Plant– and arthropod diversity of vegetable gardens along a socio–economic gradient within the Tlokwe Municipal Area
Globally urbanization has increased to such an extent that more than half of the human population currently resides in cities. In the years to come, urban expansion will especially take place in developing countries through efforts to improve economic growth and poverty alleviation. This may have a negative effect on native biodiversity within and surrounding urban environments. However, residential areas with a high proportion of gardens form a significantly large part of urban environments and these domestic gardens contribute to the maintenance and preservation of biodiversity in cities. Although the preservation of biodiversity in these gardens is important in the overall conservation of urban green spaces, little is known about how these gardens can possibly contribute to conservation purposes in urban areas. Bearing in mind that anthropogenic activities are possible drivers of urban biodiversity, it is vital to quantify socio-economic aspects within urban ecological research. In developing countries, such as South Africa, the inclusion of socio-economic aspects are especially important because there is a wider gap between poor and wealthy households. There are also a larger number of people that are dependent on their gardens for subsistence purposes, such as vegetable gardening. In the Municipal Area of Tlokwe, South Africa, there exists a definite socio-economic gradient from the poorer western to the more affluent eastern part of the city. Five socio-economic status (SES) classes, primarily based on % unemployment, were used in this study. The ultimate aim of this study was therefore to determine the plant- and arthropod diversity within urban domestic gardens along a socio-economic gradient. Vegetable gardens within domestic gardens were selected to quantify plant- and arthropod biodiversity. Biodiversity of adjacent lawns were also sampled for comparative purposes. The study also attempted to determine to what extent socio-economic aspects of city residents may be possible drivers of biodiversity within the gardens. Various other factors that might have an effect on the plant and/or arthropod diversity were included such as soil characteristics, specific management factors of the gardens and other land-uses surrounding domestic gardens. Arthropod diversity was surveyd by means of pitfall traps and suction sampling in eight 0.25 m2 squares along an 8 m transect in each representative garden. Arthropods were identified up to morphospecies level. Vegetation was surveyed along the same transect and total species composition was determined. Plants were identified up to species level. The plant and arthropod surveys were conducted in both the vegetable gardens and lawns of all SES classes. For the soil samples a 1:2.5 water analysis was conducted. A social survey was conducted in all representative gardens by means of a questionnaire and a SPOT 5 satellite imagery was used to determine the land-use types in the areas surrounding the participating gardens. All the above mentioned factors were compared between the different SES classes. Diversity indices for the arthropods, multivariate statistical analyses and ANOVA analyses were applied to test for meaningful variables between socio-economic status classes as well as vegetable gardens and lawns. From the results it was evident that the more affluent SES classes had significantly higher arthropod diversity values, whilst the lower income classes had higher plant diversity. The factor analysis between the plants and arthropods with the surrounding land-uses revealed two significant factors. Firstly, arthropod diversity was influenced by domestic gardens in the surrounding landscape and there was a positive correlation between these two variables. This indicates that a high percentage of surrounding domestic gardens were possible drivers of arthropod diversity. No correlations were evident between plant and arthropod diversity. Secondly, the other significant factor showed that one SES class had a significantly higher percentage of woodlands and grasslands as opposed to two of the other classes that had a significantly higher percentage of built structures within the surrounding area. Differences were also apparent between the SES classes concerning management regimes, financial stability and level of education. The two more affluent SES classes had obtained a higher level of education and income and had management practices that were uncommon in the three poorer SES classes. This study proposes that domestic gardens are a means to conserve biodiversity in cities. Vegetable gardens in domestic gardens will also be able to harbour a larger diversity of plants and arthropods than the lawns. The socio-economic status of residents also had a significant effect on biodiversity and therefore it should be included in studies on urban domestic gardens. This study also provides additional knowledge to the fundamentals of the field of urban ecology and the importance of using domestic gardens as an urban green space for conservation purposes.