The distribution and habitat associations of non-native plant species in urban riparian habitats

Questions: 1. What are the distribution and habitat associations of non-native (neophyte) species in riparian zones? 2. Are there significant differences, in terms of plant species diversity, composition, habitat condition and species attributes, between plant communities where non-natives are present or abundant and those where non-natives are absent or infrequent? 3. Are the observed differences generic to non-natives or do individual non-native species differ in their vegetation associations? Location: West Midlands Conurbation (WMC), UK. Methods: 56 sites were located randomly on four rivers across the WMC. Ten 2 m × 2 m quadrats were placed within 15 m of the river to sample vegetation within the floodplain at each site. All vascular plants were recorded along with site information such as surrounding land use and habitat types. Results: Non-native species were found in many vegetation types and on all rivers in the WMC. There were higher numbers of non-natives on more degraded, human-modified rivers. More non-native species were found in woodland, scrub and tall herb habitats than in grasslands. We distinguish two types of communities with non-natives. In communities colonized following disturbance, in comparison to quadrats containing no non-native species, those with non-natives had higher species diversity and more forbs, annuals and shortlived monocarpic perennials. Native species in quadrats containing non-natives were characteristic of conditions of higher fertility and pH, had a larger specific leaf area and were less stress tolerant or competitive. In later successional communities dominated by particular non-natives, native diversity declined with increasing cover of non-natives. Associated native species were characteristic of low light conditions. Conclusions: Communities containing non-natives can be associated with particular types of native species. Extrinsic factors (disturbance, eutrophication) affected both native and non-native species. In disturbed riparian habitats the key determinant of diversity is dominance by competitive invasive species regardless of their native or non-native origin.