Abstract
Invasive alien parasites and pathogens are a growing threat to biodiversity worldwide, which can contribute to the extinction of endemic species. On the Galápagos Islands, the invasive parasitic fly Philornis downsi poses a major threat to the endemic avifauna. Here, we investigated the influence of this parasite on the breeding success of two Darwin's finch species, the warbler finch (Certhidea olivacea) and the sympatric small tree finch (Camarhynchus parvulus), on Santa Cruz Island in 2010 and 2012. While the population of the small tree finch appeared to be stable, the warbler finch has experienced a dramatic decline in population size on Santa Cruz Island since 1997. We aimed to identify whether warbler finches are particularly vulnerable during different stages of the breeding cycle. Contrary to our prediction, breeding success was lower in the small tree finch than in the warbler finch. In both species P. downsi had a strong negative impact on breeding success and our data suggest that heavy rain events also lowered the fledging success. On the one hand parents might be less efficient in compensating their chicks' energy loss due to parasitism as they might be less efficient in foraging on days of heavy rain. On the other hand, intense rainfalls might lead to increased humidity and more rapid cooling of the nests. In the case of the warbler finch we found that the control of invasive plant species with herbicides had a significant additive negative impact on the breeding success. It is very likely that the availability of insects (i.e. food abundance)is lower in such controlled areas, as herbicide usage led to the removal of the entire understory. Predation seems to be a minor factor in brood loss.
Highlights
Invasive alien pathogens and parasites are a major and growing threat to biodiversity worldwide
On 11 ha of our 30 ha study site, the National Park applied strong herbicides to control the invasion of R. niveus, which results in the removal of the whole forest understory
In 2012, we found that the only factor to negatively influence breeding success significantly in both species was the percentage of heavy rain days during the nestling period
Summary
Invasive alien pathogens and parasites are a major and growing threat to biodiversity worldwide. Small host populations of endemic species are vulnerable because extinction can occur before the hosts have a chance to evolve effective defences [1,2]. This is especially true if the parasite is a generalist because it can switch to another host if it has driven one close to extinction [3]. The avifauna of almost all islands of the Pacific Ocean has been dramatically altered by introduced species or other human impact [5] In this respect the Galapagos archipelago is one of the exceptions, as no bird species has become extinct in modern times. The introduction of predators, pathogens, and parasites have led to increasing pressures affecting several Darwin’s finch species [6,7,8,9,10]
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