Body size and biomass distributions of carrion visiting beetles: do cities host smaller species?

The question how animal body size changes along urban-rural gradients has received much attention from carabidologists, who noticed that cities harbour smaller species than natural sites. For Carabidae this pattern is frequently connected with increasing disturbance regimes towards cities, which favour smaller winged species of higher dispersal ability. However, whether changes in body size distributions can be generalised and whether common patterns exist are largely unknown. Here we report on body size distributions of carcass-visiting beetles along an urban-rural gradient in northern Poland. Based on samplings of 58 necrophages and 43 predatory beetle species, mainly of the families Catopidae, Silphidae, and Staphylinidae, we found contrary patterns of necrophages and predatory beetles. Body sizes of necrophages decreased towards the city centre and those of predators remained unchanged. Small necrophages and large predators dominated in abundance in the city centre. Necrophage body sizes appeared to be more regularly spaced in the city centre than expected from a random null model and in comparison to the rural pattern, pointing to increased competition.