Conversion from environmental filtering to randomness as assembly rule of ground beetle assemblages along an urbanization gradient.

Urbanization fragments, isolates or eliminates natural habitats, and changes the structure and composition of assemblages living in the remaining natural fragments. Knowing assembly rules is necessary to support and/or maintain biodiversity in urban habitats. We hypothesized that forest communities in rural sites are organized by environmental filtering, but this may be changed by urbanization, and in the suburban and urban forest fragments replaced by randomly organized assemblages, influenced by the colonization of species from the surrounding matrix. Evaluating simultaneously the functional and phylogenetic relationships of co-existing species, we showed that at the rural sites, co-existing ground beetle species were functionally and phylogenetically more similar than expected by chance, indicating that environmental filtering was the likely process structuring these communities. Contrary to this, in urban and suburban sites, the co-occurring species were functionally and phylogenetically not different from the null model, indicating randomly structured assemblages. According to our findings, changes in environmental and habitat characteristics accompanied by urbanization lead to assemblages of randomly colonized species from the surrounding matrix, threatening proper ecosystem functioning. To reassemble stochastically assembled species of urban and suburban fragments to structured, properly functioning communities, appropriate management strategies are needed which simultaneously consider recreational, economic and conservation criteria.