 Abstract | Endemic avian populations on islands may experience increased risks associated with introduced pathogens. This study examines the impact and molecular ecology of an invasive fly (Philornis downsi), which is a haematophagous ectoparasite of nestling birds on the Galapagos Islands, Ecuador. This parasite causes extreme mortality and fitness costs in Darwin's finches and threatens vulnerable and declining finch species across the archipelago.
This study may be divided into two complementary parts; (1) ecological factors affecting the impact of P. downsi on its avian hosts (chapters 2-4); and (2) molecular ecological insights into the genetic structure and reproductive behaviour of P. downsi (chapters 5-7). With six years of data across six finch species, P. downsi intensity was found to be higher in years with increased rainfall, and finch species with high adult body mass had more parasites in their nests. The percentage of nests with mortality was between 40 % and 100 % for all six host species. Darwin's small tree finches that nested in mixed species aggregations had increased P. downsi intensity, and larger nests had more parasites. Evidence is therefore presented for parasite-mediated selection pressures on nesting behaviour and nest characteristics that interact with climate, habitat and host species. Using nine novel microsatellite markers for P. downsi, gene flow and dispersal was examined across two climatically contrasting habitats and three islands of the Galapagos. Low genetic differentiation across habitats and islands indicated high dispersal in P. downsi, though evidence for population genetic bottlenecks and fine-scale genetic structure within islands was observed. Genetic analyses of P. downsi broods within nests revealed a high frequency of multiple mating in female flies, and an almost ubiquitous occurrence of multiple infestations within nests. Patterns of host distribution, parasite intensity, and genetic relatedness of P. downsi broods across habitats on Floreana Island provided evidence for host density-dependent oviposition behaviour in female flies.
The scope and approach of this study is unmatched by previous investigations of dipteran ectoparasites of birds, and represents a seminal contribution to the fields of avian parasitology and invasive species biology. The results are particularly applicable to the conservation management of the Galapagos avifauna, and future efforts to control and eventually eradicate the severe threat of P. downsi to endemic island populations.
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