PROBABLE DISPLACEMENT OF RIFFLE-DWELLING INVERTEBRATES BY THE INTRODUCED RUSTY CRAYFISH, ORCONECTES RUSTICUS (DECAPODA, CAMBARIDAE), IN A NORTH-CENTRAL WISCONSIN STREAM

The rapid northward range expansion of the rusty crayfish, Orconectes rusticus, and its negative effects on Wisconsin lakes have been the su bjects of intense study throughout the last fifteen years. In this stu dy, we investigated the possible impact of rusty crayfish on the benth ic macroinvertebrate community structure of the Prairie River in north -central Wisconsin. Rusty crayfish and other invertebrates were collec ted during August and September, 1994, from three sections of the Prai rie River. Rusty crayfish relative abundance increased significantly f rom the upper to middle, and middle to lower sections; and correlated negatively with a significant 77% decrease in total density of aquatic invertebrates between sections. Mean density of all important inverte brate families and trophic guilds decreased significantly between the upper and lower sections. Due to the similarity of most environmental conditions between river sections, decrease of invertebrates is attrib uted to the increased abundance of rusty crayfish and its interactions with the native fauna. Our results suggest that a high abundance of r usty crayfish may negatively impact Wisconsin lotic systems.