Experimental evidence of invasion facilitation in the zebra mussel-killer shrimp system

Invasion facilitation, whereby one species has a positive effect on the establishment of another species, could help explain the rapid colonisation shown by some freshwater invasive species, but the underlying mechanisms remain unclear. We employed two-choice test arenas to test whether the presence of zebra mussel (Dreissena polymorpha) could facilitate the establishment of the killer shrimp (Dikerogammarus villosus). Killer shrimp preferred to settle on mats of zebra mussel, but this was unrelated to mat size, and was not different from attraction shown to artificial grass, suggesting that zebra mussel primarily provides substrate and refuge to the killer shrimp. Killer shrimp were strongly attracted to water scented by zebra mussel, but not to water scented by fish. Chemical attraction to the zebra mussel’s scent did not differ between sympatric and allopatric populations of killer shrimp, suggesting that chemical facilitation is not an acquired or learned trait. Our results have implications for managing the spread of killer shrimp, and perhaps other freshwater invasive species, because invasion facilitation could significantly increase establishment success. Failure to consider invasion facilitation may underestimate the risk of establishment and likely impact of some aquatic invaders.