Characteristics of the naturalized flora of Southern Africa largely reflect the non‐random introduction of alien species for cultivation

Ali Omer,Qiang Yang,Jan Pergl,Petr Pyšek,Mark Van Kleunen,Noëlie Maurel,Franz Essl,Wayne Dawson,Jonas Bleilevens,Patrick Weigelt,Trevor Fristoe,Holger Kreft

doi:10.1111/ecog.05669

Abstract

Biological invasions are one of the most defining features of the Anthropocene. Most studies on biological invasions focus on the later stages of the invasion process, that is 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 assess such introduction biases by analyzing 5317 plant species introduced for cultivation (i.e. primarily as ornamental garden plants) in the 10 countries composing 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 contributes to patterns in geographic origin, phylogenetic composition and traits 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 driven by their higher likelihood of introduction for cultivation. The same is true for the overrepresentation of free‐standing woody species in the naturalized cultivated flora. The strong phylogenetic clustering of the naturalized cultivated flora is also, to a large extent, driven by introduction bias. Although functional traits explained little variation in naturalization success of cultivated plants, naturalization success was more likely for plants with intermediate seed mass and height and high specific leaf area. Thus, despite strong biases in which species have been introduced to Southern Africa, there are significant patterns in the species characteristics related to naturalization probability. Our quantification of introduction biases demonstrates that they are huge, and that accounting for it is important to avoid over‐ or under‐emphasizing the characteristics of successfully naturalized alien plants.

Highlights

Anthropogenic activities are increasingly changing abiotic and biotic components of the biosphere (Lewis and Maslin 2015)
Some of the cultivated species are native to Patterns in native range size of introduced and naturalized cultivated plants Compared to the global flora, the native range sizes of the introduced and naturalized cultivated floras were significantly larger than expected
When the average native range size of the naturalized species was compared to that of the introduced cultivated flora of Southern Africa, it was still larger than expected (SES = 16.7, p < 0.001, Fig. 3a), but the standardized effect size (SES) was lower than when compared to the global flora

Summary

Introduction

Anthropogenic activities are increasingly changing abiotic and biotic components of the biosphere (Lewis and Maslin 2015). Over 13 000 vascular plant species have invaded regions outside their known native geographic ranges (van Kleunen et al 2015a). With a few exceptions (Bradley et al 2012, McGregor et al 2012, Moodley et al 2013, Gippet and Bertelsmeier 2021, Palma et al 2021), it is not known whether species from certain continents and families and with certain native range sizes are over-represented among naturalized species because they were more likely to be transported and introduced elsewhere (i.e. they show an introduction bias), or because they have a higher inherent ability to naturalize once introduced. Accounting for biases resulting from the characteristics of species that have been introduced is an essential, but rarely considered, step for understanding geographic, ecological and phylogenetic patterns in naturalization success (Palma et al 2021)

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Discussion

Conclusion