When invaders collide: competition, aggression, and predators affect outcomes in overlapping populations of red swamp (Procambarus clarkii) and rusty (Faxonius rusticus) crayfishes

Non-native crayfishes can have large impacts on biodiversity and the provisioning of ecosystem services in freshwaters. In 2015 we discovered an established population of the globally widespread red swamp crayfish ( Procambarus clarkii ) in the North Shore Channel of the Chicago Area Waterway System. This population overlaps with a population of rusty crayfish ( Faxonius rusticus ), a previous invader that is widely distributed and usually the dominant crayfish species across the Great Lakes region. If P. clarkii continues to spread in the Great Lakes region it will frequently encounter F. rusticus . Factors such as water clarity, competition for food when limited, and susceptibility to predation may alter P. clarkii ’s ability to become established and spread. We sampled the overlapping populations and found that P. clarkii are significantly larger than F. rusticus . Next, we conducted lab experiments to examine the outcomes of competition between these species for shelter and food. F. rusticus were significantly more likely to seek shelter when threatened, while P. clarkii were significantly more likely to respond aggressively. P. clarkii won more competitions for food. Finally, we conducted field experiments to investigate rates of predation on each species and found that P. clarkii are predated significantly more often. Our results suggest that P. clarkii is dominant in interactions with F. rusticus but that higher rates of predation, likely occurring because P. clarkii is less likely to flee from threats, mitigate these benefits. We suggest that P. clarkii will dominate crayfish communities in water with low clarity, but not in clear-water habitats where visual predators are more effective.