Wetland diversity and ecosystem services in the Tlokwe Municipal Area
Abstract
The last two decades saw rapid urbanization and development in South Africa, resulting in highly fragmented sprawling cities. In the Tlokwe Municipal Area (TMA), agriculture and urbanization are responsible for most of the irreversible transformation of natural areas including wetlands. The quantification of urbanisation change and the transformation of natural areas in the TMA over a 61 year time period revealed extensive land-cover changes. Changes in the coverage of wetlands designated a further study of wetland delineation within the TMA. Wetlands provide a range of ecosystem goods and services, but are also the least conserved ecosystems in South Africa. The alarming rate at which wetlands are being lost and degraded is increasing due to the lack of understanding of their ecological and socio-economic importance by planners, policy makers ad developers. The main aim of the study was to investigate the floristic composition and ecosystem service delivery of wetlands within the endangered Rand Highveld vegetation unit containing the Eastern Temperate Freshwater wetland type, in the TMA along an urban-rural gradient.
During the field investigation, wetlands within the TMA were identified, delineated and classified and the plant species composition and diversity thereof were determined. Plant species composition and diversity within the wetlands were determined along a number of 100m line transects across the wetland sites, where the vegetation was surveyed at each 10m interval along the transect, within a 1m2 quadrant. The plant species data collected is used to assess the Floristic Quality Assessment Index (FQAI) – used for evaluating habitat condition or status (with specific reference to the habitat) using the conservatism of the plant community; secondly the Wetland Index Value (WIV) – which measure changes in plant species that are indicative of changes in hydrology processes due to anthropogenic disturbance for each wetland. The potential ecosystem services each wetland may deliver, were scored according to the WET EcoServices rapid assessment method.
A total of 102 plant species (68 indigenous, 34 alien species) were identified within the 14 wetland study sites. Urban wetlands had a higher total vegetation cover and plant species richness than rural wetlands, as well as a different species composition. One of the rural wetlands can be seen as a ‘transition’ wetland, as it almost equally shares its species with both urban and rural wetlands. All wetlands surveyed was determined to be ‘true wetlands’ (all WIV <2.5). Two rural wetlands had lower WIV values than the others. The other rural wetlands had higher WIV values, mostly due to the higher abundance of facultative wetland species and lower abundances of obligate wetland species. The FQAI calculated for each wetland did not differ substantially between most of the rural sites, and all had relatively low FQAI values. Urban wetlands are considered to have relatively high FQAI values when compared to rural wetlands. The quality of the overall wetland habitat of the TMA could be considered as degraded.
Urban wetlands scored higher for direct ecosystem services (provisioning ecosystem services), which is ecosystem services such as tourism and opportunities for education and research (cultural ecosystem services), whereas rural wetlands scored higher for regulating ecosystem services (sediment and phosphate trapping, nitrate and toxicant removal and erosion control). From all the wetlands, it is evident that the presence and cover of vegetation can be considered a major driving force for delivering the ecosystem services, rather than the type of plant species (types of species) present in the wetlands.
The identification of ecosystem services being delivered by natural areas, has become a policy tool to protect biodiversity, however this concept is not readily implemented. The increase in urbanisation and anthropogenic activities surrounding wetlands, has decreased the available wetland area to deliver ecosystem services, thus reducing the ability of the wetlands to deliver ecosystem services. Several management and conservation measures are recommended for implementation by local residents and stakeholders of the municipality. Public participation of inhabitants surrounding the wetlands is encouraged; conservation of the entire wetland habitat, rather than focusing on a specific species, is encouraged; and regular monitoring of invasive plant species is recommended. The findings of this study should be tested in other urban areas in South Africa, to indicate potential general trends and strengthen the basis of ecosystem service delivery research of wetlands which is needed to adequately understand and ultimately limit further wetland loss in South Africa