Evolutionary responses of aquatic macroinvertebrates to two contrasting flow regimes

Natural disturbances are agents of natural selection that drive multiple biological adaptations along evolutionary time. Frequent, high magnitude disturbances are expected to select for morphological and behavioral traits to resist or to avoid them. In contrast, predictable and seasonal disturbances are expected to select for synchronized life cycles to avoid unfavorable periods. We assessed the effect of flood disturbances on aquatic macroinvertebrates in two rivers with contrasting flow regimes: the Gila (USA) with seasonal floods and droughts, and the Thur (Switzerland) with a high frequency of aseasonal floods. Macroinvertebrates were analyzed based on 46 biological trait categories classified into morphological, life-cycle synchronization, and behavioral strategies. Flood effects on diversity and composition were much clearer for the Gila than for the Thur. Overall, biological adaptations were related to the flood regime of each river. Morphological adaptations to resist or avoid floods prevailed under frequent and aseasonal disturbances (the Thur), whereas life-cycle synchronization and behavioral adaptations were associated with highly seasonal, predictable, and low-frequency disturbances (the Gila). Given that forecasted future flow regimes differ between regions, our results suggest that the effects of future flow regime alterations will ultimately depend on the adaptation strategies to current flow regimes.