Habitat diversity and species diversity: testing the competition hypothesis with juvenile salmonids

I experimentally tested two predictions of the hypothesis that the positive relationship between habitat diversity and species diversity arises from a reduction in the negative effects of interspecific competition. By allowin g species to partition habitat and avoid competition. habitat diversity sho uld 1) facilitate the addition of an ecologically intermediate species into an existing community, and 2) reduce the negative effects of that species on existing members of the community. I tested these predictions with juven iles of three sympatric salmonid species: coho salmon (Oncorhynchus kisutch ), steelhead trout (O. mykiss), and cutthroat trout (O. clarki), which in n atural streams occupy deep low-velocity pools, shallow high-velocity riffle s. and intermediate habitats, respectively. I introduced two (coho and stee lhead) and three species communities into each of three artificial stream h abitats: pools, riffles, and diverse. The results provide partial support f or the predictions. Cutthroat trout grew fastest in the diverse stream habi tat. Though habitat diversity did not eliminate the negative effects of com petition, in the three species community coho and steelhead grew as fast in the diverse habitat as in either homogeneous habitat. The results are cons istent with data on species number and evenness from natural communities, w here variation along other niche axes confounds the relationship between ha bitat diversity, interspecific competition, and species diversity.