Weed management increases the detrimental effect of an invasive parasite on arboreal Darwin's finches

The detrimental effects of invasive parasites on hosts often increase under poor environmental conditions. Both natural fluctuations in environmental conditions and habitat management measures can temporarily cause adverse environmental effects. In this study, we investigated the interaction between the invasive parasitic fly Philornis downsi, control of invasive plants and precipitation on the breeding success of Darwin's finches. Introduced plant species have invaded a unique forest on the Galapagos island of Santa Cruz, which is a key habitat for Darwin's finches. The Galapagos National Park Directorate applies manual control and herbicides to combat this invasion. We hypothesized that these measures led to a reduction in the arthropod food supply during chick rearing, which in turn caused mortality in chicks that were already weakened by parasitism. We compared food availability in three study sites of varying degrees of weed management. To assess the interaction of parasitism and weed management, we experimentally reduced P. downsi abundance in nests in all three study sites. The key result suggests that there is an increase in the adverse effect of parasitism under unfavourable habitat conditions. As predicted, we found a negative additive effect of parasitism and weed management on the breeding success of the insectivorous warbler finch (Certhidea olivacea), but not on the omnivorous small tree finch (Camarhynchus parvulus), which was strongly affected by parasitism independently of weed management. Management measures can temporarily decrease environmental quality and reduce resistance/tolerance to invasive species. This should be considered when applying management measures to habitats of vulnerable species.