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Bothalia - African Biodiversity & Conservation

On-line version ISSN 2311-9284
Print version ISSN 0006-8241

Bothalia (Online) vol.50 n.1 Pretoria  2020

 

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Correspondence:
Sanet Janse van Vuuren
sanetjansevanvuuren@nwu.ac.za

Submitted: 20 September 2019
Accepted: 29 April 2020
Published: 24 February 2021

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ORIGINAL RESEARCH

 

Plant diversity and conservation value of wetlands along a rural-urban gradient

 

 

M.J. du Toit; C. du Preez; S.S. Cilliers

Unit for Environmental Sciences and Management, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa

Correspondence

 

 


ABSTRACT

BACKGROUND: Wetlands are the most threatened ecosystem in South Africa despite the range of ecosystem goods and services they provide. A significant cause of wetland destruction and degradation is a lack of understanding, by planners, policymakers and developers, of their ecological and socio-economic importance
OBJECTIVES: This study assessed the floristic composition and diversity of wetlands in the former Tlokwe Municipal area along a rural-urban gradient
METHODS: Fourteen wetland sites were surveyed along an urbanisation gradient. Vegetation surveys were done in quadrats along transects in each wetland recording the cover-abundance of each species. The data were analysed by using ordinations, similarity percentages, and the adjusted Floristic Quality Assessment Index
RESULTS: Overall, the proportional species composition of urban and rural wetlands was mainly similar. Trends indicated that the alpha diversity increased with both habitat size and heterogeneity along a rural-urban gradient. In all wetlands, indigenous species were the most abundant, with the highest score in the largest urban wetland. The floristic quality varied widely along the gradient with none of the sites in pristine condition
CONCLUSION: The similarity in species composition and floristic quality of the wetlands, as well as the high levels of indigenous species richness, indicated that urban wetlands are worthy of conservation. However, the signs of disturbances and the presence of alien species means that restoration strategies need to be implemented to improve the quality of the wetlands

Keywords: wetlands, rural-urban gradient, plant diversity, conservation value, floristic quality.


 

 

Introduction

Wetlands play an essential role in biodiversity conservation and in the supply of ecosystem services to humanity (Ramsar Convention Secretariat 2013). They fulfil several ecosystem services such as climate regulation, carbon storage, water reservoirs, runoff containment and flood risk reduction (e.g. Cimon-Morin & Poulin 2018; Mclnnes & Everard 2017). Moreover, they filter pollutants, conserve unique biodiversity, and act as a refuge for species (e.g. Bateganya et al. 2015; Mclnnes & Everard 2017). Important cultural ecosystem services include increased well-being of residents, eco-tourism, recreation and environmental education (e.g. Pedersen et al. 2019; Ramírez & Santana 2019).

History showed us that since the Iron Age, extensive drainage of the land took place so that it could be utilised for other purposes such as agriculture and settlements (Everard 1997; Hoeksema 2007). Land drainage happened due to an established idea that wetlands were only sources of disease and danger with no intrinsic value in themselves (Purseglove 1989). The Ramsar Convention of 1971 was the primary catalyst for the recognition of the importance of wetlands, and lobbied for global action towards their protection. However, Hettiarachchi et al. (2015) argue that this framework has key weaknesses that contribute to failures in urban wetland governance. Wetlands in urban areas are often regarded as wastelands (Panuccio et al. 2017), and subsequently, urbanisation is recognised as a significant cause of wetland loss (Panuccio et al. 2017). Not only can urban development cause destruction of wetlands, but it often also alters hydrological cycles, increases pollution that transforms wetlands, and influences species composition and species diversity (e.g. Baldwin 2011; Ehrenfeld 2000).

Local perceptions and the direct use of wetlands in urban areas vary. In an urban study in Canada, where residents did not visit local wetlands regularly, they still identified with the aesthetic value of wetlands and its importance as a habitat for biodiversity (Manuel 2003). In Cape Town, residents placed a high value on the provisioning services, mainly grazing for livestock, supplied by a peri-urban wetland and they derived 82% of their income from this wetland (Lannas & Turpie 2009). Wetlands were also found to be critical in reducing urban wastewater pollution in areas with malfunctioning or inadequate treatment plants (Bateganya et al. 2015). Moreover, wetland planning is seen as a critical element to be included in urban master plans due to its beneficial functions of flood control, water purification, microclimate regulation, and aesthetic and cultural value (Jia et al. 2011).

Notwithstanding the benefits mentioned above, urban wetlands are also important in urban biodiversity conservation. A study undertaken in Rome, Italy, on the importance of urban wetlands as a habitat for birds, recorded regular observations of species of conservation concern (Panuccio et al. 2017). Construction of urban wetlands in Greensboro, North Carolina, increased bat species richness and diversity (Parker et al. 2018). In addition, in Canada, some urban stormwater ponds had the same plant species, dragonfly and damselfly assemblages as natural ponds, which underlines the importance of urban wetlands to enhance local biodiversity (Perron & Pick 2020). Moreover, Semlitsch and Bodie (1998) have established that even if small or isolated, wetlands are integral for connectivity and maintaining biodiversity.

In South Africa, Working for Wetlands (2019) estimates that between 35% and 60% of the country's wetlands have been destroyed through drainage for crops and pastures, poorly managed burning regimes, overgrazing, disturbances to wetland soils, vegetation clearing as well as industrial and urban development (including mining activities). The latest National Biodiversity Assessment described wetlands as the country's most threatened ecosystem, stating that 88% of wetland areas are threatened and less than 2% are well protected (Skowno et al. 2019). Realising the importance of wetlands in South Africa, recent research efforts include the National Wetland Vegetation Database (Sieben et al. 2014) and the updated National Wetland Map 5 (van Deventer et al. 2020). Urban wetlands in South Africa have also seen an increase in research efforts and realisation of their importance, e.g. phytosociological studies of urban wetlands in Potchefstroom (Cilliers et al. 1998) and the Durban municipal area (Roberts 1993), the monetary valuation of provisioning services in a peri-urban wetland in Cape Town (Lannas & Turpie 2009), amphibians in urban wetlands (Kruger et al. 2015), health effects in fish in wetlands in Soweto (Bengu et al. 2017), wetlands as a habitat for birds (Calder et al. 2015), and the detrimental effects of urban development on wetlands (Gov-ender-Ragubeer et al. 2014). The current study aimed to add to the developing body of knowledge on urban wetlands in South Africa. The primary objective was to assess the floristic composition, quality and diversity of wetlands in the former Tlokwe Municipal area along a rural-urban gradient. The specific research questions were: (1) do urban and rural wetlands differ based on their floristic composition and quality? and (2) are the local urban wetlands worthy of conservation?

 

Materials and Methods

Study area

The study was carried out in the former Tlokwe Municipal area, which now forms a part of the larger, recently amalgamated, JB Marks Local Municipality. The former Tlokwe Municipal area covers 2 672 km2, which includes the urban area of Potchefstroom and its rural surroundings (Figure 1) and is located in the North West Province of South Africa. The population estimate of the study area was 179 604 in 2018 (JB Marks Local Municipality 2018). Research on the land-use transformation in the study area indicated significant changes in the cover of natural and urban areas over a period of 61 years (Pretorius et al. 2013). This inventory revealed a 23% increase in urban land coverage and a 68% increase in cultivated land-uses, decreasing the coverage of natural habitats by 12% and impacting the coverage of wetlands in the area.

 

 

The mean annual rainfall of the study area is 600 mm, falling mainly in the summer months with average temperatures between 0°C and 30°C, and frequent frost in winter (www.weathersa.co.za). The Mooi River flows through Potchefstroom and includes rural upstream and downstream segments with a city segment influenced by decades of urban development. There are various dams situated in the Mooi River system of which the Potchefstroom Dam and Boskop Dam are located within the municipal area. The water of the Mooi River system is contaminated by agricultural and mining pollutants, of which the impact of mining is of particular concern to the water quality in the system (Barnard et al. 2013). A recent study on phytoplankton assemblages and the measurement of physico-chemical variables in the Mooi River and its tributaries, confirmed that the system was polluted and that the water quality is declining (Koekemoer et al. 2021).

The study area lies within the Grassland Biome on the high central plateau of South Africa and at the confluence of three vegetation types: the Carletonville Dolomite Grassland, the Rand Highveld Grassland, and the Andesite Mountain Bushveld (Mucina et al. 2006). The wetlands in the study area are classified as grass lawn wetland vegetation (Sieben et al. 2016) and temperate grassy wetland vegetation (Sieben et al. 2017).

Site selection

All possible wetland sites along the Mooi River (excluding its tributaries) within the study area (Figure 1) were identified using satellite imagery. All sites accessible via roads and on private property, where owners granted permission, were visited. Wetlands chosen for this study were those ecosystems defined by the National Water Act (No. 36 of 1998) (South Africa 1998), as 'land that is transitional between terrestrial and aquatic systems where the water table is usually at or near the surface, or land which is periodically covered with shallow water, and land which, under normal circumstances, supports or would support vegetation typically adapted to live in saturated soils'. To delineate physical wetland boundaries of this study, wetland vegetation was used as an indicator. Fourteen wetland study sites were selected from the city and downstream segments of the Mooi River (Figure 1, Table 1). Approximately 80% of all the downstream wetlands were included in this study. Table 1 shows the selected wetland sites with their respective sizes and the number of transects surveyed per wetland.

Quantification of the rural-urban gradient

The rural-urban gradient was objectively quantified using four urbanisation measures namely: edge density, percentage vegetation cover, percentage urban land cover, and density of dwellings as selected by van der Walt et al. (2014) using Hawth's analysis tools version 3.27 (Beyer 2007) and ArcGIS 10 (ESRI 2010). The urbanisation measures were calculated for matrix areas represented by a 500 m buffer surrounding each selected wetland. Edge density is the sum of the length (m) of all edge segments divided by the area (in hectares) (McGarigal & Marks 1995). Percentage vegetation and urban land cover were both calculated as the total area covered by each land cover type divided by the total matrix area multiplied by 100 (McGarigal & Marks 1995). Urban land cover was digitised as all impervious built-up surfaces within the matrix, e.g. roads, buildings. The density of dwellings was calculated by digitised point counts of all the buildings in the matrix divided by the matrix area (McGarigal & Marks 1995).

Hierarchical, agglomerative cluster analysis in PRIMER 6 software (Clarke & Gorley 2006) and a subsequent Non-metric Multi-Dimensional scaling (NMDS) ordination indicated two clear groupings in the data (Figure S1 and S2, Supplementary material). Table 1 provides the specific urbanisation measure values for each selected wetland study site. A Pearson r correlation matrix was calculated to see whether one measure could be used as a proxy to represent the rural-urban gradient. All the measures correlated highly with each other, so we chose the percentage urban land cover to represent the gradient as the most intuitive measure (Table S1, Supplementary material). Based on the NMDS and cluster analysis, two sites were classified as urban and 12 as rural. The rural wetlands are all situated in matrix areas that have less than 3% urban land cover (Table 1), and the two urban sites had a percentage urban land cover ranging between 35 and 45%.

Vegetation surveys

Vegetation surveys were conducted from January to March 2014, during the flowering season of most plants. Plant species composition and abundance within the wetlands were determined by laying 100 m line transects across each wetland (Ruto et al. 2012). Transects were aligned along the longest axis of each wetland. Where sites were wide enough, adjacent transects were sampled parallel to one another, 20 m apart. The number of transects per selected wetland was determined by the size of the wetland under observation. A minimum of three transects were done per wetland (since the smallest wetlands could only fit three transects with 20 m between each transect). The largest wetland (U2) had 38 transects (Table 1). The presence and estimated percentage crown cover of each plant species were determined in a 1 m2 quadrant placed at 10 m intervals along the 100 m transects (Ruto et al. 2012) situated in homogenous areas of each wetland site. A total of 130 transects (1 254 quadrants, not all transects had 10 sample plots due to open water areas) were sampled in the 14 wetland sites.

The soft traits used in this study included the origin of each species (indigenous/alien), life history (annual/perennial), growth form (tree, shrub, forb, graminoid and geophyte) and wetland indicator status. The wetland indicator status divides plants into categories based on their expected frequency of occurrence in wetlands namely obligate wetland (>99%), facultative wetland (67-99%), facultative (34-66%), facultative upland (133%) and upland (<1%) (Tiner 2006).

Data analysis

The floristic composition of each of the wetlands was compared using NMDS. The ordination was performed using the Primer 6 software (Clarke & Gorley 2006). The average percentage cover of species per transect was used as input for the ordination. The sample data was first square root transformed to allow a greater contribution from the rare species, and then sites were compared using the Bray-Curtis dissimilarity coefficient. To determine the percentage dissimilarity between urban and rural sites based on the cover-abundance data, a similarity percentages (SIMPER) analysis was done in Primer 6. This analysis compared sites based on respective species composition and also indicates which species account for dissimilarities between sites.

The Wetland Index Value or WIV for each wetland site surveyed (i.e., community weighted mean) were calculated using the abundance of plant species and their ecological index value based on their wetland indicator status (Carter et al. 1988). The WIV provides a useful way of interpreting the status of wetlands based on their vegetation composition and is based primarily on the relevant species' wetland indicator status (Cowden et al. 2014). The values represent a wetness gradient with values less than 2.5 indicating a true wetland and values above 3.6 a non-wetland area (Carter et al. 1988)

Data on the abundance of plant species and their classification status were used to determine the adjusted Flo-ristic Quality Assessment Index (adjFQAI), as defined by Miller and Wardrop (2006). The adjFQAI addresses the problem of sensitivity to species richness and the contribution of non-native species (Miller & Wardrop 2006). The adjFQAI is an evaluation procedure that indicates the quality of the wetland habitat based on the relative abundance of indigenous, weedy, pioneer or alien invasive species within each surveyed site. Moreover, the adjFQAI calculates the percentage of the maximum value attainable by the site if all the species present were low tolerance indigenous species indicative of pristine wetland communities (Miller & Wardrop 2006). Species were assigned a 'coefficient of conservatism' that is 'a subjective rating indicating a species' preference for non-degraded natural communities' (Tiner 1999). Within the selected wetland sampling sites, each plant species was allocated a specific coefficient of 0 to 10 based on its conservation value relative to other native species in the surrounding area. Values ranged between alien species (0) and indigenous species with very low tolerances, to disturbance and high fidelity to habitat integrity (10) (Miller & Wardrop 2006). The allocation of the coefficient was based on available literature (Retief & Herman 1997; Van Ginkel et al. 2011).

 

Results

Plant species composition and diversity

The total number of species recorded in the wetlands along the rural-urban gradient was 102 (for the complete species lists and the list of invasive alien species recorded in the sites refer to Tables S2 and S3 in the supplementary material). Rural sites had a slightly higher gamma diversity than urban sites, but proportionately their overall species composition, diversity, origin, life history and wetland indicator status types were mostly similar (Table 2).

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