Potential forage production
Carrying capacities for domestic stock, expressed as number of ha required per large stock unit (LSU), were determined for pristine examples of the 32 habitat types defined in Vlok et al. (2005). This service was mapped by overlaying the carrying capacity recommendation map of the Department of Agriculture (DA) with those of the habitat map prepared by Vlok et al. (2005) for the Little Karoo domain. It is important to note that not all habitat types of the Little Karoo are covered by the DA map; however it does provide clear recommendations for the habitat types with the highest (valley thicket with spekboom) and lowest (Proteoid fynbos) carrying capacity, as well as several other clear recommendations at other carrying capacities (e.g., for Apronveld, Gannaveld, and Sandolienveld). For habitat units not recognized by the DA map, carrying capacity recommendations for pristine examples of such types had to be interpolated. This was done by estimating the degree to which plants palatable to domestic stock would increase or decrease in the habitat type in relation to the DA recommendation for the most similar habitat type. These estimates were reviewed in terms of the range recommended by the DA, as well as by officers from the DA. We assigned a medium level of certainty to these reviewed and well understood data.
Potential carbon storage
Carbon storage refers to the number of tons of carbon locked up in the above and below ground biomass of plants; most of this carbon would be released if these intact ecosystems were transformed or degraded. In mapping this service, we (similar to Chan et al. 2006), focused on carbon storage rather than sequestration as an ecosystem service, mostly because of the data gaps and uncertainty in estimating sequestration. Most Little Karoo habitat types were assigned zero carbon storage values due to their arid, fire prone nature. For the remainder, carbon storage values were extracted for the habitat types of arid thicket with spekboom based on research on carbon storage in the region (Mills et al. 2005, Mills and Cowling 2006). Through a process of expert consultation, the more mesic thicket with spekboom types were assigned higher values based on higher predicted biomass. Similarly, arid thicket types without spekboom (Portulacaria afra) were assigned lower values owing to the large contribution of this species to carbon stocks (Mills et al. 2005). Three remaining habitat types (Randteveld, Gravel Apronveld, and Thicket Mosaics) were assigned small values to reflect the small amount of carbon they potentially store. The ecosystem service was mapped as tons of carbon stored per ha per habitat type. We assigned a high certainty to the carbon storage values of the arid thicket with spekboom type, and low certainties to the remaining values where scientific understanding is still in development.
Potential erosion control
In mapping this ecosystem service, we assessed the interaction between rainfall, soil depth and texture for each habitat type. This information was used to assign habitat types to classes of high, medium, and low erosion hazard. These classes were determined using the vegetation descriptions in Vlok et al. (2005) and through expert consultation. We identified high erosion hazard habitat types as all of those belonging to the aquatic source (streams and seepage areas) and drainage (river and floodplains) biomes, as well as the Gannaveld types which are located in valley bottoms and often form large open plains just above the river and floodplain habitat type. Gannaveld types have deep, fine-fractured soils very prone to erosion, with rainstorms transferring soils to the riverine and floodplain habitats causing declines in water quality and nutrient enrichment. These habitat types are associated with high runoff (high rainfall mountain catchment areas) and high run on areas (lowlands with vulnerable soils plus other functions (e.g. nutrient retention)) and are areas where the maintenance of pristine vegetation cover is essential. These areas form the focus of this study. Areas of medium hazard include the remaining mesic and montane habitat types, which are important for water run-off and drainage. We assigned a high certainty to these qualitative ranks based on a sound expert understanding of the service.
Potential water-flow regulation
In mapping this service, we used data on both water-flow regulation and water-quality regulation. The former is a function of how much water infiltrates the soil, passes beyond the root zone, and recharges the groundwater stored in the catchment (Sandström 1998). Infiltration is primarily regulated by the texture of the soils (rapid in sandy soils and slow in clays) and inputs from the vegetation and fauna which maintain the soil porosity and protect it from the erosive forces of raindrops and unhindered surface run-off (Dean 1992, Bruijnzeel 2004, Ludwig et al. 1997). From the human use perspective, the most important component of the water flows is the sustained flows which meet needs in the dry season and also increase yields from storage dams. One measure of sustained flows is the river baseflow which is the main component of the flow during the dry season and is typically generated by groundwater discharge (Farvolden 1963). The most appropriate dataset for estimating these flows was gridded data on groundwater recharge extracted from the Department of Water Affairs and Forestry (DWAF 2005). This estimate combines data on rainfall, geology (lithology), and estimates of recharge (e.g., from chloride profiles) to provide a grid on recharge depth at a 1 km x 1 km resolution. These estimates take into account losses due to evaporation from the soil, interception, and transpiration of soil water by plants (i.e., green water), but not the losses during the groundwater discharge into rivers (e.g., through riparian vegetation).
In mapping the water-quality component of the service, we used data on the relationship between geology (primary lithology) and groundwater-quality (electrical conductivity) because high sodium chloride (salinity) concentrations make the water unfit for domestic use. Data on groundwater-quality were extracted from borehole water analyses stored in the Water Management System database of the Department of Water Affairs and Forestry. The results were summarized by the primary lithology taken from the 1:1 million geological data (Council for Geosciences 1997). Formations where the electrical conductivity exceeded the target water-quality range for acceptability for domestic water supplies (DWAF 1996) were used to identify and exclude areas where water-quality was deemed unacceptable for domestic consumption. We assigned a high certainty to these well understood and peer-reviewed data.
Using ArcGIS 9.2 (ESRI 2008), we modelled a 10 km viewshed of the major tourist routes of the Little Karoo. The distance was determined based on visual assessments in the region. This viewshed was extracted and used as the ecosystem service of tourism. We assigned a medium level of certainty to these data due to our limited understanding of the full suite of drivers of tourism in the region.
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Supported by Grant No 1064410304. Critical Ecosystem Partnership Fund. Cape Town, South Africa. [online] URL: http://bgis.sanbi.org/littlekaroo/index.asp.