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Here, using a new comprehensive compilation of data on plant distributions in the 48 conterminous US states, we determine the degree to which domestic exotic species contribute to the exotic fraction (or degree of invasion). We also quantify the [http://www.selleckchem.com/products/at13387.html Onalespib cost] statistical relationship of exotic fraction to state area, human population density and land use within each state to identify mechanisms that potentially influence the exotic fraction. Because different habitats naturally support different numbers of species, invasibility is often measured relative to native diversity, and the exotic fraction is taken as the degree of invasion (DI; Lonsdale, 1999; Guo & Symstad, 2008). From the standpoint of assessing invasibility, one might instead quantify established exotic species relative to the total number of introductions, but this quantity is more difficult to ascertain (see, however, Blackburn et?al., 2008). For each of the 48 conterminous US states, we compiled the number of species introduced from outside the United States (foreign exotics) and those introduced to states from within the country (domestic exotics). Numbers of exotics were obtained for each [http://en.wikipedia.org/wiki/Cisplatin Cisplatin] state from Kartesz & Meacham (1999) and USDA & NRCS (2004). Their status as foreign or domestic was determined for each state by researching their precise origins through vouchered herbarium records, supplemented by literature (for more details, see Kartesz, 2010). Exotic fractions are clearly higher in smaller than in larger states within the United States (Fig.?1a), partly because native species richness increases with area but the number of exotics bears no consistent relationship to the size of a state (i.e. exotic floras are more homogenized than native floras, see Qian & Ricklefs, 2006; but also see Blackburn et?al., 2008; Fig.?1b). Moreover, domestic exotics inflate the exotic fraction more in smaller than in larger states (Fig.?1a). In continental areas, using a larger external boundary to estimate exotic richness in smaller, internal units leads to increasing bias with progressively smaller area (Fig.?2). [http://www.selleckchem.com/erk.html ERK inhibitor manufacturer] Comparable data for islands are less likely to suffer this bias because all introduced species typically are considered exotic. How we define exotics also influences the relationship between exotic and native species. For example, exotic species richness based on each states�� own boundary is not significantly related to native species richness (r2?=?0.04, P?>?0.05), although previous analyses using the US border to define exotics (and with Hawaii and Alaska included) reported significant positive correlations (r2?=?0.11, P?