Exotic invasive species in urban wetlands: environmental correlates and implications for wetland management.
Wetlands in urban regions are subjected to a wide variety of anthropogenic disturbances, many of which may promote invasions of exotic plant species. In order to devise management strategies, the influence of different aspects of the urban and natural environments on invasion and community structure must be understood. The roles of soil variables, anthropogenic effects adjacent to and within the wetlands, and vegetation structure on exotic species occurrence within 21 forested wetlands in north-eastern New Jersey, USA, were compared. The hypotheses were tested that different vegetation strata and different invasive species respond similarly to environmental factors, and that invasion increases with increasing direct human impact, hydrologic disturbance, adjacent residential land use and decreasing wetland area. Canonical correspondence analyses, correlation and logistic regression analyses were used to examine invasion by individual species and overall site invasion, as measured by the absolute and relative number of exotic species in the site flora. Within each stratum, different sets of environmental factors separated exotic and native species. Nutrients, soil clay content and pH, adjacent land use and canopy composition were the most frequently identified factors affecting species, but individual species showed highly individualistic responses to the sets of environmental variables, often responding in opposite ways to the same factor. Overall invasion increased with decreasing area but only when sites >100 ha were included. Unexpectedly, invasion decreased with increasing proportions of industrial/commercial adjacent land use. The hypotheses were only partially supported; invasion does not increase in a simple way with increasing human presence and disturbance. Synthesis and applications. The results suggest that a suite of environmental conditions can be identified that are associated with invasion into urban wetlands, which can be widely used for assessment and management. However, a comprehensive ecosystem approach is needed that places the remediation of physical alterations from urbanization within a landscape context. Specifically, sediment, inputs and hydrologic changes need to be related to adjoining urban land use and to the overlapping requirements of individual native and exotic species.