Functional response of aquatic invertebrate communities along two natural stress gradients (water salinity and flow intermittence) in Mediterranean streams

Functional trait diversity can provide insight into ecosystem function beyond that provided by species diversity measures. The relationship between functional diversity and natural stressors has received less attention compared to anthropogenic stressors. In this study, we investigated how two natural stressors, water salinity and flow intermittence, affect functional richness and functional redundancy of aquatic invertebrate communities using seven biological traits and 39 modalities. For this purpose, we characterized these functional diversity measures in 22 Mediterranean streams with a gradient of natural salinity and flow intermittence. Our findings showed that both functional richness and functional redundancy decreased with increased stress by water salinity and flow intermittence for all the studied traits but more rapidly for the former, suggesting that water salinity is a stronger environmental stressor than flow intermittence. Our study also described an antagonistic interaction of the two study stressors, in which the net effect of both is less than the sum of their independent effects. This study emphasizes that in saline streams, characterized by lower functional richness and functional redundancy, the loss of any taxon can have a huge impact on community functioning. In particular, the functional singularity of saline intermittent streams makes them extremely sensitive to additional anthropogenic impacts. In the context of future global change scenarios, which predict higher flow intermittence and water salinity, this study gives a better understanding of the functional features of these types of ecosystems.