Dominant species in communities can negatively affect the abundance of other taxa via predation and competition.  Alternatively, these same dominant taxa can have positive effects on co-occurring species through habitat modification.  In small streams, leaf packs can be buried by sediment after flood events.  If large taxa (e.g., large caddisflies) can uncover these buried leaf packs then this could have a positive effect on the abundance of other taxa.  We hypothesized that Pycnopsyche could influence leaf pack assemblages through both positive effects (sediment reduction) and through negative effects (competition).  Leaf packs were placed in enclosures that were permeable to small taxa but not the larger Pycnopsyche.  Leaf packs were covered with a known volume of sediment and then 0, 3 or 6 large Pycnopsyche were added to the enclosures.  Pycnopsyche significantly reduced the volume of sediment covering leaf packs.  Some taxa only responded to sediment removal, i.e., their abundances increased in the presence of Pycnopsyche.  Other taxa, however, exhibited declines in abundance at the higher Pycnopsyche density which suggests that the negative effects of Pycnopsyche (e.g., encounter competition) outweighed the positive effects (sediment removal) at higher densities.