Relationships among introduced parasites, host defenses, and gut microbiota of Galapagos birds

Parasites can cause significant damage to their hosts. Hosts can reduce the damage that parasites cause by either resisting parasites (e.g., with an immune response) or tolerating parasites (e.g., with energy compensation). Recent work suggests that host‐associated microbiota (bacteria) affect parasites, and vice versa, which can be mediated by host defense mechanisms. Although there are many possible interactions among parasites, host defenses, and host‐associated microbiota, few studies have explored these interactions in non‐model systems. Such studies are important for understanding disease risk, especially when considering emerging diseases. The parasitic nest fly Philornis downsi , which was introduced to the Galapagos Islands, causes considerable damage to Darwin's finches. In contrast, P. downsi has little effect on tolerant hosts, such as Galapagos mockingbirds, which increase nestling provisioning to compensate for energy lost to the parasite. Although host defenses against P. downsi have been relatively well‐studied, the importance of host‐associated microbiota in host– P. downsi interactions is unknown. In this study, an experimental manipulation of P. downsi was used to test the effect of parasitism on the fecal bacterial community (proxy for the gut) of Galapagos mockingbird and Darwin's finch nestlings. The relationships between the bacterial community and nestling provisioning or the antibody response were also determined. Galapagos mockingbirds and Darwin's finches harbor different gut bacterial communities. P. downsi decreased bacterial diversity of mockingbird nestlings, presumably in response to increased provisioning, which was negatively correlated with bacterial diversity. Lower bacterial diversity was related to lower antibody levels in mockingbirds. In contrast, P. downsi did not affect bacterial communities of finch nestlings, but bacterial diversity was negatively related to antibody levels. Overall, this study demonstrates that introduced parasites can affect host‐associated bacterial communities and that host defenses can correlate with changes in the gut microbiota.