Land-use change from urbanization can affect environmental conditions and have subsequent impacts on the species communities within urban landscapes. Urban areas also tend to contain a high number of non-native plant species, which may be pre-adapted to such conditions and contribute to reductions in native species. The purpose of this study was to evaluate plant community diversity and environmental variation in wetlands occurring across a gradient of rural to urban land-use, relating environmental conditions to native and non-native diversity. Patterns of plant diversity across the urban to rural gradient have been largely ignored in wetlands, although wetlands provide valuable ecosystem services beneficial to humans. To assess variation in plant communities and environmental conditions, we conducted vegetative field surveys in 45 wetlands throughout Ohio that were categorized based on their surrounding land-use as either intense urban, moderate urban, or rural. We arranged wetlands into 15 distinct triads that represented separate urban to rural gradients across the landscape, with one wetland from each land-use category sampled within each triad. Within each wetland, we classified species as either native or non-native and collected soil cores to measure both soil mineral content and plant-available ions.
Results/Conclusions
Intense urban wetlands had higher concentrations of magnesium, boron, sulfur, sodium, and arsenic compared to moderate urban and rural wetlands. Additionally, both the proportion of bare ground and mean temperature for the warmest quarter were greater in intense urban wetlands compared to rural wetlands. Non-native richness was 56% greater in intense urban sites compared to rural sites, while non-native relative abundance was 74% greater in intense urban sites compared to rural sites. Lower native richness was primarily associated with high relative abundances of non-native species, whereas low native diversity was related to high concentrations of soil contaminants that varied across our land-use gradient, including cadmium and sodium. Our results correspond to both alternatives of the driver-passenger model of invasion: native richness is apparently directly limited by competition with non-native species (the driver model), while native diversity may be limited more by the environmental stressors present in intense urban sites (the passenger model). Such limitations to native communities may be more prevalent in intense urban wetlands compared to other land-use types, since wetlands are particularly likely to accumulate soil contaminants as well as propagules of ecologically dominant non-native species.