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.