Invasive parasitic flies may cause extinction of Darwin’s finches

Welcome to the middle of a post series about parasite-induced host extinction! Today I’m blogging about one particular example of a parasite that might drive a host species extinct, and next week I’ll move on to the more conceptual/review-type stuff. So, if you haven’t yet, take a guess at this question to help me prepare for next week: In our current mass extinction, what percentage of species extinctions do you think have been caused (extinct species) or nearly-caused (endangered species) by parasites, pathogens, or viruses? Round your answer to the nearest 10%.

Around 1997, a parasitic nest fly (Philornis downsi) was found in the nests of several species of Galapagos bird species. The fly larvae feed on the blood of nestlings, and this can have profound effects on nestling survival for some bird species. In some years, the fly larvae can even reduce the probably of nestling survival to zero! So, Koop et al. (2016) created a population projection model to explore what impacts the introduced nest fly may have on the long-term survival of medium ground finches on Santa Cruz.

Koop et al. (2016) parameterized their model using five years of data regarding nestling survival in the presence and absence of parasitic nest flies. This was complicated by the fact that the impact of parasitic nest flies on nestling survival varied by year. So Koop et al. (2016) treated the “fly effect” stochastically, where each year of their simulations was given a “fly effect size” from one of the five observed years, and observed years were either weighted evenly or weighted towards “good” (small fly effect) or “bad” (big fly effect) years. Here’s the bad news: when weighting evenly or towards bad years, simulated medium ground finch populations went extinct within a century.


The good news is that the probability of long-term finch population viability could be greatly increased by reducing the proportion of nests that have the parasitic nest fly. At this point, it isn’t clear what methods would be best for reducing the proportion of nests that have the parasitic nest fly, but Koop et al. (2016) talk about some possible management activities. My favorite one was providing a source of permethrin-treated cotton that finch parents could incorporate into their nests.


Koop, J.A.H., P.S. Kim, S.A. Knutie, F. Adler, and D.H. Clayton. 2016. An introduced parasitic fly may lead to local extinction of Darwin’s finch populations. Journal of Applied Ecology 53: 511–518.

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