Seed dispersal by vertebrates promotes invasion risk in the southern African grassland biome

Abstract

Vertebrate-mediated seed dispersal plays a crucial role in enabling the spread of invasive alien plants that threaten biodiversity in ecosystems such as the southern African grassland biome. Understanding the dynamics of the seed dispersal process by animals is important for assessing invasion risk and designing management interventions. This study investigated whether the southern African grassland biome is vulnerable to future plant invasions facilitated by frugivorous vertebrates in two known plant invasion hotspots in the Free State province: Boshof, dominated by Opuntia engelmannii (Cactaceae), and Fouriesburg, invaded by Pyracantha angustifolia (Rosaceae). We (1) monitored, documented, and compared the diversity of dispersal vector species in sites invaded by non-native woody and cactus species with non-invaded sites; (2) quantified the seed rain from vertebrates’ faecal samples and classified seeds and disperser species into functional groups to identify drivers of plant invasion of the grassland biome; and (3) assessed the effectiveness of vertebrate-mediated seed dispersal by measuring their post-ingestion viability. We predicted that frugivorous vertebrates are key drivers of fleshy-fruited plant invasions in this biome. We found taxonomically diverse dispersers (i.e. 27 families), with Bovidae and Numididae being dominant. Overall vector species diversity was high (H > 3.0), assuring effective population recruitment, although moderate in invaded sites (H > 2.5), and lower in non-invaded sites (H < 2.5). Over 9 months (March–November 2024), mammals dominated dispersal services (74%; N = 328 visits), followed by birds (26%), supporting our study prediction. We noted a high seed rain (i.e. 107,031 seeds) collected from 11,295 faecal samples associated with 43 vertebrate species. Invaded sites experienced significantly higher seed rain than non-invaded sites (P < 0.001) and were dominantly comprised of non-native species. Many seeds were medium-sized (1–5 mm), typical for successful invaders, and had a hard seed coat. Germination trials showed that ingested P. angustifolia seeds retained viability (46.7 ± 8.61%), similar to de-pulped controls (P = 0.53), while O. engelmannii seeds significantly lost viability (P = 0.002). The predicted seed dispersal distances were greater than 15.7 km for birds and ranged between 1.7 km and 8.6 km for mammals. We concluded that the grassland biome is vulnerable to further invasion by non-native plants dispersed by vertebrates, coupled with their long dispersal distances, and that integrating management of dispersal pathways into control plans is essential to optimise control efforts.