Non-native crayfish species pose a serious threat to aquatic ecosystems and have been shown to decrease macroinvertebrate density and diversity, displace native crayfish, and alter fish and plant assemblages. We have identified a reproducing population of red swamp crayfish (Procambarus clarkii) that stretches along ~5 miles of channel in the Chicago region. This population overlaps with a population of rusty crayfish (Orconectes rusticus), a previous and widely distributed invader of freshwater ecosystems across the Great Lakes region. We believe this is the first time that overlapping populations of these species have been found. The competitive interactions between these species, both of which have large ecosystem impacts, may provide a guide to impacts if P. clarkii continues to spread. We used competition assays to examine interactions between the two species. Size matched individuals collected from the overlapping populations were used to examine interspecific competition for food (n=21 trials) and shelter (n=23). During shelter trials a fish attack was simulated to test if crayfish would seek shelter in a tank with available shelter for a single crayfish. For food competition trials we starved crayfish before adding a single food item. We analyzed which species ate the food and how long it took.
During shelter trials we found that O. rusticus fled the attack and/or sought shelter more often (n=23/23) than P. clarkii (n=9/23). Those P. clarkii that did not flee showed aggression by raising their claws towards the fish. O. rusticus were more likely to spend time in the shelter during the trial period (n=13/23) than P. clarkii (n=4/23). P. clarkii ate the food in more of the food competition trials (n=14/21) and did so more quickly (average time to consumption of 58.21 minutes) when compared to the trials in which O. rusticus ate the food (n=7/21, 81.40 minutes). It is almost certain that O. rusticus were already established in the channel when P. clarkii were introduced. The success of P. clarkii in the field, combined with our experimental results, indicate that this new invader may be competitively dominant and capable of spreading into freshwater ecosystems where O. rusticus is well established. If this occurs, the total impact of crayfish in rivers and lakes across the Great Lakes region is likely to grow with the potential to cause widespread ecosystem changes.