Species-specific responses to roads and urbanization predict invasion of wetlands by non-native plants in the Chicago, Illinois region

Background/Question/Methods

In urban areas human modification of the landscape, including land development, road construction and species import, has facilitated the spread of non-native species. The effects of urbanization are likely to be species specific and an understanding of these species-specific responses is needed to assist management of non-native species. We examined the influence of urbanization and roads on the occurrences of 16 non-native plant species in over 2000 wetlands within the Chicago region in northeastern Illinois.  Our primary objective was to determine whether landscape context, road characteristics, distance among wetlands, or a combination of those variables best predicted occurrences of selected species. Our study species included a group of halophytes, which we expected to be associated with major roads where de-icing salt is applied at higher rates.  We also expected that non-native species would occur more frequently as the amount of urban land cover increased and proximity to the city of Chicago decreased. Species occurrences were modeled using multiple logistic regression with model selection based on an information theoretic approach. 

Results/Conclusions

We found that species, or groups of species, responded differently to the effects of roads, urbanization 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 and state highways, road types that are likely to receive greater applications of de-icing salts. Several species were negatively associated with distance to Chicago. Distance from Chicago may serve as a proxy for the degree of urbanization. Alternatively, the negative association with distance to Chicago 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.  This finding suggests that propagule pressure from occupied wetlands may be an important factor in the spread of non-native plants along road corridors.  Our findings support the need to understand urbanization and dispersal along roads as predictors of the spread of non-native plant species, and particularly, a need to understand how individual species vary in response to these factors.