Interference competition between alien invasive gammaridean species

The relative abundances of gammaridean species in the river Rhine have profoundly changed since the invasion of Dikerogammarus villosus in 1994/1995. This study tested whether these changes in gammaridean dominance could have been determined by interspecific competition and unequal mortality, for example by intraguild predation (IGP). Single and two species tests have been carried out in aquariums provided with all substrata present in the main channel of the Rhine. Changes in substratum choice, increased swimming activity and increased mortality of a species were used as indicators of interspecific competition during interaction between gammaridean species. Interspecific competition and mortality between the most abundant invasive gammaridean species in the Rhine, viz. Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus were tested. In single-species experiments, G. tigrinus and D. villosus showed similar preferences for a stony substratum, whereas E. ischnus mostly occupied the water column. The two-species aquarium experiments indicated direct interference competition for substratum and unequal mortality between G. tigrinus and D. villosus, with D. villosus being the stronger competitor. Competitive stress was influenced by population density, was size-dependent and varied between the different types of substratum due to substratum choice. G. tigrinus did not show any behaviour indicative of interference competition in the presence of E. ischnus, and neither did E. ischnus or D. villosus in the presence of any of the other gammarideans. Swimming in the water layer may already enable E. ischnus to minimise its encounters with the stone-dwelling D. villosus and G. tigrinus. To maximise the encounters between E. ischnus and D. villosus, a fish (Lepomis gibbosus) was added to occupy the water layer during the aquarium experiments. E. ischnus showed a higher mortality in the presence of both D. villosus and fish, probably due to increased stress, as shelter opportunities to escape the predators had been minimised. The study shows that interference competition between gammaridean species can explain the replacement of the North American invader G. tigrinus by D. villosus in the river Rhine. E. ischnus and D. villosus both Ponto-Caspian invaders did not show interference competition in our experiments and co-exist in the Rhine.