Land conversion induced by urbanization leads to taxonomic and functional homogenization of a river macroinvertebrate metacommunity

Conversion of forests to urban land-use in the processes of urbanization is one of the major causes of biotic homogeni-zation (i.e., decline in beta diversity) in freshwater ecosystems, threating ecosystem functioning and services. How-ever, empirical studies exploring urban land-use shaping patterns of taxonomic and functional beta diversities and their components in subtropical urban rivers are limited. Here, by leveraging data for 43 sampling sites from urban and forest rivers in Shenzhen, a megacity showing rapid urbanization, we determined the spatio-temporal dynamics and associated drivers of taxonomic and functional beta diversities of river macroinvertebrates. Our results showed that, from the forest to urban rivers, taxonomic beta diversity (wet: 32.9%; dry: 17.1%) declined more significantly than functional beta diversity (wet: 17.4%; dry: 9.5%) in different seasons. We further found that these compositional changes were largely driven by decreased roles of species/traits replacement. Although replacement was also domi-nant for taxonomic beta diversity (60.4%-68.4%) in two sets of rivers, richness difference contributed more to func-tional beta diversity in the urban river (52.6%-60.5%). Both deterministic and stochastic processes simultaneously affected beta diversity, with stochastic processes being more important in the urban (3.0-19.0%) than forest rivers (0.0%-3.0%). Besides, db-RDA and variation partitioning results showed that local-scale environmental variables ex-plained considerably large fractions of variation in beta diversity. We hence recommended that biodiversity conserva-ti on should focus on improving and restoring local environmental conditions. Despite no significant seasonal differences in beta diversity were detected in this study, we found that the roles of deterministic (i.e., local-scale and land-use variables) and stochastic processes varied considerately across seasons. This result highlights the viewpoint that urban river biodiversity monitoring should go beyond one-season snapshot surveys. As the ongoing trend of urbanization in developing countries, the findings of this study are relevant in guiding urban river environmental monitoring, biodiversity conservation and land-use planning.

Automated summary

Conversion of forests to urban land-use during urbanization is a major cause of biotic homogenization in freshwater ecosystems. This study explores the patterns of taxonomic and functional beta diversities in urban and forest rivers in Shenzhen, China. The results show that taxonomic beta diversity declines more significantly than functional beta diversity in urban rivers, driven by decreased species/traits replacement. Both deterministic and stochastic processes affect beta diversity, with stochastic processes being more important in urban rivers.