Effects of climate variability, land-use change and transformation on grassland plant diversity in South Africa

Abstract

Grasslands as open ecosystems are not successional stages or degraded forests but are ancient old-growth systems supporting a high diversity of fauna and flora. It is now known that forests and grasslands are two distinct ecosystem types that have existed for millennia. In South Africa, approximately 27.9% of the terrestrial land surface is covered by the Grassland Biome for which less than 25% of untransformed old-growth grassland are being moderately to well protected. This biome is one of the most threatened biomes in South Africa, with 40-60% of its area already irreversibly modified, whilst 60% of the remaining grassland is threatened. Grasslands are subjected to disturbances which can be grouped into two main disturbance types, namely endogenous and exogenous. Endogenous disturbances occur naturally, and grasslands evolved in the presence of these disturbances, including climate variability, fire, and herbivory. Exogenous disturbances are those that disrupt the soil surface and are usually of anthropogenic origin, including afforestation, agriculture, and urbanisation. The overarching aim of this study is to investigate, describe and critically evaluate the community composition and diversity responses of grassland ecosystems to direct and indirect anthropogenic disturbances. Two long-term datasets were used to investigate the effect of transformation on grasslands of South Africa. The first dataset was used to investigate land-use change effects on grasslands by comparing untransformed and transformed grasslands in terms of species diversity and composition. The second dataset was used to compare two adjacent grassland communities, of which one is encroached. These two communities were further investigated to determine the effect of a severe drought on species diversity and composition. Findings from the present study confirmed that exogenous disturbances transform grasslands by removing above- and belowground plant biomass, thus reducing the potential of perennial grassland plants to persist through resprouting. There is also an increase in exotic species which transform the community composition. Both exogenous and endogenous disturbances result in grasslands transforming to encroached sites. The origin of encroacher species can be savanna or forest as both are bordering grasslands. However, in this study, neither savanna or forest woody species seemed to dominate, and it is hypothesised that the ‘encroached community’ might be a transformed grassland, or alternatively a shrubland. Woody grasslands are somewhat less resistant and resilient to drought than an old growth grassland. Woody grasslands could therefore be seen to be more vulnerable to drought, but the results are not definitive.