Alien species can have massive impacts on native biodiversity and ecosystem functioning. Assessing which species from currently cultivated alien floras may escape into the wild and naturalize is hence essential for ecosystem management and biodiversity conservation. Climate change has promoted the naturalization of many alien plants in temperate regions, but whether outcomes are similar in (sub)tropical areas is insufficiently known. In this study, we used species distribution models to evaluate the current naturalization risk of 1,527 cultivated alien plants in 10 countries of Southern Africa and how their invasion risk might change due to climate change. We assessed changes in climatic suitability across the different biomes of Southern Africa. Moreover, we assessed whether climatic suitability for cultivated alien plants varied with their naturalization status and native origin. The results of our study indicate that a significant proportion (53.9%) of the species are projected to lack suitable climatic conditions in Southern Africa, both currently and in the future. Based on the current climate conditions, 10.0% of Southern Africa is identified as an invasion hotspot (here defined as the top 10% of grid cells that provide suitable climatic conditions to the highest numbers of species). This percentage is expected to decrease slightly to 7.1% under moderate future climate change and shrink considerably to 2.0% under the worst-case scenario. This decline in climatic suitability is observed across most native origins, particularly under the worst-case climate change scenario. Our findings indicate that climate change is likely to have an opposing effect on the naturalization of currently cultivated average plants in (sub)tropical Southern Africa compared to colder regions. Specifically, the risk of these plants’ naturalizing is expected to decrease due to the region’s increasingly hot and dry climate, which will be challenging for the persistence of both native and alien plant species.

Most studies on biological invasions focus on the later stages of the invasion process, i.e. after species have already become naturalized. It is frequently overlooked, however, that patterns in origin, phylogeny, and traits of naturalized alien species might largely reflect which species have been introduced in the first place. Here, we quantify and account for such introduction biases by analyzing 5,317 plant species introduced for cultivation in Southern Africa. We show that this cultivated alien flora represents a non-random subset of the global flora, and that this bias at the introduction stage largely drives patterns in origin, growth form and phylogenetic composition of the naturalized flora. For example, while species from Australasia are, compared to the global flora, disproportionally overrepresented in the naturalized cultivated flora of Southern Africa, this pattern is solely driven by their higher likelihood of having been introduced for cultivation. We also show that among cultivated aliens, naturalization success was correlated with intermediate seed mass and height, as well as high specific leaf area. Our quantification of introduction biases demonstrates that they are huge, and that accounting for it is essential to avoid over- or under-estimation of the characteristics of successfully naturalized alien plants.