17 December 2015 (University of Utah) – Mathematical simulations at the University of Utah show parasitic flies may spell extinction for Darwin’s finches in the Galapagos Islands, but that pest-control efforts might save the birds that helped inspire the theory of evolution.
The new study “shows that the fly has the potential to drive populations of the most common species of Darwin’s finch to extinction in several decades,” says biology professor Dale Clayton, senior author of the study published online Dec. 18 in the Journal of Applied Ecology.
But the research “is not all doom and gloom,” he adds. “Our mathematical model also shows that a modest reduction in the prevalence of the fly – through human intervention and management – would alleviate the extinction risk.”
Several approaches may be needed, such as introducing fly-parasitizing wasps, removing chicks from nests for hand-rearing, raising sterile male flies to mate with females so they can’t lay eggs in finch nests, and using insecticides, including placing pesticide-treated cotton balls where birds can collect them to self-fumigate their nests.
Pigeons were more important to Charles Darwin’s theory of evolution than finches, partly because he failed to label finches he collected in the Galapagos to denote the islands where he collected the birds. Nevertheless, Darwin observed how different Galapagos finch species evolved varying beak and body sizes.
“Darwin’s finches are one of the best examples we have of speciation,” says the new study’s first author, Jennifer Koop, who did the research as a University of Utah doctoral student and now is an assistant professor of biology at the University of Massachusetts Dartmouth. “They were important to Darwin because they helped him develop his theory of evolution by natural selection.”
Darwin’s finches live only in the Galapagos Islands, off the coast of mainland Ecuador. The finches began as one species and started evolving into separate species an estimated 3 million to 5 million years ago.
The new study dealt with medium ground finches, Geospiza fortis, among the most common of at least 14 species and perhaps 18 species of Darwin’s finches. One of them, the mangrove finch, already “is facing potential total extinction because it is present in only two populations on a single island, Isabela,” Koop says.
Clayton says that if the parasitic nest fly, Philornis downsi, “can lead to extinction of such a common species, then the less common species – which have the same fly problem – are likely at risk as well.”
In addition to the medium ground finch, other abundant species of Darwin’s finches are the small ground finch, cactus finch and small tree finch.
The case of the flies and finches exemplifies how “introduced pathogens and other parasites pose a major threat to global diversity,” especially on islands, which tend to have smaller habitat sizes and lower genetic diversity, the researchers write. [more]
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 finch Geospiza fortis populations on Santa Cruz Island. Under two of the three scenarios tested, the model predicted medium ground finches are at risk of extinction within the next century.
However, sensitivity analyses reveal that even a modest reduction in the prevalence of the parasite could improve the stability of finch populations. We discuss the practicality of several management options aimed at achieving this goal.
Synthesis and applications. Our study demonstrates the predicted high risk of local extinction of an abundant host species, the medium ground finch G. fortis due to an introduced parasite, P. downsi. However, our study further suggests that careful management practices aimed at reducing parasite prevalence have the potential to significantly lower the risk of host species extinction.