There is a growing recognition that omnivory is a pervasive phenomena on real food webs, but empirical demonstrations about the role of omnivory in food web structure and properties are still contradictory. We conducted a meta-analysis with data from 330 experiments that independently manipulated the presence of omnivorous and strict zooplanktivorous fishes and examined their effects on biomass distribution and stability of planktonic communities.

Results/Conclusions

In accordance with the predictions of food web theory, the presence of a third trophic level represented by both omnivorous and zooplanktivorous fishes consistently reduced zooplankton biomass and increased phytoplankton biomass, but the strength of this trophic cascade was significantly weaker in the presence of omnivorous fishes. Fish trophic guilds did not affect differently the probability of cascade occurrence, but they have distinct interactive effects with trophic state and fish biomass on cascade strength. Furthermore, omnivorous and zooplanktivorous fishes did not affect temporal variability of zooplankton biomass, but in contrast with the predictions of theory zooplanktivorous fish stabilized temporal variability of phytoplankton biomass. Cascade strength and biomass variability of phytoplankton were not significantly correlated. These results improve the general understanding about the importance of omnivorous fish on aquatic food web structure and dynamics. If in one hand the higher connectance among trophic levels promoted by omnivores modifies cascade strength, in another hand it does not affected food web stability.