How to Protect an Ecosystem on Life-support from Destructive Invaders? Understanding the Spatial Ecology of an Invasive Predator, the Domestic Cat, to Restore the Unique Ecosystems on Norfolk Island

Author: Andrew Tilley – Bachelor of Environmental Science and Management (Honours)

The ecosystems present on Norfolk Island evolved under unique conditions. The island is geographically isolated, and while it is an Australian territory, it is located closest to New Caledonia (767 km to the north) and is part of the mostly submerged Zealandia continental mass. This has enabled the speciation of endemic flora and fauna. Living under the shadow of the towering Norfolk Pine, iconic fauna such as the Norfolk Island green parrot, Morepork owl, and Norfolk robin evolved in this insular ecosystem, naïve to predation by mammals. Norfolk Island National Park protects 460 hectares of the habitat of these species on the island.

The Polynesian rat was introduced by early Polynesian visitors, followed by the introduction of the black rat, house mouse and domestic cat by European settlers. These phases of introduction have led to the explosive proliferation of invasive species populations throughout the entire island, including the national park. These invaders have decimated fauna populations and have led to the extinction of three endemic bird species and four subspecies, three endemic snail species and the local extinction of two reptile species.

In the national park, rodent species have been a focus of management by the Parks Australia team due to their high populations and rapid reproduction rates continually posing an existential threat to birds and their nests in the national park as well as invertebrate and flora species. Baiting and other lethal controls occur intensively across the park but largely limit rodent populations rather than drive a net reduction in their population. Baiting occurs on a monthly rotational basis within six sectors or ‘baiting catchments’.  While populations of feral cats are known to prey on native bird species, their role in limiting rodent populations on Norfolk Island is less understood. By modelling the spatial ecology of feral cats in the national park, we may be able to understand how feral cats respond to changes in the distribution of the rodent population in response to periods of baiting and significant seasonal variations in rainfall. Such information may inform the management of invasive rodents as well as the feral cats themselves.

My honours project aims to utilise a gridded array of 51 camera traps, deployed since July 2024 across the national park to model the occupancies of rodents and cats and population density of feral cats in the national park. I am aiming to produce models for each month of the camera trap deployment to examine the impact of monthly baiting rotations and seasonal rainfall variations. By comparing the occupancies of rodents and cats in the national park, I may be able to develop an understanding of the interspecific dependence of feral cats on rodents, providing an insight into the nature of the predator-prey relationship in the national park. Additionally, the development of an accurate population density model is likely to contribute toward a more detailed understanding of the spatial ecology of feral cats in the park, which may inform their effective management.  My results may require the consideration of the top-down or bottom-up population control factors in future invasive species management measures. Additionally, an accurate population density model may inform the targeted management of feral cats in Norfolk Island National Park.

I have just concluded my field trip to Norfolk Island to collect all 51 camera traps. All images have been downloaded and backed up, and I am utilising the artificial intelligence model integrated within Wildlife Insights (www.wildlifeinsights.org), developed by World Wildlife Fund and Google to assist in the identification of species and in filtering out blank photos. During this process, I have sighted several photos of cats preying on rats and have produced rare photographic evidence of feral cats preying on seabirds on Norfolk Island. In these images, a white tern and a black-winged petrel have very clearly been predated on by a feral cat. The camera trap which captured these images was located to the north of the island, where there is relatively high seabird activity. These images reinforce the need for the effective management of feral cats in the national park, to protect the seabird populations on the island, but also for the responsible ownership of cats as pets, as domesticated free-roaming cats can also endanger native bird species. The unique ecosystems on Norfolk Island are incredibly important breeding grounds for seabirds in the Pacific. It is therefore troubling to find evidence of their predation by feral cats.

These images reinforce the necessity for developing a greater understanding of the population dynamics of rodents and feral cats in Norfolk Island National Park so that the unique flora and fauna that remains can continue to exist on Norfolk Island.

This project was funded by the Commonwealth government through the National Environmental Science Program’s Resilient Landscapes Hub.