Direct predation on developing insects undoubtedly reduces the number of emergers, right? In this paper, the answer is maybe. We blocked off 10 entire pools of a small stream in Oklahoma, and created a 13X gradient of fish density. After 44 days, there was a clear negative relationship between fish density and benthic insect density, evidence that fish predation reduced benthic insect density. However, this did not translate into a reduction in emergence. The reason may lie in a lag effect between measured benthic standing crop, and subsequent emergence. Unfortunately, the fish effect was detected only in the last week of the experiment, and it’s likely that the remaining insects were not developed enough to emerge immediately.
But experiments always have to end, especially when you have to return to campus to teach. There is, however, some evidence of a lag effect – benthic standing crop before the experiment predicted emergence at the end of the experiment. In other words, had we waited a few weeks, the story may have been different, and emergence may have reflected fish density, revealing a predation effect.