Will black rats benefit from cat control on Christmas Island with negative consequences for endemic birds?

(Previous title: The ecological impacts of invasive species and invasive species control on Christmas Island)

A collaborative project undertaken by Rosalie Willacy, NESP and Parks Australia, and supervised by Drs Sarah Legge, Eve McDonald-Madden and Michaela Plein

(Previous title: The ecological impacts of invasive species and invasive species control on Christmas Island)



Christmas Island is a unique environment, home to a suite of endemic fauna and internationally significant seabird colonies. Feral cats, black rats and other invasive species, have had a devastating impact on the ecosystem since their introduction over 100 years ago. They have contributed to the extinction of at least 7 species and continue to impact on remaining native, nationally threatened species, including the endemic Christmas Island flying fox, several endemic forest birds, and some seabird species. To mitigate against potential threats posed by cats, Parks Australia (in conjunction with project partners, the Department of Biodiversity, Conservation and Attractions, Western Australia) is undertaking a cat eradication program. Studies on other islands and theoretic modelling of multi-species interactions on Christmas Island have shown, however, that cat control could result in increased rat abundance, with perverse outcomes for native species. Predicting whether cat control could result in negative consequences for black rats and native species on Christmas Island is difficult, due to existing knowledge gaps on the nature of interactions between feral cats and black rats and the impact of black rats (which may be unusual due to the unique ecology of Christmas Island, considering the role of land crabs and now extinct endemic rats). The focus of this PhD is to fill these knowledge gaps and inform decision making and management for the Christmas Island ecosystem, to ensure that the cat eradication program delivers the maximum possible conservation benefit for threatened Christmas Island species.
The overarching aims of this research are to determine

  • whether cat control is likely to lead to increases in rat abundance on Christmas Island; and if so,                          
  • whether that rat increase could cause increased negative impacts to native species.                                                               

Specifically, our project aims to:

  • Determine the distribution and relative abundance of invasive black rats across Christmas Island;
  • Determine the absolute density of black rats in areas of high and low activity and calibrate these to relative abundance estimates;
  • Determine the relative impacts of black rats and cats on the breeding success of a suite of Christmas Island bird species including the Christmas Island white- eye (Zozopteris natalis), Christmas Island Thrush (Turdus poliocephalus erythropleurus), Christmas Island Emerald dove (Chalcophaps indica natalis), Red-tailed Tropicbird (Phaethon rubricauda) and Brown Booby (Sula leucogaster);
  • Model the impact of reduced cat densities on rats and native birds.            

The research will fill important knowledge gaps that are currently limiting our ability to predict the outcomes of cat control for the invasive black rat population and native avifauna in the Christmas Island ecosystem and monitor changes in the ecosystem over time.


 

Figure 1. Rosalie Willacy checking a motion sensor camera on a red-tailed tropic bird nest

Figure 2. Red-tailed tropic bird chick

Figure 3. Island thrush nest and eggs

Figure 4. Island thrush chicks

Figure 5. Island thrush juvenile