Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands

Aims Although biological invasions occur throughout the world, and some invaders are widespread in many habitats, few studies on the ecological impact of invaders have examined multiple sites. We tested how the impact of three widespread plant invaders changed depending on the identity of the species and the invaded island. We also tested whether relative species loss was lower in species-rich communities than in species-poor ones. Location We conducted floristic surveys and soil analyses in eight Mediterranean Basin islands: Crete and Lesbos (Greece), Sardinia (Italy), Corsica, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain). Methods We compared native species richness and diversity, proportion of life forms, soil percentage nitrogen, percentage organic carbon, C/N, and soil pH in nearby paired plots of 2 x 2 m: one control and one invaded by either the deciduous tree Ailanthus altissima, the succulent subshrubs Carpobrotus spp. or the annual geophyte Oxalis pes-caprae, across eight Mediterranean Basin islands. Results On average, the presence of invaders reduced species diversity, Carpobrotus spp. exhibiting the largest impact and Oxalis the least. However, the relative impact was island-dependent, and was positively but weakly associated with the species richness of the recipient community. Therophytes were the life form that experienced the largest decrease across islands. The effects of invasion on soil properties were very variable. Total N changed (increased) only in plots invaded by Ailanthus, significantly decreasing the C/N ratio. The presence of this tree increased soil pH, whereas the opposite was found in plots invaded by the other two species. Organic C increased in plots invaded by Ailanthus and Carpobrotus species. Main conclusions By conducting an analysis at multiple sites, we found that the three plant invaders had an impact on plant community structure not entirely concordant with changes in soil properties. The impacts depended on the identity of the species and of the invaded island, suggesting that impact of invaders is context-specific. The impact in terms of species loss was not lower in species-rich than in species-poor communities.