Apparent competition is a feature of many natural systems, but complex combinations of direct and indirect interactions can often obscure the mechanisms producing strongly asymmetric outcomes which result between prey. Microcosm experiments offer a means of disentangling how direct and indirect effects can complicate the dynamics of apparent competition even when using relatively simple community modules. The role of positive effects, such as facilitation among prey species, has only recently become incorporated into the framework of apparent competition. The role of both facilitation and apparent competition in contributing to community structure and biodiversity makes determining how these two processes influence one another crucial.
We used a factorial experiment to explore the complexities that can arise in a simple community module consisting of two non-competing prey, the green alga Chlamydomonas reinhardtii and the ciliate Colpidium kleini, and their shared predator the ciliate Euplotes eurystomus. We monitored species abundance, habitat modification by species, and plastic defense phenotypes expressed by Chlamydomonas to interpret the outcome observed through the lens of the interactions of all three species.
Results/Conclusions
Apparent competition between the two prey species was strongly asymmetric, with Chlamydomonas emerging as the superior apparent competitor. Our experimental design allowed us to isolate the factors contributing to this asymmetric outcome. Facilitation of one prey Chlamydomonas, by the other prey, Colpidium, enhanced the negative effect of the predator on the facilitating prey. Chlamydomonas received an additional advantage through an inducible colonial defense phenotype, which promoted its persistence with the predator. The facilitation of Chlamydomonas by Colpidium was associated with reductions in bacterial abundance in all treatments containing Colpidium, suggesting that the mechanism of facilitation involved the moderation of negative interactions between the alga and bacteria.
These results show that even in relatively simple community modules studied in highly controlled laboratory settings, unexpected interactions can emerge that complicate the interpretation of apparent competition’s role in community dynamics. The facilitation of one prey by another as seen in our experiment provides a possible explanation for the asymmetric outcome of apparent competition, which may be further augmented by differences in prey defenses. These complexities merit inclusion in simple models of apparent competition.