Ecological and genetic features of the invasion by the alien Carpobrotus plants in Mediterranean Island habitats

Invasion by introduced Carpobrotus sp. pl. (Aizoaceae) constitutes one of the most severe threats to native plant communities and ecosystems in the Mediterranean Basin. Carpobrotus acinaciformis (L.) L. Bolus and C. edulis (L.) R. Br. are trailing, mat-forming succulents that have now invaded diverse coastal continental and island habitats in Mediterranean France. The present study is focused on two Carpobrotus patches on the island of Bagaud (Hyeres archipelago, Provence, France) where the two taxa have proven to be highly invasive within natural plant communities. The purpose of this study is to (i) determine the taxonomic position of C. acinaciformis, and (ii) compare the clonal structure. genetic diversity, and reproductive alternatives for the two taxes. Multilocus isozymic variation and reproductive ecology results strongly suggest that. in site 1 on the island of Bagaud (Bagaud 1), C. acinaciformis individuals are of hybrid origin, and once established, have generated different introgressed types from C. edulis genotypes. We have found a multilocus patch structure with 121 clones detected in Bagaud 1 (34 clones for C. acinaciformis and 87 clones for the introgressed types) and 93 clones detected for C. edulis in a second site on the same island (Bagaud 2). C. acinaciformis and all introgressed types show higher levels of genetic diversity than C. edulis. The former reproduce primarily by vegetative propagation, and do not show sexual reproductive alternatives, except in interspecific hybridization. whereas C. edulis shows a mixed sexual mating system as a reproductive alternative to a weaker capacity for vegetative growth. Finally, we examine several synergetic ecological factors which may have favoured the optimal establishment and rapid spreading of the two taxes on the island of Bagaud.