Invasion by exotic plant species is a serious threat to the integrity of natural communities. The distribution of an exotic species depends upon environmental conditions, the structure of the native community, patterns of disturbance and ecological features of the species itself. This study identifies (1) associations between two exotic and seven native species in a tallgrass prairie in southeastern Wisconsin and (2) factors underlying the distribution of these species, with special regard to the effects of human disturbance. The distribution of two exotic species, Melilotus alba (white sweetclover) and Daucus carota (Queen Anne's lace), and seven native species, Potentilla arguta (prairie cinquefoil), Pedicularis canadensis (Canada lousewort), Dodecatheon meadia (Mead's shooting star), Equisetum laevigatum (scouring rush), Pycnanthemum virginianum (Virginia mountain mint), Phlox glaberrima (smooth phlox) and Solidago graminifolia (bushy goldenrod), were studied in 100, 4 m2 quadrats on five transects. Plant densities and soil characteristics were recorded for each quadrat and analyzed using nonparametric comparison of means and Spearman correlation analysis. Densities of the two exotic species were positively correlated with each other and negatively correlated with those of five of the seven natives. Most species exhibited a clear segregation between disturbed and undisturbed transects; Melilotus and Daucus dominated the disturbed transects, and native species dominated the undisturbed transects. Edaphic conditions appear to be the most important factor driving this habitat segregation between exotics and natives. Our finding is instructive for prairie restoration, because it suggests that in some cases, soil restoration may be desirable even if it results in additional mechanical disturbance. Although the prairie in its undisturbed state seems to resist invasion of these two exotic species, Daucus is able to escape disturbed microhabitats more extensively than is Melilotus.
