Grassland ecology along an urban-rural gradient using GIS techniques in Klerksdorp, South Africa

Abstract

Urban areas represent complex assemblages of unique vegetation communities. The multitude of influences on cities adds to this complexity rendering them an intriguing study object from an ecological point of view. Understanding the underlying patterns and processes operating in urban areas becomes increasingly important with large scale urbanization, making urban areas potential conservation sites of the future. The urban-rural gradient approach often used to study these patterns and processes, aims to quantify the existing gradient allowing comparisons of vegetation at different locations, each with diverse human influences. However, accurately quantifying the urban areas became difficult with the realization that gradients are non-linear and complex. Most previous studies cannot be compared with each other due to differences in measures used to quantify the gradient and a lack of a well defined definition for urban areas. Our study in Klerksdorp focused on testing a model developed in Melbourne (Australia) in an attempt to contribute towards creating a standard set of broad measures to quantify the urban-rural gradient. The methods used in Melbourne aims to set a general standard with which to globally compare urbanized areas taking into account the entire extent of the study area allowing multidimensional insights into the unknown gradients. Thereby placing individual studies into an urbanization context. At the heart of it, the main objective is to observe if any global patterns emerge to shed light on urbanization influences and drivers of ecological processes. In our study, SPOT 5 HRV satellite imagery and GIS techniques were used to calculate measures representing demographic and physical variables, as well as landscape metrics. The accuracy of the demographic measures was constrained by the scale of the available census data and subsequently more information is needed. Results showed that density of people, landscape shape index, and the percent urban land cover best quantified the observed gradient. Potential changes in grassland ecology were identified with vegetation surveys studying both the extant and the soil seed bank. Clear differences were observed in the extant vegetation composition of comparable grassland patches at different locations along the gradient, showing that urbanization does influence grassland vegetation composition and survival in the greater Klerksdorp area. The plant species richness of the existing and the soil seed bank showed significant correlations to the specific soil properties of each sample plot. Both demographic and landscape metrics also correlated significantly to some of the species subsets, emphasizing that both are needed to accurately quantify the urban-rural gradient of the greater Klerksdorp area and identifying potential patterns of species distributions. The urban-rural gradient described in the greater Klerksdoip area is not associated with an increase of exotic species towards the urban centre, but with a decrease of indigenous species richness as one nears the urban centre. Patterns and processes emerging from the current study could meaningfully influence planning and implementation actions concerning human development and conservation of a critically endangered vegetation type.