Inducible trophic polymorphisms (ITPs) are striking examples of phenotypic plasticity that allow organisms to respond dynamically to environmental conditions by changing the trophic level upon which they feed. In the case of Tetrahymena vorax, pyriform bacterivorous microstomes can transform into larger ovoid carnivorous macrostomes in the presence of intraguild prey. We tested the hypothesis that feeding on prey of intermediate size may enable macrostomes to grow sufficiently large as to feed on competitors that would otherwise be too large to serve viablely as intraguild prey.

Results/Conclusions

Our results confirmed this hypothesis. Mean cell diameter in macrostomes showed a significant increase in the presence of intermediate-sized prey. While macrostomes that had been allowed to feed on intermediate-sized prey were able to engulf the larger prey, macrostomes that had been cultured in the absence of intermediate-sized prey quickly reverted to microstomes in the presence of the larger prey. Continued monitoring of the populations over the course of 10 days revealed, however, that the expanded diet breadth that T. vorax obtained in the presence of intermediate-sized prey resulted in an unexpected decrease in fitness. In all replicates in which T. vorax had been exposed to larger prey after having been allowed to feed on intermediate-sized prey, T. vorax was competitively excluded by the larger prey. In all replicates in which T. vorax had been cultured in the absence of intermediate-sized prey, T. vorax competitively excluded the larger prey. These results show that mixed intraguild predation interactions can result in surprising outcomes that depend on the recent history of community assembly.