MAIN AND INTERACTIVE EFFECTS OF INVERTEBRATE DENSITY, PREDATION, AND METALS ON A ROCKY-MOUNTAIN STREAM MACROINVERTEBRATE COMMUNITY

Using laboratory microcosms and a 2 x 2 x 2 factorial design, I conduc ted an experiment to test the main and interactive effects of a mixtur e of metals (Cd, Cu, and Zn), invertebrate density, and predation on s tream invertebrate community structure. Metal exposure reduced the abu ndance of one species: Prostoia besametsa (Plecoptera: Nemouridae). Pr esence of the predaceous stonefly Hesperoperla pacifica (Plecoptera: P erlidae) significantly affected invertebrate community structure, as t otal abundance (p = 0.001) and taxa richness (p = 0.02) were lower in predator-added treatments. Predation intensity on Ephemerella infreque ns (Ephemeroptera: Ephemerellidae) was greater at high invertebrate de nsity than at low density (p = 0.01). Metal exposure influenced the ou tcome of predation on Hydropsyche sp., as predation intensity was grea ter in streams dosed with metals than in controls (p = 0.03). Results from this experiment suggest that metals can influence the relative im portance of biotic factors to alter stream invertebrate community stru cture.