The biogeography of alien plant invasions in the Mediterranean Basin

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By Luigi Cao Pinna, Irena Axmanová, Milan Chytrý, Marco Malavasi, Alicia T. R. Acosta, Silvia Giulio, Fabio Attorre, Erwin Bergmeier, Idoia Biurrun, Juan Antonio Campos, Xavier Font, Filip Küzmič, Flavia Landucci, Corrado Marcenò, Maria Pilar Rodríguez-Rojo & Marta Carboni

The flow of alien plants to Mediterranean Europe. (a) The map of the 14 biomes as defined by Olson et al. (2001) and the corresponding colour-coded legend. (b) The number of observed species originating from each biome. (c) The mean expected number of species from each biome, based on the biome’s species pool (the biomes that donate more species than expected are marked with an asterisk). Original image from the paper.

The Mediterranean basin is a biodiversity hotspot highly invaded by alien plants. Alien species may compete with local biodiversity and are among the major causes of biodiversity loss. It is therefore important to understand where these species are coming from and why. Alien species flows are usually tracked considering the origins of species based on countries’ political borders, but these boundaries are of scarce importance for biodiversity. In contrast, in our study, we describe alien plant species flow to the Mediterranean Basin with regard to the natural origin of species at a global scale. To define these natural origins, we used the biogeographic concept of biomes, defined as homogeneous regions with similar vegetation physiognomy and climate. We analysed the Mediterranean part of Europe and Anatolia, in which we identified 299 extra-European alien species and their biomes of origin. We found that the other Mediterranean, temperate and xeric biomes of the world, donated more alien species than expected, suggesting that species coming from these biomes have developed pre-adaptations that favour their establishment in the Mediterranean Basin (see the schema above).

We also asked which drivers better described the unexpected alien species flow between biomes, whether it was geographic distance, trade exchange or climatic similarity. Theoretically, to successfully establish in a new region, alien species should reach a novel environment and survive in these new environmental conditions (e.g. severe summer aridity typical of the Mediterranean climate). Alien plants are by definition transported by humans, so geographically closer or more intensively trading biomes may be greater exporters of alien plants. However, our results show that neither geographic distance nor trade exchanges are significant drivers of the alien plant species flow to the Mediterranean Basin. This suggests that alien species movements across biomes are not a limiting factor, and we mainly expect species with appropriate adaptations to establish. Therefore, we estimated the climatic difference between each biome and our study area, focusing on the limiting conditions of the Mediterranean biome. Indeed, we found that the flow of alien species was lower from more climatically different biomes.

In a nutshell, the biome of origin of an alien species can be sensibly used to predict whether it will successfully establish in a new region. From theory to practice, it is mandatory to control and monitor alien species coming from climatically similar regions. Nonetheless, further screening is needed for the most problematic species.

Examples of the most widespread and emblematic alien species in Mediterranean Europe. (a) Carpobrotus edulis, (b) Erigeron annuus, (c) Solidago canadensis, (d) Robinia pseudoacacia and (e) Senecio inaequidens. Flower’s beauty is part of alien species success. Source of the photographs: the Internet, under the Creative Commons License.

This is a plain language summary for the paper of Cao Pinna et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12980). The post was prepared by Luigi Cao Pinna, Marta Carboni, Alicia Acosta and Marco Malavasi.