The impact of an exotic parasite on Darwin´s finches

Invasive alien pathogens and parasites are a growing threat to biodiversity worldwide. Small host populations of endemic species are particularly vulnerable, because extinction can occur before the hosts have a chance to evolve effective defences. Darwins finches are endemic to the Galpagos Islands, a tropical Archipelago where the avifauna has remained remarkably unaltered. To date no recent bird species has gone extinct but that is about to change. Introduced predators, pathogens, and parasites are threatening several Darwins finch species. One of the biggest threats to the Galpagos avifauna is the introduced parasitic fly Philornis downsi. The larvae of this obligate bird parasite develop in the bottom of the nest of small passerines and suck the blood of the nestling. Correlative as well as experimental studies have shown that Philornis downsi has a negative impact on nestling growth, nestling haemoglobin levels and fledgling success causing very high chick mortality in some years. The highest prevalence and intensity of P. downsi infestation in the Galpagos Islands was found on Santa Cruz Island, where finch numbers have decreased continuously over the last 15 years. The most dramatic decline was observed in the warbler finch (Certhidea olivacea). This insectivorous species has a small clutch and body size and is therefore very strongly affected by Philornis parasitism. Additionally its main range of distribution covers the last remnants of humid cloud forest. These remnant Scalesia forest patches have been invaded by introduced trees and shrubs and have dramatically decreased in size during the last 25 years. Our preliminary data suggests that Philornis has a strong negative impact on the breeding success of this species but that two environmental factors, namely extreme climatic conditions such as heavy rain days or drought and control of invasive plant species with herbicides, may have an additive negative effect. We assume that both factors affect food supply during chick rearing which in turn causes mortality in chicks that are already weakened by parasitism. To investigate the interaction and impact of all three factors (Philornis parasitism, habitat change, and unfavourable weather conditions) on breeding success, we plan to conduct two experiments. In the first experiment we will manipulate Philornis intensities in the finch nests, which will enable us to quantify the impact of parasitism. In a second experiment we will measure the effect of habitat change on food availability, prey selection and breeding success by comparing four different habitat types that are affected by invasive species or control of invasive species to varying degrees We will integrate classical Behavioural Biology, Insect Ecology and Conservation Biology with an Ecosystem Research approach to understand how an introduced parasite affects native populations, especially if they already have to cope extreme environmental conditions and anthropogenic habitat change.

One of the biggest threats to the famous Darwin's finches is the introduced parasitic fly Philornis downsi who's larvae feed on the blood of the nestlings. Additionally, habitat change caused by invasive plant species is affecting their breeding success. The Scalesia forest, representing the area with the highest density of arboreal Darwin's finches, has been invaded by different invasive plant species. In some areas of the Scalesia forest the Galpagos National Park applies herbicides to control the invasive plants. These measures leads to a temporal removal of the whole understory, which may affect food abundance for birds. In our project we tested whether additional stressors such heavy rain and habitat changes increase the detrimental effects of P.downsi on the breeding success of the Small Tree Finch and the Warbler finch. We measured the effect of habitat management by comparing plant diversity, insect abundance, and breeding success in three different habitat conditions that are affected by invasive plant species or control of invasive species to varying degrees (1) 'invaded' (2) 'recently controlled' area and (3) 'long-term management' area. The positive effect of long-term management was that it led to a rapid recovery of the Scalesia forest but the removal of the understory led to a temporary reduction of arthropod biomass. In the invaded area, dominance of invasive plants led to dilution of protein in the ecosystem and in turn to low-quality arthropod food. Isotopic evidence revealed that Warbler finches utilized this low-quality resource but seem to suffer from protein deficiency. Our two focal species reacted differently to the reduced food availability. Warbler Finches could compensate for parasitism when arthropod abundance was high, but not when arthropod abundance was reduced due to control of plants . In the Small Tree Finch breeding success was very low in all habitat conditions and only increased when nests were experimentally freed from parasites. The reason for this species differences could lay in a difference in the intensity and timing of infestations due to a recent change in the behaviour of the parasite. Since 2012 we have recorded larvae already in incubating nests where they attack incubating females. During incubation the fly infested Warbler Finch nests more frequently but once chicks hatched Small Tree Finches contained more larvae which could explain the higher brood loss. One reason for the preference of the parasite for Small Tree finch nestlings could be that the larvae develop faster in Small Tree finch nests. However, Darwin's finches have developed counter strategies: We recorded that individuals of four species rubbed leaves of the endemic Guayabillo tree into their feathers. We found that leaf extracts repel both mosquitoes and P. downsi flies. This behaviour can be seen as a form of self-medication.