Investigating environmental variation and landscape characteristics of an urban-rural gradient using woodland carabid assemblages

Aim To investigate environmental variation and associated assemblage changes of carabid beetles along an urban-rural gradient. Location 'Quercus-Acer' (oak-sycamore) woodlands in the city of Birmingham, UK. Methods We collected carabid data using pitfall traps on 12 sites in the city. The traps were run from April-September in 2000, and we collected environmental data on 24 individual variables associated with the individual sites and their landscape context. Changes in carabid assemblages were analysed using repeat measures ANOVA and the environment-species relationships with a Redundancy Analyses (RDA) and Generalized Linear Modelling (GLM). Results We found that: (1) species richness and diversity were lower in the urban and suburban zone and higher in the rural zone; (2) Berger Parker dominance index was higher in the urban and suburban zones; (3) the number of woodland and woodland associated species was significantly higher at the rural end of the gradient; (4) the number of short-winged (brachypterous) species was highest in the rural zone and decreased towards the urban woodlands, whereas the long-winged species were more abundant in suburban woodlands; (5) the median weight length (WML) of the assemblage declined along the gradient from the rural to the urban zone, as did the number of large species; and (6) five of the 24 environmental variables showed a significant relationship with variation in the carabid assemblage. At site level the carabid assemblages were related to the level of site disturbance and soil penetrability, whereas site size and amount of woodland and urban land within 5 km of the site were important at a larger landscape scale. Main conclusions The results suggest that urbanization has a deleterious impact on carabid assemblages, causing a reduction in species richness from the rural fringe to the centre of the city. Changes in assemblage structure were related to woodland fragmentation, which led to variations in woodland size, woodland location and site disturbance due to trampling. Large, flightless and specialist woodland species are more susceptible to changes associated with urbanization, presumably due to their longer life spans, lower reproductive rates, more specialized niches and more limited dispersal potential.