Urbanization drivers differentially affect ground arthropod assemblages in the city of Turin (NW-Italy)

Urbanization is a complex process associated with environmental changes affecting many biotic and abiotic ecosystem components, which cause the alteration of biological communities, habitat features, connectivity and stability. In particular, the urbanization process is usually associated with a decrease in species richness that parallels a general increase in the overall abundance of individuals, mostly due to generalist and tolerant species taking advantage of the altered environmental conditions and outcompeting and excluding native species. We here adopted a hierarchically nested sampling design to investigate the response of ground arthropods to the combined effects of multiple facets of urbanization, namely increased impervious surface cover, temperature and patch isolation, measured at small, intermediate and large spatial scale. We studied 15 plots of 150 m in diameter located in the city of Turin (NW-Italy), along an urbanization-cover gradient ranging from suburbia to the city center. For each plot we considered the level of urbanization at three different scales (circular buffers with a 100, 400 and 1600 m radius). Within each plot, we identified a control and an isolated subplot, and in each subplot, we measured ground temperature using dataloggers and we sampled ground arthropods with pitfall traps. Firstly, by relating ground temperature and urbanization cover, we showed that temperature values reflected an Urban Heat Island (UHI) effect, which indicates increasing values of temperature along the urbanization gradient. This trend was particularly evident at large scale regarding daily and diurnal temperature, and at small scale regarding nocturnal temperature. Secondly, we demonstrated that different groups of ground arthropods respond differently to urbanization cover, with major effects at the largest spatial scale. Finally, we extrapolated the differential contribution of urbanization cover, UHI-effect and patch isolation to the observed activity density of the investigated taxonomic groups: Coleoptera and Hemiptera were influenced by temperature, whereas Hymenoptera, Collembola, Acari and Araneae were mainly influenced by urbanization, especially in control subplots. Our results emphasize how a multitaxonomic approach can help unravel patterns of community assembly in urban areas.