Road verges facilitate exotic species’ expansion into undisturbed natural montane grasslands

Questions: How has exotic plant species richness and composition changed in and adjacent to a montane road verge over a 10-year period? Are montane road verges conduits of exotic species' dispersal into adjacent, undisturbed hinterland? Location: Montane grassland, Maloti-Drakensberg Transfrontier Conservation Area, South Africa.
Methods: We surveyed road verge and adjacent hinterland transects (25 x 2 m; N = 80) across an elevational gradient of 1,500-2,874 m a.s.l. in 2007 and 2017. In each transect, we estimated the cover of each exotic plant species and the total cover of indigenous species. Generalised Linear Models were fitted to test if exotic species' richness and cover had changed over time. A Canonical Correspondence Analysis was used to determine if exotic species' composition had changed over time.
Results: Ten years onwards, exotic species richness had increased significantly across the entire elevational gradient, predominantly in the low-to mid-elevational range. Road verge and adjacent hinterland transects differed in species composition in 2007, but no longer in 2017.
Conclusion: Exotic species richness and composition displayed a high spatiotemporal rate of change. Over time, exotic species increasingly moved into the hinterland from the verges as a result of human-mediated colonisation pressure. These results indicate that montane road verges are far better conduits for the dispersal of exotic species than previously assumed, and that further colonisation of the hinterland by exotics can be expected.