Discussion
Numerous regional studies have indicated that alien plants are projected
to experience increased range sizes due to climate change, particularly
in the Northern Hemisphere (Adhikari, et al. 2022, Bellard, et al. 2013,
Dullinger, et al. 2017, Thapa, et al. 2018). Here, we used species
distribution models to predict the potential distribution of the
cultivated flora of Southern Africa under current climatic conditions
and assessed how it would change under a warmer future climate. The
results of our study indicate that, under current conditions, many
cultivated alien plants could thrive in many regions within the area.
Moreover, Southern Africa shows a higher prevalence of climatically
suitable areas for naturalized cultivated plants than non-naturalized
ones. However, our analysis indicates that the potential distribution of
most cultivated alien plants in Southern Africa will decline under a
warmer future climate.
The projected shrinkage of suitable climatic space for alien species
under climate warming contradicts previous studies for other regions.
Climate warming, which may allow cultivated species to escape and
naturalize outside their native ranges in cooler and humid (arctic to
temperate) regions (Adhikari, et al. 2022, Dullinger, et al. 2017,
Haeuser, et al. 2018), is unlikely to have this effect in Southern
Africa and other hot and semi-arid subtropical parts of the world
(Beaumont, et al. 2011). This result parallels findings by Broennimann,
et al. (2006), who have shown that also the native flora of Southern
Africa will experience range losses under climate change. Particularly,
endemic plant species in Southern Africa are predicted to lose
approximately 50% of their suitable ranges by 2050, even under the most
optimistic climate change scenario (Broennimann, et al. 2006). In line
with our findings, Bellard, et al. (2013) projected that the potential
distribution of the 100 world’s worst invasive species will decrease
under climate change in some regions, especially in regions like the
western part of Southern Africa. Similarly, (Bezeng, et al. 2017)
projected that climatically suitable areas for the majority of alien
trees and shrubs in South Africa would contract under climate change.
The southeastern region of Southern Africa appears to remain the
invasion hotspot for the cultivated alien flora (Fig. 2a-c) into the
future. This pattern might be explained by the predicted higher increase
in temperature and the higher aridity in the western part of Southern
Africa, which already experiences extreme climatic conditions (such as
the Namib desert) (Almazroui, et al. 2020). Our findings further support
this pattern, as we identified the Maximum Temperature of the Warmest
Month as the most influential bioclimatic variable in determining the
future distribution of climatically suitable areas of alien cultivated
plants in Southern Africa (Supporting Information Fig.S3). Temperature
was also found to be the most essential macroecological factor reducing
the species richness and diversity of native savanna flora at the plant
community scale in Kruger National Park, Republic of South Africa
(Hejda, et al. 2022). We also observed regional variation in the
specific biomes in Southern Africa; for example, the tropical biomes are
expected to experience fewer losses of alien species due to future
climate changes. In contrast, other biomes, such as semi-deserts, are
expected to undergo significant contractions in the potentially suitable
area for a large number of cultivated alien plants. The regional
variation in future climatic suitability might be again attributed to
the fact that regions that suffer more losses are subject to
increasingly hot and arid climatic conditions under climate change that
will affect the survival of most native and cultivated alien plants
(Almazroui, et al. 2020).
Naturalized plants have significantly larger climatically suitable areas
than non-naturalized cultivated alien plants under current climatic
conditions. This supports previous findings that suggest a correlation
between climatic suitability and naturalization success (Feng, et al.
2016, Haeuser, et al. 2018, Mayer, et al. 2017). However, under warmer
climatic conditions in the future, differences between naturalized and
non-naturalized species are predicted to decrease. For example, the will
be no significant difference in mean climatic suitability between
naturalized and non-naturalized (Fig. 3c, Supporting information Table
S1). This implies that as climate change progresses, some
non-naturalized cultivated alien plants may still have a high potential
to become naturalized. Moreover, it may indicate that invasion dynamics
will become more complex to predict with continuing climate change.
Despite the overall decrease in the future potential distribution of
cultivated alien species in Southern Africa, our analysis also revealed
variations among species with different native origins. Species native
to continents mainly located in the Northern Hemisphere (i.e., Europe,
other parts of Africa, and Asia Temperate) were less likely to find
suitable climatic conditions in Southern Africa than species native to
the Southern Hemisphere. In line with our findings, a recent study
showed that the potential alien ranges of European endemic plants would
shrink under climate change (R. Pouteau, et al. 2021). Our result showed
that plants of tropical origins are more likely to encounter suitable
climatic conditions in Southern Africa than those from temperate
regions. This could be both because tropical biomes in Southern Africa
are less likely to lose climatic suitability under climate change (Fig.
6) and because these species were introduced from areas with a climate
similar to Southern Africa. For example, the majority of alien trees and
shrubs in Southern Africa were introduced from regions with similar
climates, such as Australia and Southern America (Rejmánek and
Richardson 2013).
The reduction in climatic suitability for cultivated alien plants in
Southern Africa can be attributed to the future climatic conditions that
will be unfavorable to their growth. However, it is essential to note
that by the end of this century, the region is projected to experience
novel combinations of climatic factors, which could affect species
distributions (Williams, et al. 2007). It is possible that current
species distribution models (SDMs) do not appropriately account for how
these cultivated plants will respond to such novel conditions,
potentially leading to an overestimation of the effect of future climate
on species distribution (Early and Sax 2014, Fitzpatrick and Hargrove
2009).