Landscape Influences on Stream Habitats and Biological Assemblages

<em>Abstract.</em>—Biologists need to understand causal relations among key habitat elements and fishes to effectively protect and manage stream systems. Though much groundwork has been laid, development of an analytic framework that incorporates spatial hierarchy of stream characteristics to predict instream habitat and fish assemblages has been challenging. A key issue is the complex web of direct and indirect effects that arise when one attempts to include all pertinent habitat parameters in analyses. Covariance structure analysis (CSA) was specifically developed for untangling such webs. We used CSA to quantitatively describe relations between catchment- and local-scale habitat variables and fish biomass in streams of Michigan’s Lower Peninsula. Catchment-scale variables characterizing stream size, land use, and surficial geology had significant direct and indirect effects on, and explained 50–83% of spatial variation in, mean depth, velocity, July mean temperature, 90% exceedence flow yield, and total phosphorus values in streams. These variables also had significant direct effects on substrate composition at sites, accounting for up to 30% of the spatial variation in individual substrate classes. Local-scale variables—mean depth, total phosphorus, and percent cobble—had significant direct effects on total fish biomass, explaining 26% of spatial variation among sites. Catchment area and percent of agricultural and urban land uses in the catchment had significant indirect effects on fish biomass. Our findings clearly demonstrated the ecological complexity and multiscale nature of fish habitat in stream ecosystems.