Soil-dwelling arthropods as indicators of erosion in a South African grassland habitat
Introduction
Soil erosion is a natural occurrence that drives the creation of new landscape types and the formation of mountainous areas (Mhangara et al., 2012). However, soil erosion is regarded as a serious problem throughout the world’s terrestrial ecosystems, due to the increased rate of erosion during recent years (Pimentel and Kounang, 1998; Pimentel et al., 2004; Durán Zuazo and Rodríguez Pleguezuelo, 2008). Soil erosion is known to reduce the quality of soils and, in turn, decrease the productivity of agricultural, natural, and forest ecosystems (Pimentel, 2006).
In South Africa, soil erosion is regarded as one of the most significant environmental problems causing ecosystem degradation (Meadows, 2003; Le Roux et al., 2007; Dlamini et al., 2011). The levels and severity of erosion in South African landscapes are dependent on soil composition, as well as the sources of erosion, such as intensity and frequency of rainfall, anthropogenic influence such as overgrazing and agricultural practices, and wind speeds (Meadows, 2003; Le Roux et al., 2007). Research strongly suggests that over 70 % of South African landscapes are affected by soil erosion to varying degrees (Garland et al., 2000; Le Roux et al., 2008). Soil erosion by water flow is a major environmental issue that has been found to threaten the ecological function of terrestrial and aquatic systems (Mhangara et al., 2012). The Golden Gate Highlands National Park (GGHNP) in South Africa is well-known for its sandstone cliffs and formations across the western side of the park. Sandstone is known to produce shallow sandy soil with very low fertility (Roberts, 1966) that is notoriously susceptible to erosion losses (Roberts, 1969; South African National Parks (SANParks, 2013). The park comprises many smaller rivulets running through the landscape, which form part of the water bodies found within the area. Although rivers around the GGHNP are situated in the upper catchment area, South African National Parks (SANParks) Authorities are concerned that finer soils could be carried into the aquatic sectors of the park during the heavier rainfall season, and measures are taken to prevent excessive siltation of these aquatic areas (South African National Parks (SANParks, 2013). Of interest is the make up of the park landscape as it is known today. The eastern section, formally known as the Qwaqwa National Park, focused on the preservation of cultural traditions and practices, and was mainly used as grazing lands for livestock (Rademeyer and van Zyl, 2014). The parks announced a merger in 2004 and the amalgamation was finalised in 2007, incorporating the various ecosystem types into a single national park (Ministry of Environmental Affairs and Tourism, 2004).
Edaphic biota are known to play a vital role in soil functions, which include nutrient element cycling, organic matter breakdown and humus formation (Menta et al., 2011). In addition, soil arthropods also affect the aeration, porosity, infiltration and distribution of organic matter in soils, subsequently modifying soil structure and improving its fertility. As a result, below-ground diversity is essential for the function of above-ground ecosystems (van Straalen, 2004). However, soil arthropods are not readily considered when looking at erosion, despite their below-ground endemism and role in major ecosystem functions (Orgiazzi and Panagos, 2018).
Grasslands have been indicated as important habitats in the protection of soil biodiversity (Menta et al., 2011), highlighting the importance of monitoring grassland health in conjunction with soil biota assemblages. Soil arthropods are useful indicators of a number of disturbance types in varying terrains, due to their rapid responses to changes in an environment (Paoletti et al., 1991; Stork and Eggleton, 1992; Kremen et al., 1993; Convey et al., 2003; Gulvik, 2007; Nakamura et al., 2007). It is vital that bio-indicators in grasslands are identified in order to monitor areas that may be at risk of deterioration or, at least, changes that may alter the nature of ecosystems. The GGHNP is currently the only national park in the Free State Province (South African National Parks (SANParks, 2013), and may play a significant role in the conservation of soil biota groups in the area. However, soil biota assemblages in this area are poorly understood and very few studies have been done on soil-dwelling arthropods as indicators within protected areas in South Africa.
The aim of this study was to identify possible soil-dwelling arthropod indicators of the effect of soil erosion and the differences between non-rehabilitated and rehabilitated sites in the GGHNP. It was hypothesised that (1) differences in soil-dwelling arthropod assemblages would be observed between sites in the eastern and western parts of the park, (2) rehabilitated eroded and non-rehabilitated eroded sites would show differences in comparison to undisturbed sites, and (3) different possible arthropod indicators for non-rehabilitated and rehabilitated sites could be identified.
Section snippets
Study area and period
The study was carried out in the GGHNP, located in the eastern parts of the Free State, South Africa, bordering with Lesotho to the south (Fig. 1). It covers an area of 340 km2 and features several deeply eroded sandstone outcroppings and cliffs, alongside large expanses of undisturbed grassland hills and valleys. The park comprises small streams that flow through sections of the landscape, with temporary water bodies forming during peak rainfall seasons. Erosion rehabilitation methods, such as
Results
A total of 5661 individuals were sampled during the 24-month study period, representing 13 orders and 62 families. The non-rehabilitated eroded sites had the highest species richness over the study period, with the lowest species richness sampled from the undisturbed sites and the rehabilitated site located on the western side of the park (Fig. 2).
Of interest was the higher species diversity sampled from site 3. However, this could be explained by the flattening of the site halfway through the
Discussion
Soil erosion within protected areas is a major concern in regards to conservation strategy and land degradation prevention, as it vastly threatens natural environments (Rahman and Chongfa, 2009). Although soil erosion is a particular concern around the various streams running through the park, the results suggest that erosion sites provide a differing niche for species not typically found in the natural undisturbed grasslands. The high number of unique species, as well as shared species found
Declaration of Competing Interest
None.
Acknowledgements
Thank you to SANParks for approval and permission to conduct this research in the Golden Gate Highlands National Park, Free State, South Africa (Permit number: VDMES1387), as well as to the University of the Free State’s Ethical Committee (clearance number: UFS-HSD2017/0074). We would like to thank Charlene Janion-Scheepers and Mikhail Potapov for assistance with species identification and verification of Collembola, Elizabeth Hugo-Coetzee for verifying identification of oribatid mites, as well
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