Introduction

The number of species that are naturalizing outside of their native ranges continues to increase (Seebens, et al. 2020). The associated ecological and economic costs make alien species management an urgent task (Pyšek, et al. 2020). Estimations and projections of current and future distributions are important for alien species management, especially as their spread will certainly be affected by other drivers of global changes, such as climate change (Bellard, et al. 2015, Liu, et al. 2023, Northrup, et al. 2019, Vilà and Hulme 2017). These changes alter both biotic and abiotic ecosystem properties known to be critical for biological invasions (Bellard, et al. 2016, Dullinger, et al. 2017, Rodríguez-Labajos, et al. 2009).
Predicting the species that could successfully escape into the wild and naturalize from a larger pool of deliberately introduced species (e.g., those cultivated in a region) is one of the biggest challenges in invasion ecology. Apart from specific functional traits and evolutionary history (e.g., seed mass, geographical origins and phylogenetic composition; Divíšek, et al. 2018, Lenzner, et al. 2021, Maurel, et al. 2016, Omer, et al. 2021, Omer, et al. 2022), climate matching between native and alien ranges has been demonstrated to be fundamental for the naturalization success of alien plants (Feng, et al. 2016, Mayer, et al. 2017, Richardson and Pyšek 2012). Climate-suitability analyses have thus emerged as promising tools for predicting the naturalization risk of alien plants (Dullinger, et al. 2017, Haeuser, et al. 2018, Oduor, et al. 2023, Robin Pouteau, et al. 2021, Thuiller, et al. 2005, Mark van Kleunen, et al. 2018). However, as climate change alters the abiotic and biotic conditions that mediate biological invasions, projections based on future climate scenarios are increasingly important for predicting the potential distributions of alien species. In temperate regions such as those in Europe and Northern America, climate warming is predicted to generally increase the likelihood of biological invasions (Bellard, et al. 2013, Dullinger, et al. 2017, Haeuser, et al. 2018, Oduor, et al. 2023). However, biological invasion studies, including those that use climate-suitability analyses, are geographically biased towards regions in the northern hemisphere (Pyšek, et al. 2008). Most of such studies have been conducted in intensively researched regions (e.g., Europe; Dullinger, et al. 2017, Haeuser, et al. 2018, Robin Pouteau, et al. 2021) or for a specific set of species (e.g., the 100 world’s worst invasive species; Bellard, et al. 2013). These biases in research might hinder a general understanding of how a changing climate will affect biological invasions across the globe. Indeed, some studies suggest that in non-temperate regions, and especially in areas with extreme climates (e.g., hot desert), the risk of alien plant naturalization will decrease rather than increase under climate change (Bellard, et al. 2013, Fulgêncio-Lima, et al. 2021).
The link between environmental suitability and the ability of alien plants to naturalize has long been established (Darwin 1859, Elton 1958). However, with continuing climate change, invasion dynamics have become more complex to predict. For example, while the current climate might have favored the naturalization of species that have already been established in a region, it is not clear whether the climate of the future will promote those that have been introduced but have not managed to naturalize yet. A warming climate might hence constrain the spread of some established alien species but simultaneously foster expansions or new naturalizations of others. More generally, a changing climate might also alter the characteristics (e.g., geographical origins) that are associated with successful naturalization in alien plants (Madani, et al. 2018, Soudzilovskaia, et al. 2013). For example, climate change might change the currently observed patterns that the majority of naturalized alien plants are native to continents of the Northern Hemisphere (van Kleunen, et al. 2015).
The introduction of organisms outside their native range has occurred through various pathways and for different purposes (Hulme, et al. 2008). However, intentional introduction for cultivation represents the primary pathway for vascular plant species (Faulkner, et al. 2020, Lambdon, et al. 2008). The majority of these species are grown in domestic gardens or have other economic uses (M. van Kleunen, et al. 2018, van Kleunen, et al. 2020). The high prevalence of cultivated species among naturalized alien vascular plants implies that future naturalizations will likely also emerge mainly from cultivated plant populations. Therefore, regional cultivated floras further increase our understanding of assessing current and future naturalization risks under a warming climate.
Southern Africa has a tropical to subtropical climate and large (semi-)arid regions (Engelbrecht and Engelbrecht 2015). Therefore, it is not immediately clear whether climate change will result in an increase or decrease in the area suitable for a species from the large alien flora that is currently cultivated in the region. In the Republic of South Africa, for instance, alien woody trees and shrubs are projected to experience a reduction in habitat and shift their ranges towards higher elevations with lower temperatures in response to future climate scenarios (Bezeng, et al. 2017).
Here, we used species distribution modeling to evaluate the potential current and future naturalization risk of 1,527 alien species currently cultivated in the ten countries of Southern Africa, of which 371 have already naturalized in at least one part of the region. Our specific objectives were (1) to predict the potential current distribution of cultivated alien plants of Southern Africa; (2) to assess how these potential distributions could change under a changing climate in the future by using two climate change scenarios; (3) to identify hotspot areas with the highest suitability for cultivated alien plants under current and future climatic conditions in Southern Africa; (4) to compare the biomes within southern Africa with respect to changes in climatic suitability for introduced cultivated plants; and (5) to assess whether and to which degree naturalization status and geographical origins explain current and future potential range size of cultivated alien plants of Southern Africa.