Original paper

Coexistence of an invasive and a native gammarid across an experimental flow gradient: flow-refuge use, -mortality, and leaf-litter decay

Felten, Vincent; Dolédec, Sylvain; Statzner, Bernhard


The Ponto-Caspian gammarid Dikerogammarus villosus currently expands its range and replaces native gammarid populations in many large rivers of Europe. Results of small-scale, still-water experimentation fostered the idea that D. villosus reduces native gammarid populations in large rivers through predation. However, in large rivers, the prevailing flow conditions (potentially harsh or benign for predator and/or prey) and related flow-refuge use (potentially concentrating predator and prey in small spaces) could interfere in this predator-prey interaction. We combined a field survey with a mesocosm-flume experiment to assess flow-refuge use, mortality, and associated effects on leaf-litter decay through potential shredding by the gammarids of coexisting D. villosus and native Gammarus pulex across a shear stress (τ) gradient. In real running waters, both species had no well defined preference for a narrow τ range, suggesting that D. villosus would effectively prey on G. pulex across a wide τ range. In our experiment, however, the concentration of D. villosus and G. pulex in flow refuges increased with τ and decreased with the duration of the experiment (decrease of D. villosus < G. pulex). Overall mortality over the entire experiment was higher in G. pulex than in D. villosus and tended to decrease with τ in D. villosus, whereas it tended to increase with τ in G. pulex. Decay of alder leaf litter decreased with τ and time; however, the difference (low vs. high τ) of the temporal decay-rate drop was low. Our experimental results support the idea that at higher τ, both D. villosus and G. pulex are concentrated in flow refuges, which facilitates predation of the former on the latter. These flow effects on the interaction between D. villosus and a native gammarid (and potentially other rheophilic invertebrates) have particularly important implications for restoration projects that increase the discharge in by-passed sections of large regulated rivers, as the concurrent or future invasion of D. villosus could imperil the restoration of rheophilic invertebrate populations.


benthic invertebratesdikerogammarus villosusdischarge restorationgammarus pulexrunning watersshear stress