Anthropogenic influences reshape lentic fish diversity: Patterns of homogenization and differentiation across a Mediterranean biodiversity hotspot

Freshwater ecosystems are increasingly impacted by human activities, resulting in species invasions and extinctions, disrupting biodiversity and ecosystem functions. This study investigates the patterns of taxonomic and functional homogenization and differentiation in fish assemblages within 103 lentic ecosystems across a Mediterranean biodiversity hotspot (Greece). We focus on how non-native species have altered the taxonomic and functional diversity at both national and ecoregional levels while exploring the drivers and pathways behind these changes. Our analysis combined historical and current species data, evaluating the effects of species introductions and local extinctions on biodiversity of distinct freshwater ecoregions. We calculated taxonomic and functional similarities using Jaccard and Gower dissimilarity indices and employed generalized linear models (GLMs) to assess the significance of changes over time. Results reveal significant taxonomic homogenization across lentic ecosystems, primarily driven by the introduction of widespread alien and translocated fish species. In contrast, functional homogenization was less pronounced, with some regions exhibiting differentiation, including increases in species richness due to the introduction of species with distinct ecological traits. Non-native species introduced through angling, ornamental trade, and unintentional pathways were the primary contributors to homogenization. The findings highlight prominent regional differences and vulnerabilities: mainland ecoregions experienced stronger homogenization, while insular ecoregions have experienced taxonomic differentiation. Additionally, the study reveals a decoupling of taxonomic and functional changes, emphasizing the need to consider both in biodiversity assessments and conservation management. This research contributes to the broader understanding of how species invasions reshape biodiversity patterns and ecosystem functions in freshwater systems. Our approach provides a useful framework for assessing biotic homogenization and differentiation, with implications for conservation and management strategies worldwide.