Urbanization and roads drive non-native plant invasion in the Chicago Metropolitan region

Human modification of the landscape, including urbanization and road construction, has facilitated the spread and establishment of non-native plant species. The effects of urbanization and roads are expected to be species-specific due to differences in species habitat requirements. We examined the influence of urbanization and roads on the occurrences of 16 non-native plant species in over 2000 wetlands within the Chicago metropolitan region in northeastern Illinois, USA. We found that species, or groups of species, responded differently to the effects of urbanization, roads, and proximity to conspecific populations. Occurrences of halophyte species were best predicted by road variables; halophytes were more commonly associated with major roads such as interstates and federal highways, road types that are likely to receive greater applications of de-icing salts. Several species were associated with proximity to Chicago. Proximity to Chicago may serve as a proxy for the degree of urbanization; or alternatively, may reflect a pattern of outward dispersal from an initial establishment point in the urban center. All study species were positively associated with distance to the nearest occupied wetland, implying that for each species, wetlands were more likely to be occupied if closer to other occupied sites. Our results support the need for species-specific understanding of responses to urbanization and roads to facilitate management of non-native species.