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ROAD VERGES: SOURCES BUT NOT CORRIDORS IN THE DISPERSAL OF INVASIVE ALIEN PLANTS

Dr. Jesse Kalwij investigates the road verges on the R61 near Beaufort West.

The ecological functioning of road verges is ambiguous. In depleted or transformed landscapes, verges may contain the last remnants of the original vegetation. In relatively undisturbed landscapes, however, the construction of roads and the subsequent maintenance of verges often results in a permanent change in vegetation composition, water drainage system, and/or disturbance frequency and intensity. This set of ecosystem changes are thought to provide invasive alien plants the opportunity to spread from urban areas via road verges into uninvaded rangeland.

Jesse Kalwij, a post-doctoral associate at the C•I•B, tested this hypothesis for invasive and other exotic plants around three cities in the semi-arid Nama-Karoo biome of South Africa: Beaufort-West, Prieska, and Middelburg.

Road verges were found to be heavily invaded by exotic plants, especially where verges were regularly or heavily disturbed. However, urban areas were not the expected major sources from which exotic plants invaded the verges. Instead, proximity to farmhouses was an important factor contributing to the degree in which verges were dominated by exotics. This domination of verges by exotics poses a concern for the adjacent rangeland, as rangeland is now more likely to be colonized by exotics. For example, following a disturbance event such as intensive grazing or fire, as the large number of exotic seeds from the verge is continuously raining down onto the adjacent area and has a better chance of establishing than the indigenous, slow-growing vegetation.