An introduced parasitic fly may lead to local extinction of Darwin's finch populations

Introduced pathogens and other parasites are often implicated in host population-level declines and extinctions. However, such claims are rarely supported by rigorous real-time data. Indeed, the threat of introduced parasites often goes unnoticed until after host populations have declined severely. The recent introduction of the parasitic nest fly, Philornis downsi, to the Galápagos Islands provides an opportunity to monitor the current impact of an invasive parasite on endemic land bird populations, including Darwin's finches. In this paper, we present a population viability model to explore the potential long-term effect of P. downsi on Darwin's finch populations. The goal of our study was to determine whether P. downsi has the potential to drive host populations to extinction and whether management efforts are likely to be effective. Our model is based on data from five years of experimental field work documenting the effect of P. downsi on the reproductive success of medium ground...