Parasites can have important effects on the structure and composition of natural biological communities, either directly by influencing host fecundity and survival (i.e., density-mediated effects) or indirectly by influencing host traits such as behavior, life-history, morphology, and physiology (i.e., trait-mediated effects). Yet few studies have explored how these effects play out in the wild, as opposed to simplified and controlled laboratory or mesocosm settings. We addressed this information gap by translocating Gyrodactylus ectoparasites of the Trinidadian guppy into previously Gyrodactylus-free guppy populations in two rivers (Marianne and Paria) in the Northern Mountain Range in Trinidad. We then measured phenotypic and demographic changes in the guppy host and its competitor, Rivulus hartii, and compared these changes to guppies and Rivulus in control reaches just upstream. In the Marianne, where guppies invest more in reproduction (i.e., greater size and number of embryos), the introduction of Gyrodactylus decreased guppy survival but did not influence guppy density or phenotypes. In the Paria, where guppies invest less in reproduction, the introduction of Gyrodactylus reduced female growth but did not influence guppy survival or density. In neither river did the introduction of Gyrodactylus influence the phenotype or demography of Rivulus. These results indicate some density-mediated and trait-mediated effects of parasites on hosts in natural setting, but also that these effects were context-specific, were generally weak, and did not cascade to a competitor. Give these outcomes, and their difference from typical lab-based studies, it is clear that more studies are needed that experimentally manipulate parasites in natural settings.

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