The Invasion Potential of the Alien Shrub Pyracantha koidzumii in Comparison with the Well-Established Pyracantha angustifolia in the Southern African Grassland Biome

Pyracantha koidzumii (Hayata) Rehder is native to Taiwan, where it has an endangered IUCN conservation status, although it is invasive in other parts of the world. It was introduced to South Africa as an ornamental species. In South Africa, P. koidzumii is classified as having density-based negative impacts, although its distribution is sparse and no impact assessments have been conducted to date, compared with its established congener, Pyracantha angustifolia. We asked the following questions: (1) does the current distribution of P. koidzumii and P. angustifolia indicate any relationship with anthropogenic activities as the main driver of plant invasions; (2) what are the seed viability and germination capacity of P. koidzumii?; and (3) what is the current distribution and habitat suitability of P. koidzumii relative to P. angustifolia, and does P. koidzumii show any potential to be invasive in different climate change scenarios (i.e., SSP1 and SSP5). We found a significant relationship between land use and cover and species occurrence records, with disturbed areas having the highest occurrence records (43.5%; N = 4297), followed by grassland (23.4%). The seed viability and germination capacity were high (78.6–87.2%; n = 50), confirming that P. koidzumii can successfully recruit in southern Africa. Climate suitability projections across scenarios for both species yielded a strong data fit (i.e., training AUCs above 0.9), with precipitation as the most important climate predictor. The climate-based habitat suitability projections showed greater net expansion for both species in SSP1, with P. angustifolia projected to invade 254,974 km2, while P. koidzumii could invade 49,829 km2. Alternatively, there was contraction (i.e., positive net habitat loss) of suitable habitat in SSP5, characterised by significant area loss for P. angustifolia (12,0139 km2), while P. koidzumii displayed a moderate loss (13,174 km2). For the first time, this study showed that P. koidzumii has high potential to invade the southern African grassland biome, driven by high germination combined with biophysical disturbance, and that its future habitat occupancy will be moderately affected by extreme environmental temperature conditions.