Homogenisation and loss of specialisation in terrestrial isopods along an urban gradient. The role of local and landscape factors

Urbanisation is a major driver of biodiversity change; however, its impact on soil macrodetritivore communities and their functional composition still remains overlooked. Terrestrial isopods, key soil macrodetritivores involved in decomposition processes, are increasingly used as model organisms to assess environmental change. In this study, we investigated the influence of landscape-scale urbanisation and local environmental variables on terrestrial isopod communities in Rome (Italy) to test three hypotheses: (H1) community composition vary along the urbanisation gradient, leading to biotic homogenisation; (H2) local factors contribute more to these changes than landscape-scale urbanisation; and (H3) species show non-random associations with the urban gradient. We examined variations in taxonomic and functional β-diversity, multivariate composition, and species-habitat association from 48 plots across 16 sites sampled in spring and autumn. Each plot was characterised in terms of landscape characteristics and local microhabitat and edaphic chemo-physical conditions. Overall, 42 species were collected in the study area. In accordance with H1, urbanisation affected isopod communities, increasing taxonomic and functional homogenisation. Taxonomic and functional β-diversity decreased with increasing urbanisation, driven by greater contributions of richness differences and loss of the most sensitive species. Local-scale variables accounted for a general larger proportion of community variation than landscape-scale variables, partially supporting H2. Finally, we found non-random associations between species and urbanisation, with habitat specialists thriving only in semi-natural and suburban forests, while urban areas were mostly dominated by tolerant generalist species, confirming H3. In summary, our study revealed that urbanisation filters isopod communities through the combined influence of landscape-scale gradients and local environmental conditions, especially soil properties and habitat structure, highlighting the conservation importance of structurally complex urban green spaces for soil macrodetritivores and related soil ecosystem functions.