Invasion-related change in crayfish density affects a stream macroinvertebrate community.
Orconectes rusticus (Rusty Crayfish) have invaded streams of the upper Susquehanna River catchment, NY, replacing native crayfish and, in some areas, increasing overall crayfish density. Crayfish are important consumers and significant agents of disturbance in aquatic communities, so the introduction and expansion of Rusty Crayfish could affect the invaded community through the change in crayfish species composition, the increase in crayfish density, or some combination of the two. Other macroinvertebrates are prey of, competitors with, or subject to disturbance by crayfish and so are likely to be affected by changes in the crayfish assemblage. I conducted experiments in stream enclosures to determine the effects of crayfish species and density on the macroinvertebrate community. Increasing crayfish density reduced macroinvertebrate density but did not significantly affect macroinvertebrate taxon richness, diversity, or community composition. At moderate densities, the effects of native O. propinquus (Clearwater Crayfish) and invasive Rusty Crayfish did not differ significantly, although experimental power to detect small differences was low. These results suggest that the invasion of Upper Susquehanna River catchment streams by Rusty Crayfish will impact the macroinvertebrate community most strongly where or when Rusty Crayfish achieve high densities.