Background/Question/Methods Rusty crayfish (Orconectes rusticus) have invaded streams of the upper Susquehanna River catchment (New York, USA), replacing native crayfish and probably increasing overall crayfish density. Crayfish are important consumers and 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 macroinvertebrate taxa may be prey of, competitors with, or subject to disturbance by crayfish and so are likely to be affected by changes in the crayfish assemblage. We conducted both a correlative field study and an enclosure experiment to investigate the effects of invading O. rusticus on the abundance, diversity, and composition of the stream macroinvertebrate assemblage. We sampled crayfish and macroinvertebrates at 13 sites on 4 streams in the upper Susquehanna River catchment that varied in crayfish species composition and density. We are also conducting an experiment in stream enclosures to test the effects of O. rusticus density on the macroinvertebrate assemblage.

Results/Conclusions In the field samples, total macroinvertebrate abundance, taxon richness, diversity, and density of individual taxa were not significantly correlated to either total crayfish density or the relative abundance of O. rusticus. In the first year of the experiment, increasing crayfish density caused a significant decrease in total macroinvertebrate density but did not significantly affect macroinvertebrate taxon richness or diversity. The density of some individual taxa were also significantly affected by crayfish density. Our experiment shows that rusty crayfish density can affect the stream community; the mechanism of these effects, and whether they differ from those of native crayfish species, remain to be determined.