Citation:

Crabot, J., C. Mondy, P. Usseglio-Polatera, K. Fritz, P. Wood, M. Greenwood, M. Bogan, E. Meyer, AND T. Datry. A global perspective on functional responses of stream communities to flow intermittence. 2021 Symposium for European Freshwater Science, Dublin, IRELAND, July 25 - 30, 2021.

Impact/Purpose:

Biodiversity loss has been disproportionately high in freshwater environments, and biodiversity is a major factor governing the ecological integrity and ecosystem services provisioned from these environments. An increasing proportion of river networks are drying due to climate change and increased water abstraction. Streams that dry for a portion of the year (= temporary or non-perennial) represent the world's most widespread type of flowing waterbody. Streams shifting from perennial to non-perennial represents a substantial threshold for stream biota and processes because many aquatic biota have limited capacity to withstand drying. Biodiversity can be characterized in terms of the number and variation of taxonomic species as well as the functional diversity or the groups of species that harbor the same traits (e.g., body size, life span, dispersal ability). In this study we compare how taxonomic and functional diversity of aquatic invertebrate assemblages vary along gradients of flow duration across multiple river systems from three continents to test hypotheses about the impact of drying to stream biodiversity. With increased drying, invertebrates tended to have shorter life spans, smaller body sizes, and higher fecundity. Unexpectedly, functional diversity declined sharply with longer periods of stream drying and its decline was not different than that of taxonomic diversity. This indicates stream drying is likely a global threat to river biodiversity as functional redundancy was consistently lacking in invertebrate communities across naturally intermittent rivers from different continents and climate zones.  These results highlight the pressing need to consider drying as a major threat to freshwater biodiversity, ecosystem functions and services in rivers networks.

Description:

Due to global change, an increasing proportion of river networks are drying and shifts from perennial to non-perennial flow regimes represent ecological thresholds with dramatic and irreversible changes to community and ecosystem dynamics. However, there is minimal understanding of how biological communities respond functionally to drying, which can weaken the functional integrity of river networks and disrupt the services provided to society. Here, we shed light on the taxonomical and functional responses of aquatic macroinvertebrate communities to flow intermittence across river networks from 3 continents, to test predictions emerging from underlying trait-based conceptual theory. We predicted an high functional redundancy resulting from niche selection filtering taxa from adapted to drying, increasing flow intermittence to be associated with a reduction of taxonomic alpha diversity but not functional diversity, and a selection of specific trait modalities related to resistance or resilience to flow intermittence. Surprisingly, functional redundancy declined sharply with increasing flow intermittence: even weakly intermittent sites were functionally altered relative to perennial ones. Both taxonomic and functional alpha diversity decreased with flow intermittence. Last, a set of functional trait modalities including small body size, short life span and high fecundity were selected with increased flow intermittence. These results demonstrate the functional responses of river communities to drying and suggest that on-going biodiversity reduction due to global change in drying river networks is threatening their functional integrity.

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