Allochthonous plant species are considered among the main environmental issues worldwide, being especially accused in the Mediterranean basin. The present study aims to update Balearic Alien Flora and discuss its time and spatial variation and invasiveness. Information regarding allochthonous plant records of the Balearic Islands were gathered and analysed based on published checklists and online databases. For each species several general traits were assigned based on previous references to characterise the alien Balearic Flora. Records of allochthonous species between 1827 and 2023 indicate that Alien Balearic Flora is composed of 624 taxa recorded unevenly among islands, with lower records in smaller islands. Alien flora is mainly composed of therophytes and phanerophytes being held mainly in human related habitats. Introduction pathway is mainly ornamental, with over 50% of taxa entering through gardening and horticultural practices. Main biogeographical origins are American, Mediterranean, Euroasiatic and Cape origins. Invasive status of Balearic alien flora shows that 44% of all taxa can be regarded as casual, while 35% are naturalised, and 17% can be considered invasive, being the latter increased in smaller islands. Time variations trends starting mainly in the 70 s and predictive models show that alien species increase exponentially, being the most represented taxa from synanthropic habitats, introduced as ornamentals, lianas, phanerophytes and therophytes from Cactaceae, Asteraceae and Leguminosae families. The results of the present study can serve as a model to facilitate the scientifically informed decision making and management of plant invasions both in the Balearic Islands and other continental insular systems and Mediterranean regions.

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The western Mediterranean Sea is one of the most affected areas by the introduction of alien seaweed. Penicillus capitatus Lam. and the recently introduced Halimeda incrassata (J.Ellis) J.V.Lamour are tropical algae that invade native algae assemblies in the shallow sandy beds of Mallorca (Balearic Islands, western Mediterranean) where they are already settled. The aim of this study was to evaluate the effect of a potential competition between the invasive P. capitatus and H. incrassata and the native Padina pavonica (L.) Thivy, by means of biomarkers. P. pavonica samples were collected in their habitat without invasive species and areas where P. capitatus and H. incrassata cohabitated. P. pavonica densities were measured in the three investigated areas. The coexistence of the invasive algae and P. pavonica was related to a significant decrease in the densities of native algae when compared to the area without the invaders. Antioxidant enzymes, catalase and superoxide dismutase, and the reduced glutathione and polyphenols levels were significantly increased in P. pavonica in the presence of both invasive algae. Malondialdehyde, a marker of oxidative damage, and the reactive oxygen species production tended to increase in the presence of the alien species, but without significant differences. The obtained results show that the presence of P. capitatus and H. incrassata altered the normal vegetative growth of P. pavonica and caused an antioxidant response that led to a context of stress, but without evidence of oxidative damage. In conclusion, the presence of the alien H. incrassata and P. capitatus can be considered a source of competitive stress for P. pavonica, although further research regarding the increased water temperature is required.

Allochthonous plant species are considered among the main environmental issues worldwide, being especially accused in the Mediterranean basin. The present study aims to update Balearic Alien Flora and discuss its time and spatial variation and invasiveness. Information regarding allochthonous plant records of the Balearic Islands were gathered and analysed based on published checklists and online databases. For each species several general traits were assigned based on previous references to characterise the alien Balearic Flora. Records of allochthonous species between 1827–2023 indicate that Alien Balearic Flora is composed of 602 taxa recorded unevenly among islands with lower records in smaller islands. Alien flora is mainly composed of therophytes and phanerophytes being held mainly in human related habitats. Introduction pathway is mainly ornamental, with over 50% of taxa entering through gardening and horticultural practices. Main biogeographical origins are American, Mediterranean, Euroasiatic and Cape origins. Invasive status of Balearic alien flora shows that 46% of all taxa can be regarded as casual, while 36% are naturalised, and 18% can be considered invasive, being the latter increased in smaller islands. Time variations trends starting mainly in the 70s and predictive models show that alien species increase exponentially, being the most represented taxa from synanthropic habitats, introduced as ornamentals, lianes, phanerophytes and therophytes from Cactaceae, Asteraceae and Leguminosae families. The results of the present study can serve as a model to facilitate the scientifically informed decision making and management of plant invasions both in the Balearic Islands and other continental insular systems and Mediterranean regions.