A multi-faceted framework of diversity for prioritizing the conservation of fish assemblages

Floodplain waterbodies and their biodiversity are increasingly threatened by human activities. Given the limited resources available to protect them, methods to identify the most valuable areas for biodiversity conservation are urgently needed. In this study, we used freshwater fish assemblages in floodplain waterbodies to propose an innovative method for selecting priority areas based on four aspects of their diversity: taxonomic (i.e. according to species classification), functional (i.e. relationship between species and ecosystem processes), natural heritage (i.e. species threat level), and socio-economic (i.e. species interest to anglers and fishermen) diversity. To quantitatively evaluate those aspects, we selected nine indices derived either from metrics computed at the species level and then combined for each assemblage (species rarity, origin, biodiversity conservation concern, functional uniqueness, functional originality, fishing interest), or from metrics directly computed at the assemblage level (species richness, assemblage rarity, diversity of biological traits). Each of these indices belongs to one of the four aspects of diversity. A synthetic index defined as the sum of the standardized aspects of diversity was used to assess the multi-faceted diversity of fish assemblages. We also investigated whether the two main environmental gradients at the catchment (distance from the sea) and at the floodplain (lateral connectivity of the waterbodies) scales influenced the diversity of fish assemblages, and consequently their potential conservation value. Finally, we propose that the floodplain waterbodies that should be conserved as a priority are those located in the downstream part of the catchment and which have a substantial lateral connectivity with the main channel.