Determining the long-term effectiveness of an invasive alien plant removal strategy along the Bakwena toll route in Gauteng and North West, South Africa

Abstract

Alien invasive plants (AIPs) are major drivers of biodiversity loss, and roads often act as effective corridors for their spread. This study analysed long-term changes in AIPs along the Bakwena N4 Toll Route, South Africa, using annual audit data collected between 2011 and 2024. Biodiversity indices and statistical analyses were used to assess rates of change, identify persistent invasion hotspots, and predict future trends. The study further compared invasion dynamics between the open, grass-dominated Marikana Thornveld and the denser woody Savanna vegetation types along the route, examined patterns across urban, peri-urban, and rural land-use zones, and evaluated the influence of vegetation quality and soil type on AIP presence. Results showed clear contrasts between land-use zones and vegetation structures. Urban areas displayed fluctuating but broadly stable AIP counts, with no significant long-term decline, suggesting continued reinvasion linked to disturbance. Peri-urban areas remained persistent hotspots, supporting high diversity and richness despite management interventions. Rural areas showed significant reductions in AIP densities, although diversity trends were more variable, indicating ongoing species turnover. Plots within the more open Marikana Thornveld differed from those in the other, generally more wooded or structurally complex vegetation types, indicating that vegetation structure may influence invasion patterns along the route. Degraded vegetation quality was strongly associated with invasion hotspots, while soil type showed limited explanatory power. Similarity and dissimilarity indices revealed high turnover in urban assemblages, relative stability in peri-urban environments, and gradual compositional shifts in rural areas. The findings indicate that management has reduced AIP densities in rural sections of the route, but has been less effective in urban and peri-urban areas. Persistent reinvasion in disturbed areas highlights the need for targeted, adaptive management. Sustained monitoring, restoration, and context-specific interventions will be important for maintaining ecological integrity along the Bakwena N4 Toll Route.