An experimental test of antagonistic effects of competition and parasitism on host performance in semi‐natural mesocosms

Abstract

The mechanisms by which parasites can mediate the interactions between species have received increased interest in recent years. Nonetheless, most research has focused on the role of shared parasites as mediators of interspecific competition. Here, we explore the relative effects of Gyrodactylus specialist ectoparasites of Trinidadian guppies Poecilia reticulata on competition between their host and juveniles of the killifish Rivulus hartii. In mesocosms that replicate natural streams, we exposed guppies to only competitors, to only parasites, to both parasites and competitors, or the absence of both. Consistent with previous studies, we found that female guppies grew significantly less where only Gyrodactylus were present, and this was regardless of infection status or parasite load. Surprisingly, this effect of Gyrodactylus on the growth of female guppies was greatly reduced when both parasites and competitors were present in the mesocosms. We conclude that guppies can mediate the effects of Gyrodactylus on competition with Rivulus, by adaptively fine‐tuning their phenotype when simultaneously facing multiple enemies.