Biological invasions and eutrophication reshape the spatial patterns of stream fish size spectra in France.

Aim: The global patterns of body size distributions are affected by global environmental changes (GEC), but our knowledge of the interaction effects between GEC and natural drivers is still limited. In this study, we aimed to test the effects of these interactions on fish community size spectra, that is, the variation in a community property across the body size range of individuals in the community. Location: One thousand and ninety‐five stream locations across France between 2015 and 2018. Methods: We fitted size spectrum slopes for each fish community based on individual biomass size and using binning and maximum‐likelihood methods through regression models across size classes and negative log‐likelihood functions, respectively. Interactions between natural drivers and GEC were tested using model‐averaging and the best models were selected with information criterion. Results: Our results demonstrated that size spectra were steeper (i.e. higher proportion of small‐bodied individuals) in warmer than colder locations, as expected by temperature‐size rules. However, eutrophication (total phosphorus concentration) and biological invasions (percentage of non‐native individuals) modulated the effect of temperature, with flatter size spectrum slopes in streams with more nutrients and higher proportion of non‐native individuals. In addition, fish size spectrum slopes were generally steeper in upstream than in downstream locations, and this effect was more pronounced in stream locations with more nutrients. Finally, size spectrum slopes were flatter in species‐rich communities when nutrient concentration was high. Main Conclusions: Our study highlights that accounting for GEC, such as eutrophication and biological invasions, can help understanding the complex patterns of fish body size distributions in ecological communities. [ABSTRACT FROM AUTHOR]