Competition among invasive and endemic carrion fly species in the Galapagos Islands with implications for biological control risk assessment

1. The composition of the necrobiome community in the Galapagos Islands is poorly understood, and nothing is known about the dynamics between endemic species and those introduced through human activity.2. To determine the composition of the carrion fly community, specifically members of the families Muscidae, Calliphoridae and Sarcophagidae, we deployed four kinds of carrion bait traps during the cool and hot seasons at two lowland and two highland sites on Santa Cruz Island within the Galapagos archipelago. We also conducted a laboratory experiment to assess resource competition between fly species encountered in the baiting study.3. Of the eight fly species found in our baited traps, all were introduced except for the endemic sarcophagid, Sarothromyiops dasycnemis. Four endemic and one native carrion-feeding species that had been previously recorded on this island were not found. The introduced sarcophagid, Peckia chrysostoma, was the most abundant fly species, comprising over half of the collected specimens, and it was highly dominant at the lowland sites. The endemic species, S. dasycnemis, was only recorded at the lowland sites during the hot season. On the other hand, the calliphorid species were dominant at the highland sites.4. Experiments demonstrated that P. chrysostoma is a strong competitor against other carrion fly species in the Galapagos necrobiome, including the endemic S. dasycnemis.5. A comparison of our data with historical records, combined with the results of our laboratory study, leads to the conclusion that introduced carrion fly species, such as P. chrysostoma, represent a threat to endemic carrion fly species, such as S. dasycnemis.6. Three parasitoid species were reared from 19% of the collected fly puparia. Two of these species attacked fly larvae (Brachymeria podagrica and Aphaereta sp.), whereas one species attacked fly puparia (Exoristobia sp.)7. We discuss our results in light of the possibility of the purposeful introduction of a parasitoid as a biological control agent against the avian vampire fly (Philornis downsi; Diptera: Muscidae) in Galapagos.