Freshwater fish functional diversity shows diverse responses to human activities, but consistently declines in the tropics

Freshwater environments are intertwined with human activities and the consequence has been environmental degradation and biodiversity loss. Fish provide key ecological and economic benefits, and fish abundance and diversity can be affected by human activities resulting in functional diversity (FD) changes that might scale up to ecosystem impacts. Changes in FD can be expressed by quantifying its three main FD components: richness, regularity and divergence. There is no consensus about how human activities affect the main components of FD. In addition, human activities might affect the functional diversity of communities differently in temperate and tropical regions because of differences in the regional species pools and the distribution of functional traits. Here, using a meta-analytical approach, we assess how different human activities (e.g. deforestation, invasion, reservoirs) in freshwater systems affect FD components in fish communities. We compiled information from 2012 to 2023, and we found highly idiosyncratic patterns globally, but consistent loss of functional richness and regularity in face of human activities in the tropics. This idiosyncrasy could be related to high environmental heterogeneity or the multiple ways in which communities can be affected by human activities, including species loss or introduction of non-native species, or the distribution of functional uniqueness and redundancy. The reduction of functional diversity in tropical regions reveals that high redundancy alone does not prevent declines in functional diversity, and that human activities are removing specific ecological functions from natural environments. Despite the general patterns of reduction observed, local features play a crucial role in shaping how communities respond to human activities. Therefore, it is essential to understand these patterns at a local scale and to investigate the mechanisms by which specific activities impact FD.