Ecological interactions, especially those that are beneficial (i.e. mutualism) or detrimental (i.e. parasitism), play important roles during the establishment and spread of alien species. This chapter explores the role of these interactions during biological invasions in South Africa, covering a wide range of taxonomic groups and interaction types. We first discuss the different ways in which interactions can be reassembled following the introduction of alien species, and how these depend on the eco-evolutionary experience of the alien species. We then discuss documented examples of parasitism and mutualism associated with invasions in South Africa and how these relate to various ecological and evolutionary hypotheses aimed at explaining species invasiveness. Selected examples of how invasive species impact on native species interactions are provided. A diverse array of biotic interactions (e.g. pollination, fish and mollusc parasitism, plant-soil mutualistic bacteria, seed dispersal) have been studied for various invasive species in South Africa. Surprisingly, only a few of these studies explicitly tested any of the major hypotheses that invoke biotic interactions and are commonly tested in invasion ecology. We argue that many invasions in South Africa are promising candidates for testing hypotheses related to species interactions and invasiveness.

Seed dispersal effectiveness (SDE) for vertebrates is determined as a product of quantity (numbers of seeds dispersed) and quality (probability that a dispersed seed becomes a new adult plant) components. There is a limited understanding of seed dispersal effectiveness for different avian vectors in South Africa, yet birds are important for long-distance dispersal for native species, and are implicated in the dispersal of many invasive plant species such as Acacia cyclops. Consequently, SDE was investigated in four bird species that are common seed dispersers in South Africa, of which two are frugivorous species (the Knysna turaco Tauraco corythaix and the red-winged starling Onychognathus morio), and two are granivorous species (the red-eyed dove Streptopelia semitorquata and the laughing dove Streptopelia senegalensis). Individuals of these species were caged and fed mature seeds of A. cyclops to determine quality of seed treatment in the gut. SDE was computed as a product of germination rates of gut-passed seeds of A. cyclops (i.e. quality) and the average bird body mass (i.e. proxy for seed load as quantity component) for the four bird species. Results show that frugivorous birds had significantly greater SDE than the granivorous bird species. SDEs for respective bird species also showed notable differences: the Knysna turaco had highest SDE followed by the non-significantly different red-eyed dove and red-winged starling, while the laughing dove had the lowest SDE of all bird species. However, it is likely that the two ubiquitous dove species, and both the colonial and nomadic red-winged starling, might have higher SDEs associated with large home ranges than the Knysna turaco with its more restricted geographic range. The results highlight the previously overlooked importance of doves and the other birds in mediating the invasion of A. cyclops, and the dispersal processes for the seeds of other plants included in their diet.

As a conservation strategy, the South African National Biodiversity Institute (SANBI) establishes biodiversity gardens in areas with unique vegetation types that are vulnerable to extinction. The study aimed to (1) determine the vegetation cover dynamics of the Free State National Botanical Garden (FSNBG) over a 30-year period (1987–2017), focusing on different vegetation classes; (2) evaluate the ecological integrity of the Critical Biodiversity Area 1 (CBA1) vegetation using species abundance and vegetation cover; and (3) quantify potential conservation threats that may be drivers of vegetation cover changes. The “moderate vegetation cover” and “dense vegetation cover” had increased by 25.1 ha and 8.6 ha respectively in the FSNBG. Woody vegetation cover expanded significantly over the past 30-year period, suggesting “bush” encroachment. Shannon–Wiener diversity indices showed high overall plant species diversity of CBA1 vegetation type (H = 3.5), with the vegetation cover remaining high (79.6 ± 15.9%), 50 plant species no longer existing, suggesting reduced taxonomic richness. Major conservation threats included the presence of 27 alien and invasive plant species interspersed within different vegetation patches and anthropogenic habitat fragmentation in the past 19 years (i.e. covering ~ 18% of the buffer zone). We conclude that increased vegetation cover is associated with bush encroachment and we recommend interventions to reduce the population density of woody plants and establish permanent vegetation monitoring plots.