Abstract
The invasive parasitic fly, Philornis downsi (Muscidae), is one of the greatest threats to the avifauna of the Galapagos Islands. The larvae of this fly feed on the blood and tissues of developing nestlings of at least 18 endemic and native birds. The aim of the current study was to investigate biotic and abiotic factors that may influence the population dynamics of this invasive parasite. To study the influence of vegetation zone and related climatic factors on fly numbers, a bi-weekly monitoring program using papaya-baited traps was carried out at a dry, lowland site and at a humid, highland site on Santa Cruz Island between 2012–2014. Female flies, a large proportion of which were inseminated and gravid, were collected throughout the year at both sites, indicating females were active during and between the bird breeding seasons. This is the first evidence that female flies are able to persist even when hosts are scarce. On the other hand, catch rates of male flies declined between bird breeding seasons. Overall, catch rates of P. downsi were higher in the drier, lowland habitat, which may be a consequence of host or resource availability. Time was a stronger predictor of adult fly numbers than climate, further suggesting that P. downsi does not appear to be limited by its environment, but rather by host availability. Seasonal catch rates suggested that populations in both habitats were continuous and multivoltine. Numbers of adult female flies appeared to be regulated chiefly by simple direct density dependence, and may be governed by availability of bird nests with nestlings. Nevertheless, confounding factors such as the existence of reservoir hosts that perpetuate fly populations and changes in behavior of P. downsi may increase the vulnerability of bird hosts that are already IUCN red-listed or in decline.
Highlights
Avian parasite invasions have been responsible for many of the observed declines or extinctions in avian biodiversity worldwide [1,2]
Highland rainy seasons coincided with rainy seasons at the lowland site, but more rain fell during the June–December cool season at the highland site than at the lowland site (Fig 1)
Our study suggests that P. downsi populations are density-dependent
Summary
Avian parasite invasions have been responsible for many of the observed declines or extinctions in avian biodiversity worldwide [1,2]. Endemic avian species on oceanic archipelagos that are already threatened by habitat reduction and introduced predators are especially at risk from new parasite incursions [1,3,4]. Such is the case in the Galapagos Islands where the recent invasion of an avian nest parasitic fly, Philornis downsi (Diptera: Muscidae), has seriously affected the stability of indigenous Galapagos passerine populations [5,6,7]. The aim of the current study was to close gaps in knowledge about the biotic and abiotic factors that influence the population dynamics of this invasive parasite
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