Impact assessment of Ailanthus altissima, Carpobrotus spp. and Oxalis pes-caprae on eight Mediterranean islands
A study was done on eight Mediterranean islands to assess how the impact of three widespread plant invaders (Ailanthus altissima, Carpobrotus spp. and Oxalis pes-caprae, all on the EPPO list of invasive alien plants), varied according to the species and the invaded island. Floristic surveys and soil analysis were conducted in Crete and Lesbos (Greece), Sardinia (Italy), Corse, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain).
On average, a reduction in native plant diversity and richness was found associated with the invasion of these three species. Compared with uninvaded controls, plots invaded by Carpobrotus (Aizoaceae) exhibited 36% lower species richness, those invaded by Ailanthus (Simaroubaceae) showed 23% lower species richness and by Oxalis pes-caprae (Oxalidaceae) a reduction of 10%. When community was species-rich, species loss was not higher, suggesting that areas of higher native plant diversity are not protected against species loss.
As regards life forms, therophytes appeared to be the most negatively influenced by the presence of an invader, but the effect among islands was significant only for Ailanthus and Carpobrotus. The therophytes decrease is of concern because they represent a large proportion of endemic species in the Mediterranean Basin. High rates of therophyte extinction (90%) have already been documented, especially at low altitudes. The decrease of therophytes in plots invaded by Ailanthus was possibly a consequence of allelopathic substances reducing establishment by inhibiting seed germination. In plots invaded by Carpobrotus, the dense mat of overlapping stems and considerable litter layer could be a substantial obstacle to the germination and establishment of therophytes. The low impact of Oxalis on therophytes may be explained by its early and short season phenology (rosette production in late autumn and senescence in early spring), limiting competition with annuals.
Changes in soil properties were less evident than changes in vegetation structure. Comparing habitats invaded, Carpobrotus occurs in dunes, rocky cliffs and coastal garrigues, encompassing a high number of endemic, rare or vulnerable species, thus its consequences on native species is of particular concern. Ailanthus and Oxalis occur in anthropogenic and ruderal habitats. Although in general ruderal communities are not perceived as having high conservation value, many Mediterranean endemic and vulnerable segetal plants occur in these habitats.
Sources
Vila M, Tessier M, Suehs CM, Brundu G, Carta L, Galanidis A, Lambdon P, Manca M, Médail F, Moragues E, Traveset A, Troumbis AY, Hulme PE (2006) Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands. Journal of Biogeography, 33, 853-861.