Soil biota as bioindicators of levels of erosion and fire disturbances in Afromontane grassland areas within the Golden Gate Highlands
Van der Merwe, Sylvia Shalome
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Soil erosion and wildfires are serious problems throughout the terrestrial ecosystems of the world. The Golden Gate Highlands National Park (GGHNP) of South Africa experiences high incidences of erosion and wildfires due to its orographic nature and expansive grassland habitats. Various conservation strategies are employed by Park Management to lessen the effects of these environmental factors, including instating regular prescribed burn regimes for grasslands and construction of rehabilitation structures in eroded localities. While the effects of fire and erosion are investigated across a variety of habitats and fauna, little attention is given to their effects on soildwelling arthropods. The overall objective was to determine the state of soil-dwelling arthropods across eroded and currently rehabilitated localities, as well as their responses to fire regimes, in the GGHNP. Chapter 2 aimed at determining the impact of prescribed burning on soil-dwelling arthropods in an Afromontane grassland habitat, by comparing assemblage patterns and responses of species richness and diversity between a single burnt and non-burnt locality. Soil arthropod assemblages were more species rich and abundant in the burnt site, with a higher number of species only observed in the burnt site overall. The study hints at fires creating a preferable niche for soil arthropods adapted to frequent fires in the fire-prone landscape. Chapter 3 attempted to identify possible indicators of erosion in the GGHNP, and to determine differences in soil-dwelling arthropod assemblages found in non-rehabilitated and rehabilitated eroded sites. IndVal results indicated a single strong indicator species, the mite Speleorchestes meyerae Theron and Ryke, 1969, relevant to non-rehabilitated sites, suggesting that soil arthropods show potential use in grading changes in soils of the GGHNP. Statistical modelling identified phosphorus as having a significant negative correlation on species richness in both rehabilitated and non-rehabilitated eroded sites. These results form a basis for future investigations of erosion in the GGHNP, while also indicating that soil mineralogy in conjunction with soil arthropod richness may provide sufficient usability in monitoring strategies of erosion. In Chapter 4, the changes in soil arthropod assemblages in areas with implemented erosion rehabilitation, namely rock-wall gabions near dirt roads in the GGHNP, was examined. Results showed that arthropod species richness significantly interacted with increased sediment build-up in the rehabilitated sites as well as with sub-site position around the gabions. The findings of this study suggests that, if compared to an older rehabilitated site, rock-wall gabions could have an indirect effect on soil-dwelling arthropods, possibly through the resulting sediment build-up over the long-term. In Chapter 5, the renewal of deteriorated erosion rehabilitation structures provided a unique opportunity to study the effect of major restructuring of a site and subsequent implementation of alternative rehabilitation structures on soil-dwelling arthropods. Both soil arthropod species richness and diversity was higher after the restructuring of the site, possibly due to reallocation of species from the soil surrounding the flattened area. The results suggest that restructuring caused no significant changes on soil arthropod assemblages in this single site over a nine-month period, but is not conclusive as to the effects that a major disturbance, such as land reformation and renewed rehabilitation implementation, may have over the long-term. Fire-treated and non-rehabilitated eroded sites show a surprising attribute in that these habitats support a greater soil arthropod species richness and abundance than natural sites. These sites show potential as important environments that act as unique niches in the GGHNP, vital to supporting soil arthropod diversity in the soil environments of the park. Interestingly, themes discussed highlight the importance of fire and erosion in the GGHNP as natural ecosystem phenomena, and the association of soil arthropods to these areas.