Wetland plant community variation across replicate urban to rural gradients: non-native species as both drivers and passengers in systems impacted by anthropogenic land-use

Anthropogenic land-use change impacts ecological communities in urban and rural landscapes, and wetlands are particularly vulnerable despite the valuable ecosystem services they provide. Urbanized non-wetland systems are often enriched in non-native plant species, and similar patterns in wetlands would have implications for ecosystem function and biodiversity. We evaluated landscape-scale patterns of plant community diversity across gradients of rural to urban land-use, testing whether diversity was related to environmental conditions indicative of surrounding land-use. We surveyed vegetation and collected soil samples from 45 wetlands throughout Ohio, USA. Sites were categorized based on surrounding land-use as intense urban, moderate urban, or rural, representing 15 replicate urban to rural gradients. Non-native richness was 56% greater and non-native relative abundance 74% greater in intense urban sites compared to rural sites. Structural equation modeling indicated that high non-native relative abundance caused reductions in native plant richness but not native Shannon diversity, which was instead related to high concentrations of urban-associated soil contaminants such as cadmium and sodium. Our results support both versions of the driver-passenger model of invasion impacts, depending on the response: native richness is directly limited by competition with non-native species (the driver model), while native diversity is limited more by urban-associated stressors that also affect non-natives (the passenger model). The few wetlands remaining in highly urban areas thus experience a range of constraints affecting multiple dimensions of wetland health. We argue it is in these sites specifically where the benefits of restoring wetland ecosystems will be maximized.