Chapter 2: Small Mammal diversity and abundance in Eastern Rûens Shale Renosterveld
2.4 Study Site
The project was carried out on fragments of Eastern Rûens Renosterveld on six different farms, located between 25 to 48 kilometres northeast of Bredasdorp in the Overberg region of the Western Cape (see Figures 2 and 3). These farms include: Plaaitjieskraal, Nuwerus, Spitzkop, Now I Know, Lofdal and Haarwegskloof. The Overberg is located to the east of Cape Town, between the towns of Grabouw in the west and Heidelberg in the east. The region is bounded by the Agulhas Plains in the south and the Riviersonderend and Langeberg Mountains in the North. For ease of access fragments sampled were located near Haarwegskloof, where the Renosterveld Research Centre for the Overberg Renosterveld Conservation Trust (ORCT) is located. The sites accessed were all on private land, and were accessible only through the strong relationships the ORCT has forged with surrounding farm owners.
57 The Overberg is found within the Cape Floristic Region, and is historically characterised by Renosterveld vegetation types. This study focusses specifically on the Eastern Rûens, or Eastern Overberg Region, which is characterised solely by Eastern Rûens Shale Renosterveld.
The distribution of Eastern Rûens Shale Renosterveld extends from Bredasdorp to the Breede River, near Swellendam. The region is located between the southern foothills of the Langeberg, including the area around the towns of Malgas and Heidelberg, and the coastal limestone belt in the south (Rebelo et al., 2006). The region has an average altitude of between 40 and 320 meters. The area has been extensively transformed to agriculture with the major uses being wheat farming and pastures for livestock and game. The landscape is gently undulating, with winding ravines, along which fragments of renosterveld can still be found. The primary crops grown in this region are wheat (Triticum aestivum), barley (Hordeum vulgare) and canola (Brassica napus), which are rotated with pasture legumes, namely lucerne (Medicago sativa) (Chimphango et al., 2020). These pasture crops are grazed by sheep and cattle, but renosterveld fragments next to pasture lands are often also grazed by livestock (Chimphango et al., 2020). Within the region, five small antelope species are prominent, including bushbuck (Tragelaphus sylvaticus), Cape grysbok (Rhaphicerus melanotis), common duiker (Sylvicapra grimmia), grey rhebok (Pelea capreolus) and steenbok (Rhaphicerus campestris). These have been found to graze across agricultural lands, and in indigenous fragments (van Vuuren et al., 2022). Farms included in this study consist of cereal fields or artificial pastures with scattered fragments of renosterveld. Most fragments are seasonally grazed, with the exception of large fragments located in Haarwegskloof, 80% of which is virgin land, and which has been free from grazing since 2013, when it was set aside as a conservation area (Curtis, 2013). The cereal fields are ploughed in autumn, and are sprayed with pesticides twice per year. The small and medium sized fragments are managed as part of the surrounding cultivated land and are therefore exposed to spraying with herbicides and insecticides. Fragments in this study had not, however, been burned for at least 10 years prior to sampling.
2.4.2 Vegetation Patterns
The predominant vegetation types found in the Cape Floristic Region are fynbos and renosterveld. Fynbos is a shrubland made up of four major plant types: restioids, ericoids, proteoids and geophytes (Cowling, 1995). Renosterveld is mainly comprised of ericoid plant
58 types while not having restioids, and very few proteoids (Cowling, 1995). In the Eastern Rûens region vegetation is dominated by small-leaved, low to moderately tall grassy shrublands confined to fine-grained, clay-rich soils (Rebelo et al., 2006). Cover and diversity of succulents is higher in this type than in others, indicating a transition to the Succulent Karoo region.
Renosterveld vegetation types are transitional in nature, having strong links to the Succulent Karoo, Fynbos and Subtropical Thicket biomes (Bergh et al., 2014). Recent research has highlighted the prevalence of quartz-silcrete outcrops scattered across remnant fragments of Eastern Rûens Renosterveld, displaying similar community structure to those in the Succulent Karoo Biome (Curtis, Stirton & Muasya, 2013). Where rainfall drops below 300mm per annum renosterveld vegetation is replaced by Karoo vegetation. Where rainfall surpasses 600mm renosterveld is replaced by asteraceous fynbos (Rebelo, 1995). This region has moderately undulating hills and flat plains (Rebelo et al., 2006), supporting a decidedly mixed vegetation type, suggesting the need for further divisions in this vegetation type (Curtis, Stirton &
Muasya, 2013). Topographic features such as river valleys, which prevent the spread of fires, typically have thicket vegetation. Small quartz outcrops are located across the Eastern Rûens region, and these support a distinctly different vegetation community to that found on the shale soils, displaying high endemism and species richness (Curtis, Stirton & Muasya, 2013).
Figure 2. Photos taken during sampling indicating the floral diversity present in Eastern Rûens Renosterveld. Top row from left to right: Erica spp, Hyobanche spp, Chironia spp. Bottom row left to right: Moraea spp, Jamesbrittenia spp, Moraea spp.
59 Figure 3. Satellite image showing the study region in the context of the Western Cape (Google Earth)
Figure 4. Map indicating the locality of renosterveld fragments sampled. Fragment size class is indicated by colour, with green indicating large, blue indicating medium and yellow indicating small fragments.
2.4.3 Geology and Soils
The Cape Floristic Region lies geologically on the Cape Supergroup, a Devonian-Ordovician series of sedimentary strata, consisting of alternating layers of quartzitic sandstones and fine- grained shales (Deacon, Jury & Ellis, 1992). The region where Eastern Rûens shale renosterveld is found is predominantly comprised of fine-grained shales of the Bokkeveld Group Shales, giving rise to clay and loam-type soils, with a minor contribution from the Mesozoic Uitenhage Group sediments in the north-east.
2.4.4 Climate
The region has a mean annual precipitation of 385mm (270-540mm), with bimodal rainfall distributed evenly between summer and winter. The hottest temperatures are experienced in January, with a mean daily maximum of 26.9°C, while the coldest temperatures are experienced in July, with a mean minimum temperature of 5.9°C (Rebelo et al., 2006).