Benthic invertebrate assemblage structure in mountain streams: The influence of spatial scale of observation and organism size on relationships with environmental factors

The description of pattern in nature depends on the size and scale of both the observer and the phenomena being observed. The relative influence that different environmental factors have on organism distributions depends not only on scales of variation inherent to each factor, but also on the size and scale of the organisms.;I used data from 649 sites within 45 mountain streams in California to quantify relationships between environmental factors and organism distribution. I compared these relationships across analyses conducted at different spatial and biotic scales of resolution. I also examined relationships between environmental variation and assemblage size structure at two spatial scales.;Water temperature and chemistry, geographical location, and moss cover were related to assemblage structure at the basin scale. Riffle type, substrate size and diversity, channel width, as well as the aforementioned basin-scale factors were related to assemblage structure at the riffle scale. Multiple rifflescale analyses from independent sets of riffles from the same set of streams showed that sets of environmental factors related to a given assemblage structure descriptor differed markedly across different sets of riffles. These results indicate that analyses based on relatively large samples of riffles are unstable and could misinform managers about the factors most important to stream biota.;Size spectra based on density, biomass, and taxonomic richness in logarithmic size classes were compared at both the riffle and basin scale. Spectral shape varied less across basins than across riffles. Basin-scale spectral shape was strongly associated with water chemistry and temperature, habitat type, riparian and substrate conditions, and geography. Riffle-scale spectral shape was weakly related to substrate features and habitat-type. Assemblage size structure was linked to stream productivity and the physical architecture of the surface where these organisms live.;These results are consistent with a hierarchical interpretation of stream ecosystems where some environmental features vary only at large spatial scales and are related to organism distributions at large scales. Together, large- and small-scale factors are related to the distribution of organisms across smaller-scale pieces of the environment. Large-scale factors may constrain the influence of other factors that vary across smaller spatial scales.