Crayfish as geomorphic agents and ecosystem engineers:: Biological behavior affects sand and gravel erosion in experimental streams

Effects of animals on the transport of solids (e.g., sediments) can modify earth-surface processes and landforms (role as geomorphic agents) and resources for other organisms (role as ecosystem engineers). Therefore, we studied the impact of crayfish (Orconectes limosus) behavior on sand-gravel erosion and bottom habitat over riffle-pool sequences in experimental streams. We manipulated the availability of crayfish food and refugia. Refugia availability had clear effects on overall crayfish activity. The establishment of dominance hierarchies among the crayfish produced patterns in crayfish activity that differed between riffles and pools. Crayfish activity significantly affected sand and gravel erosion. High local sediment erosion averaged approximate to 2.8 (riffles) and approximate to 1.4 (pools) kg dry weight m(-1) d(-1) when crayfish hierarchies were established and refugia were available. Removing the refugia increased these rates to approximate to 4.0 (riffles) and approximate to 3.2 (pools) kg dry weight m(-2) d(-1). This direct erosion caused by crayfish should be Lower than that caused by hoods. Crayfish distinctly (1) increased bedform roughness (riffles), (2) decreased sand dune height (pools), (3) shifted the transition between gravel (riffles) and sand (pools) downstream, (4) decreased sand in gravel interstices (riffles), (5) decreased filamentous algae growth on gravel (riffles), and (6) decreased biofilm cover on sand dunes (pools). These sediment changes due to crayfish activity at baseflow should have opposite effects on sediment erosion during subsequent floods. Crayfish impact on physical habitat at baseflow could largely affect population and community structure of the benthos, as well as egg survival of gravel-breeding fish.