Assessing the risks to Mediterranean islands ecosystems from non-native plant introductions

The islands of the Mediterranean Basin probably represent some of the ecosystems globally most at risk from invasive species. Compared to neighbouring mainland areas, island floras have a significantly higher proportion of alien plant species. Yet the circumstances that have led to this situation and the subsequent consequences of plant invasions remain poorly understood. This knowledge deficit is addressed in this paper through a comprehensive review of recent research findings. Most alien plants occurring on Mediterranean islands have been introduced intentionally for economic purposes although there still exists a sizeable proportion that arrives by accident. A wide range of alien plant functional types have colonized Mediterranean islands. While certain traits appear important, e.g. reproductive strategies, species characteristics are closely allied to the habitats invaded. Large-scale biogeographic studies have highlighted a strong correlation between local and regional abundance suggesting a common driver of both small and large-scale invasion. Species with non-European origins appeared more successful at both spatial scales. These findings highlight the importance of estimating invasion success across a wide region thus minimizing local idiosyncrasies. Since the importance of different biological attributes may change along the dispersal, colonization and establishment phases of invasion, analyses of what makes a species invasive should also focus on specific invasion stages. For example, reproductive traits may be expected to be more relevant for long-distance colonization, while vegetative traits would prevail in achieving local dominance. Detailed mapping of species distribution highlighted that all habitats are to some extent at risk, though human disturbed areas proportionally more so. Impacts were examined for three focal species Ailanthus altissima, Carpobrotus spp. and Oxalis pes-caprae. Correlative analysis on six islands highlighted that impacts on biodiversity and soil properties are a function of both species and island with Ailanthus in general having the least impact while Carpobrotus reduced native plant diversity significantly. Although impossible to extrapolate to all invasive species, these results do highlight that significant losses in local species richness as well as ecosystem structure and function is likely to be occurring in the Mediterranean. To address this threat, mechanisms should be put in place to limit the further spread of known problem species across the Mediterranean through awareness-raising activities and better regulation of the import and disposal of alien plant material.