DISPERSAL OF THE CRAYFISH ORCONECTES RUSTICUS IN ILLINOIS, WITH NOTESON SPECIES DISPLACEMENT AND HABITAT PREFERENCE

Over the past 25 years the crayfish Orconectes rusticus has greatly ex panded its North American range. It first appeared in Illinois in 1973 . Since then it has rapidly expanded its range in the northern half of the state. We report here the results of field work conducted from Ju ne 1994 to October 1995 to determine the current range and population sizes of O. rusticus in Illinois and to ascertain whether displacement of native Illinois species is occurring. Data from 102 sample sites i ndicate that O. rusticus occurs commonly in large portions of the Kish waukee and Rock River drainages and in restricted portions of the Des Plaines, DuPage, Fox, Green, Kankakee, and Vermilion River drainages. An isolated population is also recorded from the Illinois River draina ge. Displacement of the native Orconectes propinquus and O. virilis is noted at several sites.