Invasive Parasites, Habitat Change and Heavy Rainfall Reduce Breeding Success in Darwin's Finches

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.

The impact of invasive plant management on the foraging ecology of the Warbler Finch (Certhidea olivacea) and the Small Tree Finch (Camarhynchus parvulus) on Galápagos

“…However, our data also point towards a negative influence of the habitat change (Cimadom et al 2014). On Santa Cruz Island, the Scalesia forest had been reduced to only 1% of its original distribution by previous agricultural activity (Mauchamp and Atkinson 2009), and more recently has been invaded by different introduced plant species (Renteria et al 2012).…”

“…In some areas of the Scalesia zone, the Galá-pagos National Park Directorate (GNPD) manually controls R. niveus with machetes and subsequently applies herbicides onto the regrowth to assist in the natural regeneration of the forest. The manual control caused a temporary removal of the understory, and we found that the breeding success of Warbler Finches was significantly lower in areas in which R. niveus had been recently controlled (Cimadom et al 2014). Since arthropods use plants for refuge and/or for food, the removal of the understory may reduce their abundance (Boada 2008).…”

“…From 1997 to 2010, its population on Santa Cruz Island decreased by 45% in the humid native forest (Scalesia forest) and by 85% in the agricultural zone. The closely related sympatric Small Tree Finch (Camarhynchus parvulus) has shown a population decline since 2005 (Cimadom et al 2014). We investigated the breeding success of both species in the Scalesia forest, which is the area with the highest density of arboreal Darwin's Finches, and found that both species suffered from high brood loss due to the invasive parasitic fly Philornis downsi (Cimadom et al 2014).…”

“…The closely related sympatric Small Tree Finch (Camarhynchus parvulus) has shown a population decline since 2005 (Cimadom et al 2014). We investigated the breeding success of both species in the Scalesia forest, which is the area with the highest density of arboreal Darwin's Finches, and found that both species suffered from high brood loss due to the invasive parasitic fly Philornis downsi (Cimadom et al 2014). The larval stage of this obligate bird parasite develops in birds' nests and sucks blood from the nestlings (Dudaniec and Kleindorfer 2006;Fessl et al 2006;O'Connor et al 2010a, b).…”

“…Like most small passerines, Darwin's Finches depend on insects for chick rearing (Betts 1955;Stutchbury and Morton 2001). Therefore a lower insect abundance or availability might contribute to the lowered breeding success of the Warbler Finch in blackberry control areas (Cimadom et al 2014). We hypothesized that habitat changes, due to both invasion by R. niveus and the control measures used, reduce the availability of arthropods and change their micro-spatial distribution (e.g.…”