Contrasting patterns in species richness of birds, butterflies and plants along riparian corridors in an urban landscape

Aim: Urbanization is a major driver of global land-use change, substantially modifying patterns of biodiversity. Managing these impacts has become a conservation priority. The creation and maintenance of greenways, such as river corridors, is frequently promoted as a strategy for mitigating habitat fragmentation in urban areas by bringing semi-natural habitat cover into city centres. However, there is little evidence to support this assertion. Here, we examine whether riparian zones maintain semi-natural habitat cover in urban areas and how species richness varies along such zones. Location: Sheffield, Northern England. Methods: Multiple taxonomic groups (birds, butterflies, plants) were surveyed at 105 sites spanning seven riparian corridors that transect the study system. For all groups, we model the relationships between species richness and environmental variables pertinent to an urban system. To test whether riparian zones can act to maintain semi-natural habitats within a city, we modelled the proportion of seminatural land cover within 250 m grid squares that do, and do not, contain a river. Results: Species richness varied markedly in relation to distance from the urban core. Trends differed both between taxonomic groups and between rivers, reflecting the complex patterns of environmental variation associated with cities. This suggests that biodiversity surveys that focus on a single group or transect cannot reliably be used as surrogates even within the same city. Nonetheless, there were common environmental predictors of species richness. Plant, avian and butterfly richness all responded positively to Habitat Diversity and the latter two declined with increases in sealed surface. Main conclusions: Multiple transects and taxonomic groups are required to describe species richness responses to urbanization as no single pattern is evident. Although riparian zones are an important component of the mosaic of urban habitats, we find that river corridors do not disproportionately support tree and Natural Surface Cover when compared to non-riverine urban areas.