113 J. N. Am. Benthol. Soc., 2005, 24(1):113–122  2005 by The North American Benthological Society The effects of 2 coexisting crayfish on an Appalachian river community BRIAN S. HELMS 1 AND ROBERT P. CREED 2 Department of Biology, Appalachian State University, Boone, North Carolina 28608 USA Abstract. Crayfish can act as keystone species and ecosystem engineers in small streams, but their effects in large rivers are not well known. Two species of crayfish, Orconectes cristavarius and Cambarus chasmodactylus, coexist in the South Fork of the New River in western North Carolina. We used gut- content analyses and an enclosure–exclosure experiment to investigate the influence of both species of crayfish on sediment accumulation and benthic invertebrates. Crayfish guts contained mostly sed- iment and vegetative detritus. However, C. chasmodactylus guts contained significantly more detritus and animal matter than O. cristavarius guts, and O. cristavarius guts contained significantly more sediment than C. chasmodactylus guts. In the field experiment, sediment volume was lower in open baskets and cage controls, which were exposed to crayfish and benthic-feeding fish, than in enclo- sures containing only crayfish. Sediment volume was highest in fish/crayfish exclosures. Despite their effect on sediment accumulation, crayfish did not significantly affect the density of any inver- tebrate taxon. No clear relationship was found between chironomid density and enclosure–exclosure treatment, but chironomid density was positively correlated with sediment volume. Damselflies (Cal- opteryx maculata) tended to be more abundant in crayfish enclosures than in open treatments and cage controls, and cyclopoid copepods tended to be more abundant in O. cristavarius enclosures than C. chasmodactylus enclosures. The significant differences in the diets of the 2 species of crayfish were not associated with differences in their effects on invertebrates. Our results suggest that these 2 species of crayfish may be functionally redundant in this community, despite differences in diet. Furthermore, the lack of pronounced crayfish effects on invertebrate taxa suggests that the effects of crayfish may not be as strong in large rivers as in small streams. Key words: crayfish, Cambarus, Orconectes, sediment processing, detritivory, omnivory, community structure, diet analysis. Organisms can have strong effects on com- munities via trophic interactions (e.g., keystone species) or via nontrophic mechanisms (e.g., ecosystem engineering). Ecosystem engineers can create or modify habitats and can affect the availability of resources to other organisms (Jones et al. 1994, 1997). The magnitude of the effects of an ecosystem engineer may be a result of the functional uniqueness of that organism in a community. For example, beavers have no functional analogs in their habitats and, there- fore, removal of beaver causes large changes in a system (Naiman et al. 1988, Jones et al. 1994). Conversely, many organisms inhabiting a beaver pond may be functionally similar on a com- munity level (e.g., members of the macroinver- tebrate leaf-shredder guild). Species that are not functionally unique may have only minor indi- vidual effects on community structure and can 1 Present address: Department of Biological Scienc- es, Auburn University, Auburn, Alabama 36849 USA. E-mail: helmsbs@auburn.edu 2 E-mail address: creedrp@appstate.edu be considered redundant species in the presence of functional equivalents (Navarrete and Menge 1996, Ehrlich and Walker 1998, McCann et al. 1998, Covich et al. 1999). Crayfish have strong trophic and nontrophic effects in many freshwater systems. They can influence, directly and indirectly, the distribu- tion and abundance of sediment, detritus, algae, macrophytes, and macroinvertebrates (e.g., Creed 1994, Lodge et al. 1994, Charlebois and Lamberti 1996, Parkyn et al. 1997, Nystrom et al. 1999, Statzner et al. 2000, Usio 2000, Schofield et al. 2001, Creed and Reed 2004). For example, the crayfish Orconectes rusticus directly con- sumes periphyton. It also dislodges the floccu- lent detritus component of periphyton and con- sumes other periphyton grazers as it grazes. Thus, O. rusticus can have an overall positive effect on periphyton quality and chlorophyll a content (Lodge et al. 1994, Charlebois and Lam- berti 1996). Direct grazing by the crayfish O. pro- pinquus reduced the abundance of the filamen- tous alga Cladophora glomerata in deep-water habitats in a Michigan stream. The reduction in-