Plant-herbivore interactions in streams near Mount St. Helens

1. In four separate field experiments near Mount St Helens (Washington, U.S.A.) during1986, the grazing effects of two large benthic herbivores, tadpoles of the tailed frogAscaphus truei and larvae of the caddisfly Dicosmoecus gilvipes, were investigated usingstreamside channels and in-stream manipulations. In the experimental channels,abundances of periphyton and small benthic invertebrates declined significantly withincreasing density of these larger herbivores.

2. In eleven small, high-gradient streams affected to varying degrees by the May 1980eruption, in-stream platforms were used to reduce grazing by A. truei tadpoles on tilesubstrates. Single platforms erected in each tributary and compared to grazed controlsrevealed only minor grazing effects, and no significant differences among streamsvarying in disturbance intensity (and, consequently, tadpole density). However, resultsprobably were confounded by high variability among streams in factors other thantadpole abundance.

3. Grazing effects were further examined in two unshaded streams with differenttadpole densities, using five platforms per stream. In the stream with five tadpoles m-2,grazing reduced periphyton biomass by 98% and chlorophyll a by 82%. In the streamlacking tadpoles, no significant grazing effects were revealed. Low algal abundance onboth platforms and controls, and high invertebrate density in that stream (c. 30 000 m-2)suggests that grazing by small, vagile invertebrates was approximately equivalent tothat of tadpoles.

4. The influence of large benthic herbivores on algal and invertebrate communities instreams of Mount St Helens can be important, but reponses vary spatially in relation tostream disturbance history, local environmental factors, and herbivore distributionalpatterns and abundance.