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 temporally and/or spatially: 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 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, or the distribution of functional uniqueness and redundancy. The reduction of functional diversity is concerning since human activities are removing specific functions from natural environments and results in the dominance of traits related to generalist ecological strategies. Despite the general patterns of reduction, local features are determinants on how the community will answer to human activity and therefore we highlight the importance of understanding the environment and fauna at the local scale, and the mechanisms by which each activity might affect FD.

Methods
Gathering data

We did a systematic search on scopus and web of science for papers that evaluate any human activity impact on freshwater fish functional diversity and we ended with 41 articles after the exclusion of papers that were not adequate:

For the papers that compared functional diversity values from natural against impacted sites, we extracted mean, error and sample size for each study group 

For papers that evaluated changes due human activities through a regression "functional index vs impact variable", we extracted the coefficient of correlation and the sample size. Also, we extracted information about the type of human activity and local of the study.

Processing data

With this data, we calculated the effect size through mixed-effects models to evaluate the effect of human activities in functional diversity. We also tested the functional component, local (tropical vs. temperate), and type of human activity as moderators of the analysis.