Species Richness of Freshwater Fish Trophic Guilds Increases With Tropical River Discharge and Decreases With Variability.

Species-area relationships (SARs) are one of the most well-established conservation biogeography patterns, and in rivers, habitat area is mediated by discharge. Species richness and river discharge have a well-established positive relationship, but how discharge affects trophic diversity is less clear. Free-flowing tropical river ecosystems are hotspots of global biodiversity, but they are under increasing threat from water resource developments which alter river discharge regimes. Here we investigate relationships between river discharge metrics and the species richness of freshwater fish trophic guilds in tropical rivers of northern Australia, using data collated from 40 catchments. We analyzed relationships between the species richness of freshwater fish trophic guilds and discharge metrics including mean annual discharge (Q), mean daily dry and wet season discharge, and the coefficient of variation (CVQ) of Q. Invertivores and omnivores were the most species-rich trophic guilds. Our results show that the species richness of trophic guilds in north Australian freshwater fishes was correlated with multiple components of wet-dry tropical river discharge regimes. The species richness of predators, invertivores, and herbivore-detritivores increased with Q and wet season discharge, whereas omnivore and invertivore richness increased with dry season discharge. Increasing variability in discharge had a negative effect on the species richness of invertivores and omnivores, suggesting adverse effects of low discharge periods. We found no statistical support for the hypothesis that the slope of SARs increases with trophic level, as suggested by previous research. These findings suggest that decreases in wet and dry season discharge, or increases in flow variability due to water resource development or climate change, may result in the loss of trophic diversity from tropical rivers. Our results suggest that the conservation of both wet and dry season natural flow regimes in tropical rivers will be needed to protect freshwater fish trophic diversity.