INFLUENCE OF GRAZER TYPE AND ABUNDANCE ON PLANT-HERBIVORE INTERACTIONS IN STREAMS

Grazer-periphyton interactions were investigated in 11 laboratory stre ams holding a range of densities of three herbivore taxa during a 32-d experiment. Effects of grazers on algae were strongest with Dicosmoec us gilvipes caddisflies, intermediate with Juga silicula snails, and w eakest with Baetis spp. mayflies. Algal standing crop, export, and gro ss primary production declined logarithmically with increasing grazer density. Algal turnover rate, however, increased with grazer abundance . At high densities of all grazers, responses in most algal parameters converged, suggesting that high grazing pressure, regardless of taxon , will similarly affect periphyton. Growth of both Dicosmoecus caddisf lies and Juga snails was density-dependent, with the highest growth ra tes occurring at the lowest densities. Caddisflies displayed high grow th rates but low efficiency in resource use. Snails had lower growth r ates but were more efficient in resource use. The coexistence of Dicos moecus and Juga, or other competing herbivores, in natural streams may be related to these fundamental differences in life history strategie s.