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).