Testing the differential predation hypothesis for the invasion of rusty crayfish in a stream community: laboratory and field experiments.
We tested the hypothesis that the non-native rusty crayfish (Orconectes rusticus) is less vulnerable to predators than two native species (O. propinquus and O. obscurus) it is replacing in streams of the upper Susquehanna River catchment (New York, U.S.A.). We used laboratory experiments to compare species-specific predation rates by smallmouth bass (Micropterus dolomieu) on crayfish of equal size and field tethering experiments to compare relative predation rates between native O. propinquus and non-native O. rusticus by the suite of crayfish predators in our system. We predicted that crayfish size would affect predation rate but that predation rates would be equal among species when size was controlled. We also tested for two potential artefacts of tethering. We tethered crayfish in cages to test whether the ability to escape from tethers is size specific, and we tested whether tethering alters differential predation among crayfish species by the smallmouth bass. In the laboratory, smallmouth bass predation on rusty crayfish was lower than on either of the native species. In the field, predation risk for tethered crayfish was inversely related to size but did not differ among species when size was taken into account. Because rusty crayfish in the field experiment were slightly larger than the native species, as in nature, mortality was overall lower for the rusty crayfish. In cages, smaller crayfish were more probably to escape from tethers than larger ones, an artefact that may partially confound results from our tethering experiments. Unexpectedly, tethering nearly eliminated predation by smallmouth bass. This artefact prevented us from testing for an interaction of tethering with differential predation and means that the results of field tethering experiments do not include any contribution from smallmouth bass predation. Our experiments highlight the importance of explicitly considering potential artefacts that could confound results. Our results indicate that differential predation contributes to the rusty crayfish's invasion of a stream community. In our study system, predation rates on rusty crayfish are lower than for native species mostly because of selection by predators for smaller crayfish; species-specific characteristics such as behaviour that further reduce predation may also contribute, especially where smallmouth bass predation is important.