Potential and realized interactions between two aquatic invasive species: Eurasian watermilfoil (Myriophyllum spicatum) and rusty crayfish (Orconectes rusticus)

With multiple invasions, the potential arises for interactions between invasives inhibiting or promoting spread. Our goal was to investigate the interaction between two invasives, Eurasian watermilfoil (Myriophyllum spicatum) and rusty crayfish (Orconectes rusticus), which co-occur in several lakes in western Quebec, Canada, and to determine their overlap with littoral fish communities. Crayfish potentially aid milfoil dispersal by fragmentation or, alternatively, inhibit its proliferation through destruction and direct consumption. With a mesocosm experiment, we quantified milfoil fragment production versus biomass reduction by crayfish. More fragments were produced at medium to high crayfish densities, with a significant reduction of milfoil only at the highest densities, demonstrating the potential for both positive and negative interactions. Second, we determined the habitat preferences of each species by conducting a survey in the same lake. There was little overlap in the species' distributions, with each preferring different habitat features, indicating either a low probability of interaction or that interaction occurred historically, resulting in a contemporary exclusion pattern. While our experiment showed a potential for significant interaction, the low natural co-occurrence of these species suggests that they do not currently influence each other or that they previously excluded each other.