| Description |
Introduced parasites threaten native host populations in many parts of the world. For example, the introduced parasitic nest fly Philornis downsi has been implicated in the decline of Darwin's finch populations in the Galápagos Islands. Studies have just begun to rigorously test the question of why such parasites are successful, and how they devastate naïve host populations. In this dissertation, I examine the effects of P. downsi flies on two host species in the Galápagos; I explore the underlying mechanisms by which hosts are affected. Using an experimental manipulation of P. downsi, I demonstrate that P. downsi reduces nestling survival of medium ground finches (Geospiza fortis), but not Galápagos mockingbirds (Mimus parvulus) at the same location. This suggests that mockingbirds are tolerant to the effects of P. downsi. Mockingbird nestlings appear to compensate for parasite damage by increasing their energy intake through increased begging and parental provisioning. Mockingbirds also appear to differentially express more genes in response to parasitism than finches. However, mockingbirds and finches express very few of the same genes in response to parasitism, suggesting that P. downsi has different effects on gene expression in these hosts. I also investigated the effects of native parasitic flies Philornis trinitensis on hosts in Tobago to understand why P. downsi is successful in the Galápagos. I show that the effect of P. trinitensis on Tobago hosts is similar to the effect of P. downsi on Galápagos hosts. However, the prevalence of P. trinitensis in Tobago is lower than that in the Galápagos, which may be because there are more enemies of the flies in Tobago compared to the Galápagos. Thus, introducing native predators of P. downsi to the Galápagos may be a promising approach to reducing the effect of P. downsi on finches. Until such predators are identified, I present evidence that self-fumigation by finches can be used as an effective stopgap approach to control P. downsi. My work demonstrates that mockingbird reservoir hosts of P. downsi change their behavior to tolerate parasite damage. In turn, P. downsi can persist in the environment, regardless of future declines of vulnerable finch hosts, unless control methods are implemented. |
