Studies demonstrating the mechanisms regulating biodiversity are uncommon. Streams and rivers worldwide display a longitudinal gradient in fish biodiversity, and most prior research has used correlative evidence to infer that higher downstream diversity is produced by factors facilitating greater niche separation. We combine 20 years of fish abundance samples from a representative southern Appalachian stream with critical swimming velocity experiments to provide direct evidence that a shifting hydrodynamic barrier affects this gradient in Coweeta Creek. We observed increased diversity in multiple sites, produced by species immigrating upstream during periodic droughts (1985-1988,1999-2002) and a highly significant positive relationship (r2 - 0.77) between drought (Palmer index) and Shannon-Weiner diversity. Resident fish generally had smaller standard lengths during drought periods. Critical velocity measurements showed that residents could tolerate faster water velocities than drought immigrants and that upstream velocities in nondrought years were faster than those in some downstream sites. These data support the hypothesis that local fish diversity in this system is limited by the ability of drought immigrants to pass an upstream hydrodynamic barrier. Alternative hypotheses such as temperature differences between sites, increased capture efficiency during droughts, and increased productivity during droughts were not supported by the data. The relationship between drought and diversity in thissystem is counterintuitive because drought should reduce resource availability and produce lower rather than higher diversity. These results highlight the important relationship between natural flow variation and maintenance of local diversity and demonstrate the need for long-term ecological data.
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