Anthropogenic disturbance drives compositional and structural shifts in cerrado stream fish assemblages

Currently, one of the main threats to freshwater ecosystems is the conversion of natural areas into croplands, pastures, and urban landscapes. Therefore, understanding how fish assemblages respond to gradients of land use related anthropogenic disturbance is essential. This study aimed to assess shifts in species composition and structure across a stream disturbance gradient, and to examine the association of rare and non-native species with each disturbance level. We evaluated 40 tributary streams of the Upper Grande River (Paraná Basin), ranked according to the Integrated Disturbance Index (IDI), which was calculated based on human modifications on multiple scales. Species composition and structure differed between least and most disturbed streams, with no significant differences involving intermediate streams. Rare and native species prevailed in least disturbed sites, whereas tolerant and non-native species were indicators of most disturbed ones. Our findings demonstrate that stream fish assemblages are sensitive to disturbance intensity. Least-disturbed streams may serve as sources for recolonization if impacts are mitigated and connectivity restored, providing baselines for ecological assessments in highly modified landscapes lacking pristine conditions.