Distribution of seven native and two exotic plants in a tallgrass prairie in southeastern Wisconsin: the importance of human disturbance.
Invasion by exotic plant species is a serious threat to the integrity of natural communities. This study identifies associations between 2 exotic and 7 native species in a tallgrass prairie in SE Wisconsin (USA), and factors underlying the distribution of these species, with special regard to the effects of human disturbance. The distribution of 2 exotic species Melilotus alba and Daucus carota [carrots], and 7 native species Potentilla arguta, Pedicularis canadensis, Dodecatheon meadia, Equisetum laevigatum, Pycnanthemum virginianum, Phlox glaberrima and Solidago graminifolia was studied in 100 quadrats (each being 4 m3) on 5 transects. Plant densities and soil characteristics were recorded for each quadrat and analysed using non-parametric comparison of means and Spearman correlation analysis. Densities of the 2 exotic species were positively correlated with each other and negatively correlated with those of 5 of the 7 native species. Most species exhibited a clear segregation between disturbed and undisturbed transects; M. alba and carrots 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. Although the prairie in its undisturbed state seems to resist invasion of the 2 exotic species, carrots were able to escape disturbed microhabitats more extensively than was M. alba.