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

Abstract

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 humanmediated 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.

A non‐native invasive grass increases soil carbon flux in a Hawaiian tropical dry forest

“…Island ecosystems are particularly susceptible to plant invasions (Gimeno et al, 2006). In Hawaii, it is estimated that o10% of the original tropical dry forest remains (Bruegmann, 1996), primarily as a result of the introduction of invasive species and changes in disturbance regimes (Hughes et al, 1991;D'Antonio & Vitousek, 1992).…”

Focusing Ecological Research for Conservation

“…Islands can be more exposed to invasions because of small sizes compared to mainland, which facilitates colonization by invasives, once these have reached the island system (Gimeno et al 2006).…”

<i>Acacia cyclops</i> A. Cunn. ex G. Don (Leguminosae) in Italy: first cases of naturalization

“…approximately 10% of the World's vascular flora: thrive there. The highest levels of endemism have been recorded on islands (Heywood, 2011), where the ecological threat posed by alien invasive species seems to be higher than in adjacent mainland areas (Gimeno & al., 2006;Hulme & al., 2007). It has been suggested that island biota appear more vulnerable to invasions due to the presence of empty niches in unsaturated communities, the absence of key functional groups, and a generally lower competitive ability of the native species (Hulme, 2004).…”

“…Among natural habitats, wetlands and coastal areas are relatively richer in IAS than other habitats, suggesting that those habitats may suffer a greater impact (Chytrý & al., 2009;Arianoutsou & al., 2010). In contrast, Mediterranean mature and stable woodlands, dominated by evergreen shrubs and trees, seem to be quite resistant to invasion (Affre & al., 2010;Arianoutsou & al., 2010;), although human disturbance, such as wildfires or silvicultural interventions, may alter canopy structure and so understory light availability, thus allowing the establishment of heliophilous tree species (Constán-Nava & al., 2010;Badalamenti & al., 2012); invasions by such species have already caused a reduction in species richness and a significant alteration of functioning of Mediterranean ecosystems (Vilà & al., 2006;Traveset & al., 2008). However, the major ecological impacts caused by IAS have probably yet to be realized, because a worsening of the current situation is expected, both in terms of number and spread, and consequently of their likely damaging effects.…”