Domestic exotics and the perception of invasibility

Susceptibility of an area to invasion by exotic species is often judged by the fraction of introduced species in the local biota. However, the degree of invasion, particularly in mainland areas, has often been underestimated because of the exclusion of ‘domestic exotics' (those introduced to internal units from within the national border) in calculations. Because all introduced species on islands are considered as exotics, this contributes to the perception that islands are more susceptible to invasion than are continental regions. Here, we determine the contribution of domestic exotic species to the degree of invasion (exotic fraction) in mainland areas. We quantify the relationships of exotic fraction to the area, human population density and land use within each of the 48 conterminous US states to identify mechanisms that potentially influence the degree of invasion. For each of the 48 conterminous US states, we compiled the number of species introduced from outside the United States (‘foreign exotics') and the number of exotics introduced from other conterminous US states (‘domestic exotics'). The status of each species as foreign or domestic was determined for each state by researching its precise origins through vouchered herbarium records, supplemented by literature (Kartesz, 2010). We found that (1) the exotic fraction inevitably decreases with increasing area as the pool of potential exotic species decreases; (2) exotic richness of areas within large mainland regions is underestimated to the extent that species introduced among areas within a region are considered as natives; and (3) human activities contribute disproportionately more exotics to smaller than to larger administrative areas. How we define ‘exotic' influences how we count non‐native species and perceive invasibility. Excluding domestic exotics in mainland regions leads to a biased perception of increased invasibility on islands, where all introduced species are considered exotic. Thus, future documentation and interpretation of invasion patterns and management of exotics should account for these biases in quantifying the exotic fraction.