Geographic, terrestrial and aquatic factors: which most influence the structure of stream fish assemblages in the midwestern United States?

Effects of environmental or landscape factors on species composition, speci es richness and complexity of fish assemblages were examined using our coll ections of fish from 65 sites on streams in 13 drainages across the midwest ern United States. Effects of environmental factors were examined at three scales: broad geographic factors included drainage, latitude, and longitude ; local terrestrial factors included features of the riparian zone adjacent to the collecting site as well as local climate and land use; within-strea m aquatic factors related to structure and hydrology of the stream reach sa mpled. Each assemblage property was examined for its relationship to factor s at each scale separately, and then for relative importance of all factors found to be significant in the separate analyses. Assemblage composition ( summarized as sample scores on two axes of a detrended correspondence analy sis) varied significantly as a function of factors at all three scales when each scale was considered separately. With simultaneous consideration of a ll scales, however, only broad geographic factors (particularly latitude) a nd local terrestrial factors explained significant variation in assemblage composition. Species richness (the number of species we captured) was expla ined by longitude and within-stream aquatic factors both when considered se parately and together. Assemblage complexity (quantified as slope of relati ve abundance versus rank abundance) was only related to within-stream aquat ic factors. Assemblage composition and emergent assemblage properties (rich ness and complexity), therefore, were explained by factors acting at differ ent scales. The total variation explained for assemblage composition was mu ch greater than that explained for emergent assemblage properties, suggesti ng that assemblage composition may vary more as a function of environmental and landscape factors than do species richness and complexity.