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. We investigated the influence of landscape-scale urbanisation and local environmental variables on the terrestrial isopods (an important group of detritivores) in Rome (Italy), to test three hypotheses: (i) community composition varies along the urbanisation gradient, resulting in biotic homogenisation; (ii) local factors contribute more to these changes than landscape-scale urbanisation; and (iii) species exhibit non-random relationships with urbanisation. 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. In accordance with first hypothesis, urbanisation affected isopod communities, increasing taxonomic and functional homogenisation. Overall 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 larger proportion of community variation than landscape-scale variables, supporting our second hypothesis. 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 our third hypothesis. In summary, our study revealed that urbanisation filters isopod communities primarily based through variation in local environmental conditions, emphasising the conservation significance of structurally complex and diverse urban green spaces for soil macrodetritivore communities and related soil ecosystem functions.