Spatial variation in relative abundance of a widespread, numerically dominant fish species and its effect on fish assemblage structure

Collections of fish assemblages from streams in the midwestern United State s were used to examine assemblage-level effects of spatial variation in rel ative abundance of the red shiner, Cyprinella lutrensis, a widespread and h ighly abundant minnow species. This species has been widely introduced outs ide its native range and is suspected to have impacted local assemblages wh ere it has become established. Given its overall dominance of midwest fish assemblages, and its suspected impact on assemblage structure, we asked if structure of the residual fish assemblages (red shiners excluded) was a fun ction of the relative abundance of red shiners throughout the native range of C. lutrensis in the USA. Although red shiner ranked first in abundance i n half of the assemblages and numerically dominated 28% of the assemblages, red shiner relative abundance in an assemblage had no detectable effect on richness, diversity, evenness, or complexity of other (residual) species i n the assemblage. Relative abundance of red shiners did have a positive eff ect on the abundance of benthic minnows in the residual assemblage, but not on water column minnows that are ecologically most like red shiners. Envir onmental factors did not explain a significant amount of the variation in r elative abundance of red shiners, but did explain some variation in residua l assemblage structure. Although widespread and numerically dominant at man y localities, red shiners do not appear to have a strong impact on local fi sh assemblage structure within their native range. This is in sharp contras t to the reported negative effects of red shiners on fish assemblages where they have been introduced outside their native range.