We test the hypothesis that the level of omnivory (in this case the relative importance of prey and plant food in an animal's diet) is correlated with the number of prey taxa consumed and the effect of omnivores on prey. We tested this hypothesis using multiple populations of two invasive ants, the red imported fire ant (Solenopsis invicta) and the tawny crazy ant (Nylanderia fulva). We estimated the level of omnivory of each ant population by estimating the trophic position of ants using stable isotopes. Trophic position in these ants typically ranges from 2.5 to 4 and the lower the trophic position the more important plant food is in the ants diet. We estimated the number of prey taxa consumed by conducting Next Generation Sequencing-Molecular Gut Content Analysis on ant larvae (ant larvae serve as the "stomach" of the ant colony and digest all solid food collected by the workers). We estimated the effect of ants on prey populations with a sentinel prey study.
Results/Conclusions
We found a negative relationship between trophic position and the number of prey consumed by fire ants, suggesting that populations of fire ants that mix plant and prey food eat more species of prey than populations of fire ants that are strict predators. There was no relationship between trophic position and the number of prey species consumed in tawny crazy ants. Likewise, trophic position was positively correlated with the predation rate of sentinel prey by red imported fire ants, but not by tawny crazy ants. Our study suggests that diet mixing (mixing plant and prey food) is correlated with the ecological effects of red imported fire ants, but not tawny crazy ants. Species-specific effects can be frustrating when ecologists are attempting to make broad predictions about the role of omnivory in ecological interactions, but understanding species-specific effects is critical.