Global Evidence for Biotic Differentiation Amid Contrasting Freshwater β-Diversity Responses to Human Impacts

Vitor Manuel Barros Ferreira, Natália Carneiro Lacerda dos Santos, Míriam Pilz Albrecht, Bruno Eleres Soares 

Human pressures are reshaping freshwater biodiversity worldwide, yet the generality of biotic homogenization and the contexts in which differentiation instead emerges remain unresolved. These contrasting responses reflect decreases or increases in spatial β-diversity, respectively, as local assemblages within a region become more similar or more distinct in community composition. We conducted a global meta-analysis of 495 reference–impact contrasts to assess the direction and context dependence of freshwater β-diversity change across major biological groups and taxonomic, functional, and phylogenetic facets. We evaluated variation among impact types, study designs, and data types, as well as associations with five broad-scale factors: latitude, spatial extent, geographic distance, climatic heterogeneity, and temporal window. Overall, impacted assemblages had higher β-diversity than reference assemblages, indicating biotic differentiation across taxonomic and functional facets. Among impact categories, only land-use change showed a clear directional response, indicating differentiation. Across study designs, spatially interspersed and spatiotemporal comparisons also indicated differentiation, whereas spatially segregated comparisons showed no consistent direction. Global differentiation was observed despite broad variation in the five broad-scale factors, none of which showed detectable associations with β-diversity change. Presence–absence and quantitative data yielded opposite responses: occurrence-based differentiation and quantitative homogenization. Excluding the most-represented biological groups (fish and invertebrates) also reversed the pooled response from differentiation to homogenization. These findings further challenge the long-standing expectation that human impacts generally promote biotic homogenization, while highlighting that the inferred direction of freshwater β-diversity change depends on ecological context, taxonomic representation, and the type of data considered. Whether assemblages homogenize or differentiate may reflect the extent to which human pressures override local ecological differences, producing similar impacted states, or interact with local and historical contexts, generating distinct outcomes. Recognizing both ecological and methodological sources of context dependence is essential for understanding freshwater biodiversity reorganization under global change.