Concomitant impacts of climate change, fragmentation and non-native species have led to reorganization of fish communities since the 1980s

Aim: In response to climate change, species distribution shifts resulting from local extinctions, colonizations and variations in population abundances potentially lead to community-level reorganizations. Here, we assess changes over time in stream fish communities, quantify the extent to which these changes are attributable to population declines or increases and identify their main drivers. Location: France. Time period: 1980-2012. Major taxa studied: Stream fish species. Methods: We used abundance-monitoring data to quantify changes in composition and uniqueness for 332 stream fish communities between a cold historical period (1980-1993) and a warm contemporary period (2004-2012). Then, we used a model-averaging procedure to test the impacts of factors related to climate, land use and non-native species density and their interacting effects in shaping community reorganization. Results: We observed biotic homogenization over time in stream fish communities, although some communities experienced differentiation. Changes in composition mainly resulted from population declines and were favoured by an increase in temperature seasonality and in non-native species density. Population declines decreased with fragmentation and changes in non-native species density, whereas population increases were negatively driven by changes in precipitation and positively by fragmentation. Our results provide evidence that environmental changes can interact with other factors (e.g., upstream-downstream, fragmentation intensity) to determine community reorganization. Main conclusions: In the context of global change, fish assemblage reorganizations mainly result from population declines of species. These reorganizations are spatially structured and driven by both climatic and human-related stressors. Here, we emphasize the need to take into account several components of global change, because the interplay between stressors might play a key role in the ongoing biodiversity changes.