Trait-mediated apparent competition in an intraguild predator-prey system

Apparent competition is of broad interest due to its effects on community structure and potential applications in agriculture, restoration, and medicine. It is well-established that apparent competition can result from changes in predator abundance or behavior caused by interactions with alternate prey, but no previous empirical study has demonstrated that apparent competition can also result from prey-induced changes in predator morphology. This trait-mediated alternative mechanism of apparent competition would expand the range of conditions under which apparent competition might occur in nature and identify new ways to generate or modify apparent competition in applied contexts. Here, as a proof of concept, we show that trait-mediated apparent competition can arise from inducible offenses and show how it operates using experiments involving three ciliates. When it feeds on Colpidium kleini, the intraguild predator Tetrahymena vorax increases in size to the extent that it can then consume Paramecium aurelia, an even larger prey. When feeding only on bacteria, however, Tetrahymena remains smaller and is unable to consume Paramecium. This trait-mediated indirect effect leads to the predatory exclusion of Paramecium, while Tetrahymena and Colpidium coexist. Developmental expansions such as those underlying the interactions observed in our study are not limited to ciliates, such as Tetrahymena, but occur among many diverse taxa and may have a surprising degree of influence over the structure and dynamics of food webs.