Impact of an experimental disturbance on food-chain length

Background/Question/Methods Streams experience frequent natural disturbance and are undergoing considerable anthropogenic disturbance due to dam construction and water diversion.  Disturbance is known to impact community structure, but its effect on food-chain length is still a matter of considerable debate.  Theoretical models show that longer food chains are less resilient to disturbance, so food-chain length is predicted to be shorter following a disturbance event.  Here we experimentally test the effect of disturbance on food-chain length in streams by diverting stream flow.  We diverted water over a 100m reach for three streams for three months in the summer of 2006.  We also had an un-diverted 100m control reach for each treatment stream and three un-manipulated reference streams.

Results/Conclusions

In diverted reaches, August mean flow was reduced by 50-80% and the seven-day minimum flow was reduced by 65-95%.  We found that our experimental low flow disturbance did not alter food-chain length.  We did see an effect on body-size structure in our food webs with large fish absent from the disturbed reaches.  This suggests that food-chain length may be an insensitive indicator of disturbance.  We suggest that habitat heterogeneity and food web complexity buffer the effect of disturbance on food-chain length.  The theoretical predictions of disturbance on food-chain length are only likely to be seen in homogenous systems that closely approximate the linear food chains the models are based upon.