**1. Introduction**

Over the past four centuries, some alien woody plants were introduced intentionally into South Africa to meet the growing human demands for various goods and services (charcoal, timber production, ornaments, dune stabilization, medicine, etc.; [1–5]). The selection and use of plants by humans have been shown to be non-random, but this nonrandomness has been widely demonstrated for native plants (e.g., [6–10]). However, it is increasingly shown that alien species intentionally selected and introduced into new environments for human use (e.g., medicine) are also non-random selections from local floras (e.g., [11]; see [12] for further references). While some of the introduced alien plants fail to establish a viable population in their recipient environments, many others have naturalized, and some of the naturalized species have become invasive [13]. Alien invasive plants are naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from their points of introduction, and thus have the potential to spread over a considerable area [14]. Although some of the invasive species pose a severe ecological threat to their recipient systems [15–17], the levels of threat are not equal; some species are strong invaders and pose high ecological and economic threats, while others are weak invaders [12,18,19].

In the face of these threats, a massive research effort has focused on understanding the predisposition of alien plants to invasion success in a foreign environment [5,20–23]. The

**Citation:** Yessoufou, K.; Ambani, A.E. Are Introduced Alien Species More Predisposed to Invasion in Recipient Environments If They Provide a Wider Range of Services to Humans? . *Diversity* **2021**, *13*, 553. https://doi.org/10.3390/d13110553

Academic Editors: W. John Kress, Morgan Gostel and Michael Wink

Received: 20 August 2021 Accepted: 26 October 2021 Published: 30 October 2021

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findings reported in these studies are, to some extent, contradictory. This is because the drivers of invasion of some taxa in an environment do not explain the invasion of other taxa or the same taxa in different habitats [23–27], thus revealing the environment-dependent nature of invasion and the need for case-specific managemen<sup>t</sup> solutions. Here, we propose that the services that alien species provide, and which motivate their introduction into a new environment, should better predict the invasion status of these species (naturalized vs. invasive) in their recipient ranges. There are two reasons underlying this expectation.

Firstly, since functions (ecological or physiological) generate services, and functional traits correlate with the invasion status of species in South Africa [21], services should also predict the invasion status of alien species. Several studies have tested, albeit indirectly, this potential link between services and invasion by focusing only on the link between functional traits and invasion [21,28]. Secondly (and this is the most critical basis for our expectation), an alien species that provides a diverse array of services is more likely to be introduced independently multiple times and in various numbers into new environments than an alien species that provides only one or a few services [9,29]. However, we also acknowledge that multiple independent introductions in large numbers may not necessarily be due to a diverse array of services, but rather could be driven, for some species, by a single service of high use-value for local communities. These alternative scenarios match the prediction of the propagule pressure theory [30], also termed "introduction effort" [31], which is the number of individuals introduced into a new environment and how often the introduction events occur [30,32].

In the present study, our aim is to link the services of naturalized woody plants to their invasiveness status in South Africa. Specifically, we ask the following questions: Are plant species selected and used by humans a random selection with regard to the services they provide? Does the total number of services (used as a proxy for propagule pressure) of alien species predict their invasion status? We explored these questions using the alien woody flora recorded as intentionally introduced to South Africa [21].
