Annual monitoring reveals rapid upward movement of exotic plants in a montane ecosystem

There is increasing evidence that invasive species are threating montane ecosystems globally. However, trends in species distribution are difficult to observe directly due to a lack of data with suitable spatio-temporal resolution. Here, we aimed to detect spatio-temporal trends in exotic plants in a montane ecosystem, and to determine which drivers had a role affecting these trends. Each year, for a period of 7 years, we recorded the upper elevational range limit of exotic plant species in road verges along an elevational gradient of 1500-2874 m a.s.l. in southern Africa. We fitted repeated-measures ANOVA models to test if upper elevational range limits changed over time. Generalized least squares models showed that exotic richness of annuals increased by 3.9 species per year. Also, the upper elevational range limits of established exotics ascended by 24.5 m/year for annuals (N = 17 species), and by 9.7 m/year for perennials (N = 26). These upward trends were too rapid to be explained by slow-acting drivers such as climatic change or time since species introduction. The rates of increase indicate that many exotics were not yet in equilibrium with the environment and, therefore, had not been in the region long enough to have filled their potential niches. Exotic species could reach much higher elevations than expected, indicating that current prediction models are likely an underestimation of potential distributional ranges. The spatial clustering of upper elevational range limits around potential points of introduction indicates ongoing human-mediated propagule pressure as the major cause of rapid exotic range expansion, especially along roads and near dwellings. Montane road verges are regularly disturbed by erosion and maintenance, creating unoccupied habitats, while traffic in the form of vehicles and tourists facilitates the introduction of new species. This suggests that easily accessible montane ecosystems are much more susceptible to invasions than previously assumed, due to a combination of anthropogenic disturbance and ongoing propagule pressure.