Invertebrate assemblage responses and the dual roles of resistance and resilience to drying in intermittent rivers

Intermittent rivers are naturally dynamic ecosystems in which flow cessation and riverbed drying cause temporal fluctuations in aquatic biodiversity. We analysed datasets from intermittent rivers in different climate zones across the world to examine responses of aquatic macroinvertebrate assemblages to drying, in relation to both taxonomic composition and traits of resistance and resilience. First, we compared the differences in taxonomic richness and turnover and in trait diversity, richness and redundancy before and after intermittent sites dried with the differences in concurrently sampled perennial sites. We found such high levels of variation in the before-after differences at intermittent and perennial sites that we could not detect statistical differences between them. Second, we examined the effects of climate (arid, Mediterranean, temperate) and durations of dry and post-dry (flowing) periods on responses to drying at intermittent sites. Only climate had a detectable effect; the proportion of taxa at intermittent sites that persisted through drying-rewetting phases was greatest in arid-zone rivers. Regardless of climate, the invertebrates that persisted at intermittent sites were dominated by taxa resistant to drying. By contrast, taxa that persisted at perennial sites had fewer traits conferring resistance but more conferring resilience. The contributions of resistance and resilience combined with the presence of both intermittent and perennial reaches likely supports the long-term stability and persistence of communities in intermittent rivers, despite the inherently high variation in short-term responses to drying.