Vulnerability of riparian zones to invasion by exotic vascular plants

We compared the invasibility of riparian plant communities high on river banks with those on floodplain floors for four South African rivers. Analyses of abundant and significant riparian species showed that the floors have 3.1 times more exotic plants than the banks. The percent exotics ranges from 5% to 11% of total species richness for the banks, and from 20% to 30% for the floors. Species richness and percent exotics are negatively correlated for the banks, but not correlated for the floors. Despite great differences in climate, species richness, and landuse history, the percentages of exotic plants in three rivers in the Pacific Northwest of the USA and one river in southwestern France are similar to those in South Africa (24-30% vs. 20-30%, respectively). Furthermore, the high proportions of exotic species in these riparian plant communities are comparable to those reported for vascular plant communities on islands. We conclude that the macro-channel floor regions of the riparian zones of South African rivers are highly vulnerable to invasion by exotic vascular plants.