Does urbanization lead to taxonomic and functional homogenization in riparian forests?

Aim: In urbanized areas, exotic invasions, native extinctions, and the alteration of habitats and natural processes drive homogenization, which is a form of biotic impoverishment. This study examines whether urbanization and flooding induce homogenization of herbaceous communities in riparian forests and quantifies the relationships between taxonomic and functional beta-diversity. Location: Montreal, Quebec, Canada. Methods: Inventories were conducted in 56 riparian forests. Taxonomic and functional beta-diversity were calculated as between-site similarities in species or trait composition for three levels of urbanization and flooding. Differences among the disturbance levels were compared using tests for homogeneity in multivariate dispersions. We quantified the correlation between local species richness, exotic proportion, taxonomic and functional beta-diversity. We also partitioned taxonomic beta-diversity into species turnover and richness difference. Results: Urbanization led to taxonomic and functional differentiation, while increased flooding led to taxonomic and functional homogenization. We found a significant correlation between taxonomic and functional beta-diversity. Changes in beta-diversity were associated with species and trait turnover among both urbanization and flood levels, and with changes in species richness. Differentiation was associated with low species richness, and homogenization with high species richness. Exotic invasions tended to favour differentiation, but only at a low urbanization level. Main Conclusions: The effect of urbanization on plant diversity in riparian forests was twofold: first, it directly induced taxonomic and functional differentiation through its effect on species loss and turnover (higher beta-diversity at high urbanization level); second, differentiation was indirectly favoured through the reduction in flooding (higher beta-diversity at low flood level). Taxonomic and functional beta-diversity followed similar patterns, likely because species invasions and extinctions are not random, but are related to species traits. Our results underline the need to move our focus from exotic species to the true underlying factors of biodiversity loss and homogenization, notably land use changes and human disturbances.