Introduced crayfish are wiping out native species in North America, which has three-quarters of the crayfish species worldwide. New research provides the first evidence that introduced crayfish are taking over native species genetically by hybridizing with them.
“Our study is the first to document hybridization associated with the invasion and displacement of resident crayfish species,” say William Perry, who did this work at the University of Notre Dame in Indiana and who is now at Illinois State University in Normal, Illinois, and his colleagues in the December 2001 issue of Conservation Biology.
Hybridization has been implicated in the decline of many imperiled species, from North American sunflowers to Hawaiian ducks to red wolves. Perry and his colleagues suggest that hybridization may play an even larger role in non-native species invasions than biologists have realized.
North American freshwater animals are estimated to have the highest extinction rates in the world, and nearly a third of our 390 crayfish species are at risk. One of the biggest threats to local crayfish is the introduction of the rusty crayfish, which is widely used as fishing bait. Native to Ohio, Kentucky, and Indiana, the rusty crayfish has spread as far as Maine, New Mexico, and Tennessee.
The rusty crayfish was introduced to northern Wisconsin and Michigan lakes in the 1960s, where it is displacing the northern clearwater crayfish and other local species.
Although the rusty crayfish and the northern clearwater crayfish are both native to North America, they are not ecologically equivalent. Crayfish are critical to their ecosystems, and changes in the crayfish community can have severe impacts on lakes and streams. For instance, in northern Wisconsin lakes, introduced rusty crayfish have clearcut the bottom-growing aquatic weeds that provide a refuge for fish and invertebrates. Moreover, the rusty crayfish grows faster, is larger, and has more offspring than the northern clearwater crayfish.
Biologists have feared that besides outcompeting local crayfish, the rusty crayfish also hybridizes with them. So far, however, this assessment has been based on morphological studies, which can be ambiguous.
To determine definitively whether introduced rusty crayfish have been hybridizing with local species, Perry and his colleagues compared the DNA of crayfish from 30 sites in the midwestern U.S. The researchers found that hybridization is common. One of the worst cases is Birch Lake, Wisconsin, where only a fourth of the crayfish still have northern clearwater crayfish genes, and the entire population already looks exactly like rusty crayfish.
“Wherever the two taxa meet, [the rusty crayfish] ecologically displaces and, through hybridization, genetically assimilates and morphologically extirpates [northern clearwater crayfish] populations,” say Perry and his colleagues.
Several states have restricted the use of the rusty crayfish as fish bait, and there are calls to ban the use of live crayfish as bait. This can’t come too soon. “We are rarely if ever able to get rid of an exotic species, and thus introduced species like the rusty crayfish represent a permanent alteration to the system,” says Perry.
Perry, W.L. et al. 2001. Implications of hybridization between introduced and resident Orconectes crayfishes. Conservation Biology 15(6):1656-1666.
Perry’s co-authors are David Lodge and Jeffrey Feder, both of the University of Notre Dame in Indiana.