Compared to other facets of invasion science, the impacts of biological invasions have been understudied, but many studies have been published in the last decade. This paper reviews the growing body of evidence of impacts of invasions in South Africa. We classified information for individual species into ten ecological and four social categories of impact. We also reviewed studies that upscaled this information to larger spatial scales, as well as progress with assigning invasive species to impact severity categories. We identified 123 studies that documented the impacts of 71 invasive alien species, about 5 of the country’s naturalized alien biota. The most frequently reported impact category was species interactions (changes to habitat suitability, pollination networks or seed dispersal mechanisms), followed by direct competition, changes to ecosystem functioning (hydrology or nutrient dynamics), hybridization and predation. Trees and shrubs accounted for more than half of the species studied, but there were examples from most other groups of plants and animals. The social consequences of invasions have been less well studied at the level of individual species. Most studies (72%) considered the impacts of a single species, based on data collected on < 1 ha, and were completed in less than a year. Space-for-time substitution was widely used, but widespread collection of data from numerous small plots allowed for reporting impact over larger spatial scales. We also identified seven studies that either monitored impacts over longer periods (up to 40 years), or repeated surveys in the same area to assess change over time. Prominent landscape-scale impacts included reductions in water resources, the attrition of native biodiversity, reductions in rangeland productivity, predation of marine birds and freshwater fishes, and disease organisms affecting native mammals and trees. Nineteen studies at broader scales estimated substantial impacts on landscape-scale water yield, habitats and biodiversity, rangeland productivity, and the economic value of ecosystem services. Despite considerable progress, our understanding remains fragmentary. Impacts are expected to grow as invasions enter exponential phases of spread and densification and as the duration of invasions increases. A robust understanding needs to be developed to provide justification for management costs.
