Community resilience in a river network: the roles of connectivity and drying regime

The cross-scale resilience model suggests that resilience, the amount of disturbance an ecosystem can absorb before collapsing and reorganizing, can be measured by evaluating the diversity and redundancy of functions performed by species at different spatiotemporal scales. Yet, little is known about the effects of flow intermittence and associated hydrological connectivity on the resilience capacity of instream communities, and the ecosystem functions they perform. We expected lower resilience capacity in non-perennial and isolated reaches. Here, we used fish and invertebrate community data and litter decomposition rates across 20 sites in a river network naturally fragmented by drying to characterize the drivers of resilience at the river-network scale. Using discontinuity analysis, a set of resilience indicators were calculated from body size distribution and species traits, and related to flow intermittence, network fragmentation and position in the stream network. We found that non-perennial reaches were characterized by lower resilience with fewer species, lower levels of functional redundancy of five out of eight functional feeding groups. Functional redundancy and response diversity in shredders were decoupled, translating into low litter decomposition rates in non-perennial reaches. Upstream reaches were characterized by low community resilience, likely reflecting their isolated position in the river network, but flow intermittence affected more strongly the resilience of downstream than upstream reaches. Cross-scale functional redundancy and grazer response diversity were driven by network fragmentation, meaning that the functions these groups perform might be at risk when facing other anthropogenic pressures. Finally our study suggests that reliable resilience assessments need to be based on several standardized indicators and call for more studies comparing these indicators in diverse ecosystems.