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Presented at the NABS Annual meeting, Vancouver, British Columbia, 2004
in Food webs 1
How do grazers affect periphyton heterogeneity in streams? Perspectives from small-scale experiments
M. Alvarez1 and B.L. Peckarsky1,3. 1Departamento de Ecología y Biologia Animal, Universidad de Vigo, 36200 Vigo, Spain, 2Department of Entomology & Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853 , 3Rocky Mountain Biological Laboratory, PO Box 529 Crested Butte, Co 81224 USA
The effects of grazing on algal biomass and spatial heterogeneity were tested experimentally in flow-through microcosms. The first experiment examined the effects of fixed densities of three species of grazers (the caddisfly Allomyia sp. and two mayflies, Epeorus deceptivus and Baetis bicaudatus) on periphyton. Baetis was tested with and without chemical cues from fish predators, which reduce grazer foraging activity. Mean algal biomass (chl a) was reduced in all treatments compared to ungrazed controls, but there were no differences among grazer treatments. Algal heterogeneity (Morisita Index, Id) increased with grazer mobility; with the highest heterogeneity occurring in the Baetis - no fish treatment. The second experiment followed a three factorial design, with initial resource distribution (homogeneous vs. heterogeneous), Baetis density (high vs. low) and fish odor (present vs. absent) as factors. Distribution of Baetis and chl a on individual rocks were recorded to explore the mechanisms responsible for the observed distributions of algae. Initial algal distributions were maintained despite being subjected to grazing. Mean chl a was highest in controls and effects of Baetis on algal biomass increased with density. There were no fish effects on algal biomass and no effects of grazer density or fish on algal heterogeneity. At the scale of individual rocks Baetis was unselective when food was homogeneously distributed and it chose high food rocks when food was heterogeneously distributed Results of these mechanistic experiments showed that Baetis can track resources at the scale of single rocks and that even at high densities, mobile grazers potentially maintain periphyton distributions observed in natural streams
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