INFLUENCE OF TRIBUTARY SPATIAL POSITION ON THE STRUCTURE OF WARMWATERFISH COMMUNITIES

We found a significant and positive relationship between fish species richness and four measures of stream size (drainage area, stream order , link magnitude, and downstream link) in three Illinois drainage basi ns. Downstream link (incorporating both stream size and size of stream at the next downstream confluence) explained the greatest portion of the variance. This suggests that downstream processes significantly in fluence the structure of fish communities inhabiting warmwater streams . Significantly higher numbers of fish species were collected from tri butary streams (< 259 km2 drainage area) located lower in a drainage n etwork and connected to a main channel system than from similarly size d streams located in the headwaters of a drainage network. The differe nce in species richness among station treatments was not due to a diff erence in the number of individuals collected among treatments. We wer e unable to accept or reject the hypothesis that differences in fish, species richness were due to differences in physical habitat. The immi gration-extinction hypothesis appears to provide a plausible explanati on for the observed pattern in fish community structure within a drain age. The location of a stream channel within a network may provide a g eneral template for fish community structure in warmwater drainages by regulating potential species richness.