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20 April 2023 Investigating the impact of black rats (Rattus rattus) on the endemic and threatened avifauna of Christmas Island
Rosalie Willacy, Samantha Flakus, Eve McDonald-Madden, Sarah Legge
Author Affiliations +

Context. Invasive mammalian predators are a primary cause of extinctions, especially on islands. Baseline data on the impact of invasive mammalian predators are critical for deciding whether their eradication or control should be attempted, and for monitoring ecosystem changes over time if control does occur. Christmas Island is a tropical island with multiple invasive species, including two mammalian predators (black rats, Rattus rattus and feral cats, Felis catus). Black rats are known to prey upon island bird species (including their nests) elsewhere, but the severity of their impacts on Christmas Island birds is unclear. Describing these impacts would help managers decide whether rat control is warranted, especially as a cat eradication program underway on the island could conceivably result in increased rat density.

Aims. In this study, we quantify the impacts of rats on Christmas Island bird abundance and nesting success.

Methods. The abundance of four endemic forest bird species/subspecies (Christmas Island imperial pigeon, Ducula whartoni, emerald dove, Chalcophaps indica natalis, thrush, Turdus poliocephalus erythropleurus and white-eye, Zosterops natalis) was measured using transect surveys. Nest success was measured using remote sensing camera surveillance for one forest bird species (thrush) and one seabird species (red-tailed tropicbird, Phaethon rubicaudra westralis). We explored whether these measures were related to spatial variation in rat density and activity (measured by trapping and inkcards), the presence of other invasive species, habitat and seasonal variables.

Key results. Neither rat density nor activity explained the abundance of any forest bird species. Instead, seasonal and habitat features were more consistent predictors of bird abundance. White-eyes were more abundant near yellow crazy ant (Anoplolepis gracilipes) supercolonies. Nest success for thrushes and red-tailed tropicbirds was either not, or only very weakly, influenced by rats.

Conclusions. Black rats currently have little effect on the population sizes and nesting success of Christmas Island birds.

Implications.This study suggests that rat control is not currently a management priority, but ongoing monitoring of rat density, activity, and impacts is needed to ensure management can respond promptly if rat density or impacts change as cat control progresses towards eradication.

Rosalie Willacy, Samantha Flakus, Eve McDonald-Madden, and Sarah Legge "Investigating the impact of black rats (Rattus rattus) on the endemic and threatened avifauna of Christmas Island," Wildlife Research 51(1), (20 April 2023).
Received: 11 April 2022; Accepted: 7 March 2023; Published: 20 April 2023
black rat impact
forest birds
invasive species management
island birds
mesopredator release
nest predation
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