Is Competition with the Invasive Crayfish Orconectes neglectus chaenodactylus Responsible for the Displacement of the Native Crayfish Orconectes eupunctus?

The crayfish Orconectes eupunctus, a species of special concern, appears to have been displaced from part of its former range in the Spring River drainage of Arkansas and Missouri by the invasive crayfish O. neglectus chaenodactylus. We conducted preliminary field experiments to determine whether Orconectes eupunctus were displaced from their former range and whether interspecific competition with O. neglectus chaenodactylus was responsible for the displacement. To determine whether Orconectes eupunctus were displaced from their former range by negative biotic interactions with other species we placed single adult male O. eupunctus in cages at two sites: an upstream site where O. eupunctus was formerly abundant and a downstream site where it is currently abundant. Orconectes neglectus chaenodactylus were additionally paired alongside O. eupunctus at the upstream site. We also conducted a field competition experiment with treatments consisting of (1) 3 O. eupunctus, (2) 6 O. eupunctus, and (3) 3 O. eupunctus and 3 O. neglectus chaenodactylus at a site where both species occur. In the first experiment, O. eupunctus gained significantly more weight at the downstream site than the upstream site during 2003, but not during 2002. At the upstream site, O. eupunctus showed significantly greater growth rates than O. neglectus chaenodactylus during 2002, but not during 2003. In the competition experiment, O. neglectus chaenodactylus did not significantly reduce the growth or survival of O. eupunctus. These results suggest that O. eupunctus have been displaced from their former range by biotic interactions, but interspecific competition between adult male O. eupunctus and O. neglectus chaenodactylus does not appear to be the mechanism responsible for this displacement. Further study is needed to investigate the disappearance of O. eupunctus.