Interactions between sculpins, net-spinning caddis larvae and midge larvae

Meta-analyses have shown that benthic feeding fish tend to exert strong neg ative effects on densities of stream-living chironomids, but it is not know n whether this pattern reflects variation in consumption rates or prey move ment rates. We reanalysed data from an experimental study that showed stron g effects of a benthic Feeding fish (Cottus gobio) On chironomid densities. Gut content data suggested that chironomids were rarely consumed by sculpi ns. Path analysis suggested that indirect interactions mediated by net-spin ning polycentropodid caddis larvae (fish-net-spinners-chironomids) were muc h more important than the direct interaction (fish-chironomids). The effects of the net-spinning caddisfly larva Polycentropus flavomaculatu s on chironomid densities were examined in a manipulative field experiment. Artificial substrates, stone-filled plastic baskets, were exposed in a str eam for 5, 14 or 26 d. P. flavomaculatus had a pronounced positive effect o n the densities of most chironomid taxa. The experimental design allowed us to separate the effects of P. flavomaculatus nets from the effects of the larvae. Observed effects were primarily due to the presence of the nets. Ne gative effects of P. flavomaculatus larvae, which could be caused by direct predation, were not found. We conclude that sculpins reduced chironomid de nsities through an indirect interaction involving a negative effect of scul pins on net-spinning caddis larvae and a positive effect of caddis larval n ets on chironomid densities. To examine the potential generality of this indirect interaction, we review ed experiments examining effects of benthic feeding fish on chironomid dens ities. A majority of the experiments were performed in systems where net-sp inning caddis larvae either were absent or occurred in low numbers. However , one third of the experiments reported negative effects on net-spinners an d was performed in systems with reasonably high densities of net-spinners, suggesting that this mechanism may be more common than is presently believe d.