Comparative responses of spider and carabid beetle assemblages along an urban-rural boundary gradient

The urbanization process is the motor of deep environmental changes at both local and landscape levels. Although more and more studies are investigating the ecological consequences of urbanization, only a few have studied small-scale responses of biodiversity to urban-rural boundary gradients, and even fewer have compared different model groups synchronically. In this study, we compared the responses of two invertebrate groups often used as bioindicators, spiders and carabid beetles, along small-scale boundaries (around 1 km). The following parameters were estimated: assemblage composition, species richness, and activity-densities overall and per life history trait (habitat preference, dispersal abilities for carabid beetles and hunting guilds for spiders). The field data were collected in 2009 using pitfall traps set randomly in hedgerows within urban, boundary and rural zones (30 traps in total). 924 adult spiders belonging to 78 species were collected, whereas the 330 captured carabid beetles belonged to 25 species. We found no evidence of any significant change in carabid beetle activity-density (overall and for most life history traits) or in species richness along the urban-rural gradient. Conversely, there was a significant change in spider activity-density, both per habitat preference and per hunting guild. We also found a progressive change in community composition for spiders. Our results suggest that studying different model groups can provide complementary information about urbanization.