The Role of Hydraulic Disturbance in Twelve Streams in Southern Michigan and Northern Indiana: An Application of the Harsh-Benign Hypothesis
In order to understand interactions that take place in stream ecosystems, many investigations have focused on biotic relationships among periphyton, herbivores and predators. Research has also incorporated abiotic factors into the picture that stream ecologists attempt to develop. In this study, special attention was given to hydraulic disturbance, an abiotic factor that has been shown to have special importance in lotic communities. Substrate tiles were placed in twelve streams with similar characteristics in southern Indiana and northern Michigan over a six week period. Water velocity was measured and tiles were sampled for periphyton abundance and macroinvertebrate herbivore (grazer) abundance. In this investigation, hydraulic disturbance may have related to grazer abundance in accordance with the harsh-benign hypothesis. When water velocity was equal to or above a certain critical point, hydraulic disturbance controlled grazer abundance and affected trophic structure. When water velocity was below this point, biotic factors such as food availability may have regulated grazer abundance and affected trophic structure. For the twelve sampled streams, the possible critical flow point was calculated to be = 0.48 m/s. Conflicting evidence from different streams indicates that this threshold may not be absolute; it may be stream-specific in that grazers adapted to a faster stream may be more resilient to hydraulic disturbance, thus having a higher current threshold. Further investigation, with monitoring of predators and spatial competition, is needed to confirm these hypotheses.