Non-native plant commonness and dominance in the forests, wetlands, and grasslands of Illinois, USA

Non-native species constitute one of the greatest threats to our indigenous biota. We used data from vegetation surveys of ground, shrub, and canopy strata from 1997-2001 at 399 randomly selected forests, wetlands, and grasslands to elucidate non-native plant pervasiveness throughout Illinois. The dominant non-native species in the ground layer of forests was Lonicera japonica Thunb., in wetlands it was Phalaris arundinacea L., in prairie grasslands it was Poa pratensis L., and across all grasslands it was Festuca arundinacea Schreb. Though rarely recognized as exotic, several of the most prevalent non-natives were introduced ecotypes or cultivars of cosmopolitan species. Conversely, some well-known exotics were surprisingly uncommon. Non-native species were more dominant in the ground cover (9% forests, 33% wetlands, 36% prairie grasslands, 76% secondary grasslands) than in the shrub and canopy strata across the state. Non-native ground cover varied regionally in forests and wetlands while, overall, the southern third of the state had significantly lower non-native cover. These regional patterns may be related to isolation from metropolitan areas, historical disturbances, current land use, unique edaphic features, as well as species-specific distributions for the most successful invaders. Our results show the extent to which non-native species have permeated Illinois habitats and replaced native plant communities.