Comparison of Oxalis pes-caprae spread in islands and mainland areas of Spain
Islands are generally thought to be more vulnerable to biological invasions than continents. Oxalis pes-caprae (Oxalidaceae, EPPO list of invasive alien plants), a South-African geophyte, is considered invasive in the Mediterranean region. In Spain, a survey on Oxalis pes-caprae was conducted at 2000 sampling points in different habitats (coastal, urban, orchard, field margin, shrubland, forest, grassland, old fields, tree groves and ruderal) in Menorca, Mallorca (Islas Baleares) and in Murcia and Valencía, (two administrative provinces of Spain). The aim was to compare the spread of the plant between islands and the mainland. In this study, both the occurrence (number of sites where Oxalis pes-caprae is found) and abundance (percentage cover) were recorded.
In both island and mainland areas, the plant was most frequently found in ruderal habitats, tree groves and old fields. O. pes-caprae invaded a wider range of habitats and was observed to occupy a greater proportion of these vulnerable habitats on the two islands compared to the two mainland regions. Indeed, it was never found in shrubland and grassland in the mainland areas, while it was found in these habitats on the islands.
However, the abundance of O. pes-caprae within the sites where it occurs was similar in both islands and mainland. Therefore, at the local scale and for this species, island communities do not appear to be more easily invaded than mainland communities.
The fact that O. pes-caprae occupies a larger proportion of available habitats on islands may be due to:
- its strong dependence on domestic animal and human-mediated dispersal; these spread factors are probably greater on the islands than in mainland areas,
- the smaller surface area of islands and higher density of road networks that, over a comparable period of time, enables a greater proportion of available habitats to be colonized (and therefore the occurrence of the species is higher) than in equivalent larger mainland areas.
Sources
Gimena I, Vilà M, Hulme PE (2006) Are islands more susceptible to plant invasion than continents? A test using Oxalis pes-caprae L. in the western Mediterranean. Journal of Biogeography 33, 1559-1565.