Are islands more susceptible to plant invasion than continents? A test using Oxalis pes‐caprae L. in the western Mediterranean

Aim We tested the relative vulnerability of islands to Oxalis pes‐caprae L. invasion compared to mainland regions. Oxalis pes‐caprae is a South African annual geophyte that reproduces via bulbils, and has spread in many Mediterranean and temperate regions of the world where introduced. Our study is one of the first detailed regional analyses of the occurrence and local abundance of a non‐native plant. Methods We conducted an extensive survey (2000 sampling points) to examine local and coarse‐scale patterns in both the occurrence and abundance of O. pes‐caprae on islands and in neighbouring mainland regions of Spain. Location We analysed occurrence (number of samples where present) and abundance (percentage cover) on two Balearic Islands (Menorca and Mallorca) and in two mainland administrative provinces of Spain (Murcia and València). Results Oxalis pes‐caprae occurrence was consistently higher on islands. Occurrence varied among habitats, being the highest in tree groves and the lowest in forests and shrublands. It was never found in these two habitats on the mainland. Mean O. pes‐caprae abundance was greatest in tree groves on the mainland, and in field margins and old fields on the islands. However, in general there were not significant differences in local abundances between island and mainland locations. Main conclusions These findings suggest that local processes (such as the biotic resistance of plant communities) are less important than coarse‐scale phenomena (such as environmental driving forces) in explaining differences in the invasion patterns observed between islands and adjacent mainland regions. We suggest that O. pes‐caprae has occupied a larger proportion of available habitats on islands due to: (1) its strong dependence on domestic animal and human‐mediated dispersal which are probably greater on the islands than in mainland areas, and (2) the smaller area encompassed by islands that, over a comparable period of time, enables a greater proportion of available habitats to be colonized (and hence higher occurrence) than equivalent larger mainland areas.