Data from: Occupancy and niche dynamics of Iberian freshwater fish across twenty-five years

These processes are especially relevant in freshwater ecosystems, where vertebrate declines are particularly severe and dispersal is naturally constrained by drainage basins. Here, we investigated changes in freshwater fish species occupancy, realized niches, and their relationships with species traits across the Iberian Peninsula between two periods (2001–2010 and 2015–2024) using data from 204 sampling sites.

We combined monitoring data, environmental variables, species traits, and complementary statistical approaches, including the within outlying mean index (WitOMI), an ordination method that quantifies niche metrics such as marginality and breadth while identifying the main environmental gradients structuring communities. No significant changes were detected in mean species occupancy or niche parameters between periods. However, this apparent stability masks species-specific dynamics.