Local and global controllers of grassland ecosystem stability during global change.
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Date
2019
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Abstract
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.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.