Species turnover and geographic distance in an urban river network

Aim: Understanding the relationships between species turnover, environmental features and the geographic distance between sites can provide important insights into the processes driving species diversity. This is particularly relevant where the effective distance between sites may be a function of the habitat or topographic features of the landscape and the means of dispersal of the organism. River networks, in particular in human-modified landscapes, are a striking example of such a situation. Here, we use data for both aquatic and terrestrial organisms across an urban river network to examine patterns of species turnover and to determine whether these patterns differ between different taxonomic groups. Location: Sheffield area, UK. Methods: Aquatic (macroinvertebrates, diatoms) and terrestrial (birds, plants, butterflies) organisms were surveyed at 41 sites across an urban river network. We assessed the relationship between turnover and three alternative geographic distance measures (Euclidean, network and flow distance), whilst also taking into account the environmental distance between sites, using Mantel and partial Mantel tests. Results: Turnover of all taxonomic groups apart from butterflies was significantly correlated with at least one measure of geographic distance. The aquatic taxonomic groups showed the strongest correlations with the geographic distance measures, and in particular with network distance. Terrestrial taxa were more closely associated with environmental than any of the geographic distance measures, although network distance remained significant for birds and some plant groups after removing the effect of environmental distance. Water-dispersed and neophyte plant groups were significantly related to network and flow distance. Main conclusions: The results suggest that aquatic communities are strongly influenced by spatial processes occurring within the river network. Terrestrial taxa have a more complex relationship with distance, with different components of these communities displaying different responses. Nevertheless, it is clear that connectivity along the river corridor is important for both aquatic and terrestrial communities. © 2013 John Wiley & Sons Ltd.