LAUSR

Abstract Fishes are a speciose and ecologically important group within severely threatened or impacted freshwater habitats of Sundaland, whose contributions to ecosystem functioning can be inferred from their functional diversity. However, there is limited understanding of the potential impacts of anthropogenic species loss on ichthyofaunal functional diversity in the region. We therefore aimed to characterise the functional diversity of fishes across freshwater ecoregions in Sundaland, and predict potential impacts of anthropogenic species losses on functional diversity, in order to identify priority ecoregions for freshwater fish conservation. To this end, data comprising nine functional traits of 893 Sundaic freshwater fish species were compiled and analysed to compute measures of functional richness, functional redundancy and functional vulnerability. We then modelled the loss of extinction-prone species (identified using a trait-based conditional inference forest approach) within each ecoregion, and quantified the resulting impacts on functional diversity. Both functional richness and functional redundancy scale positively with species richness across ecoregions, but functional vulnerability is widespread. Functional entities comprising moderate- to large-sized omnivores or higher carnivores are particularly vulnerable, and the same is true for fishes with superiorly-oriented and toothed mouthparts. Removal of susceptible endemic species in each ecoregion resulted in the erosion of functional redundancy and exacerbation of functional vulnerability, but functional richness was relatively less impacted. Overall, our findings suggest a trajectory of accelerating impacts on functional diversity (and by extension, ecosystem function) if species losses continue unabated. From these results, we identify the Malay Peninsula Eastern Slope ecoregion as a potential conservation priority.