Domestic exotics and the perception of invasibility

The proportion of exotic species (the exotic fraction) on islands has been reported to be almost three times that of mainland sites of similar size (Lonsdale, 1999). The disproportionately larger fraction of exotic species has been cited as indicating that islands are highly vulnerable to invasion (Elton, 1958; but see Bellemain & Ricklefs, 2008). Although the generality of this claim has been questioned (e.g. D’Antonio & Dudley, 1995; Lonsdale, 1999; Sol, 2000), this perception persists in the literature on biotic invasions. Here, we revisit the definition of ‘exotic species’ and the associated boundaries delimiting areas of concern, and propose two additional explanations for the higher exotic fractions and perceived higher invasibility, of smaller regions, including islands. First, with smaller area, particularly combined with the discrete boundaries of islands, more species are ‘external’ and thus represent potential invaders relative to the numUSDA FS, Eastern Forest Environmental Threat Assessment Centre, 200 WT Weaver Blvd., Asheville, NC 28804, USA, Department of Biology, University of Missouri-St. Louis, St. Louis, MO 63121, USA