. The findings of these three investigations highlight the serious impacts that human activities which result in increased nutrient deposition in grasslands are having on grassland ecosystems. In relation to the prevailing consensus identified in the review literature concerning the positive effects of grassland diversity on ecosystem stability and functioning, this dissertation advocates for the increased preservation of intact grasslands.


Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.