Changes in the cover of selected invasive alien plant taxa between 2008 and 2023 in South Africa

In 2007, the South African government commissioned a national-scale survey of invasive alien plants. The survey estimated the cover of 14 plant taxa on 47,830 plots of 100 x 100 m, using observers in low-flying aircraft. The survey was completed in 2008 and repeated between 2016 and 2023. An estimated 1.6% of the country was invaded at the end of the second survey, an increase of 10.6% between the two surveys. Trees in the genera Acacia, Eucalyptus, Pinus and Prosopis accounted for almost three quarters (72.4%) of the cover at the end of the second survey. The cover of most taxa increased between the two surveys, but some Acacia species, as well as Hakea shrubs and cacti (family Cactaceae), all of which are under biological control, decreased in cover. While other species increased despite biological control, there were no decreases in species that did not have biological control. The Indian Ocean Coastal Belt was the most invaded terrestrial biome, with 11% of the remaining natural vegetation invaded, followed by the Fynbos, Albany Thicket and Grassland biomes with 5.2, 2.8 and 2.5% alien plant cover respectively. For the remaining biomes, cover of invasive taxa was 1.1% or less. Wattles (Acacia dealbata, A. decurrens and A. mearnsii) and Eucalyptus species occupied the largest area and spread more rapidly than other taxa, but the relative contribution of taxa differed between biomes. Pinus and Prosopis trees were substantial threats in the Fynbos and arid area biomes respectively. Our results suggest that control efforts have not managed to stem the spread of invasive alien plants at a national scale, other than for some of those under biological control. This suggests that scarce funds should be focussed on priority areas, and that there should be a greater emphasis on expanding the use of biological control.