Effects of introduced species on floristic similarity: Comparing two US states

This study aims to examine the effects of introduced species on increasing (homogenizing) or decreasing (differentiating) floristic similarity of plant composition. We calculated the Jaccard index for each pair of counties within two states of USA, California and Florida. We computed the Jaccard index separately for all (native plus exotic) species, for native species, and for exotic species. We further calculated a homogenization index between all species and native species for each pair of counties by subtracting similarity index for native species from that for all species. We correlated the Jaccard and homogenization indices to geographic distance, latitude separation, and longitude separation between pairs of counties and to average human population density and average land area of the two counties. We find a very strong pattern of differentiation for introduced species among nearly all Florida counties. In California, introduced species have a differentiating effect in about half the comparisons. We also find that introduced species tend to have a more homogenizing (or less differentiating) effect with increasing distances between counties. Our results do not show a clear relationship between human population density and the homogenization index.