Effects of invasive crayfish on fine sediment accumulation, gravel movement, and macroinvertebrate communities

Crayfish are one of the most prolific freshwater invaders, yet the role of crayfish as ecosystem engineers that structure benthic substrates in their nonnative habitat is rarely studied. We used an in situ experimental manipulation to document changes in sediment dynamics and macroinvertebrate density and richness caused by the presence or absence of invasive rusty crayfish (Orconectes rusticus). Rusty crayfish reduced fine-sediment accumulation and increased pit and mound structures in the river bed by moving gravels at the bed surface. We predicted that this omnivorous species would directly reduce macroinvertebrate density and richness, but we found that macroinvertebrate density was higher in treatments with rusty crayfish present and that macroinvertebrate richness did not differ between treatments with and without crayfish. Our findings suggest that rusty crayfish, which are spreading throughout the northeast USA, are probably important engineers of physical habitat structure in stream ecosystems. Rusty crayfish may also have unexpected, facilitative consequences for other benthic species by reducing fine-sediment accumulation in streams that are sediment impaired. Our study illustrates that biological forces can influence benthic gravel-size distributions and topography and suggests that crayfish may be important drivers of substrate-sorting dynamics and disturbance regimes, with consequences for community structure in streams where they are invasive.