Do exotics homogenize or differentiate communities? Roles of sampling and exotic species richness

It is commonly asserted that exotic species promote the homogenization of biological communities. However, theoretical evidence implies that exotic species may often have the opposite effect, of differentiating biological communities where different exotic species become established in different areas. Furthermore, few empirical studies have directly measured the homogenizing effects of exotics. In this study, I used the plant inventories of 20 localities in the United States to measure whether exotic plant increased the similarity of those localities. I calculated Jaccard's index of similarity (JI) for exotic species and then for native species to see if the exotic JI was consistently higher when comparing two localities. I found that JI for both exotic and native species decline exponentially with increasing distance and latitude separation between localities so that localities that share many native species also tend to share many exotic species. More importantly, in nearly half of the pairwise locality comparisons (87 of 190), the (JI(exotic)/JI(native)) ratio is less than one, indicating a slight tendency for exotic species to differentiate rather than homogenize the localities analyzed. Also, the pattern of differentiation versus homogenization is strongly related to exotic species richness. When both localities have relatively few exotics, there is a clear tendency for exotics to produce biotic differentiation (JI(exotic)/JI(native) < 1). Much of this pattern seems related to the right-skewed frequency distribution of the geographic ranges of exotic species. As with native species, most exotics occur in few localities so there is a high probability that localities with low numbers of exotic species will share very few, if any, of those exotics. As exotic richness increases, the homogenizing effects become increasingly pronounced.