Rodent project awarded 2021 Ecological Impact Award

The impacts of invasive rodents on biodiversity presents a global challenge, with invasive rodents being a key driver of species decline and extinction.

An innovative project to combat invasive rodent populations on Norfolk Island has been awarded the Ecological Society of Australia’s 2021 Ecological Impact Award.

Predation by invasive rodents such as the black rat (Rattus rattus), pacific rat (R. exulans), and house mouse (Mus musculus) is implicated in the extinction of multiple plant and animal species endemic to Norfolk Island. It is seen as a key threat to the islands 56 remaining threatened species including the Critically Endangered Morepork owl, breeding seabirds and the endemic land snails.

The impacts of invasive rodents on biodiversity presents a global challenge, with invasive rodents being a key driver of species decline and extinction. These threats are especially heightened for islands systems, where species endemism is nine times greater than that of mainland systems. Despite arboreal behaviour of invasive rodents being documented in many different systems, ground-based delivery of control methods remains standard for population control. The work on Norfolk Island provides tangible evidence that above-ground deployment of multiple rodent control mechanisms can result in improved outcomes.

In 1993 a large-scale rodent baiting program was established by the Norfolk Island National Park (NINP). This system of control grew to consist of approximately 1800 ground-based bait stations covering the entire National Park and until recently, remained largely unchanged since it was first established.

The project is a partnership between Monash University and NINP. PhD candidate Allie Nance in the School of Biological Sciences is directly involved in the project. She is supervised by Drs Rohan Clarke and Carly Cook. Allie is also part of the Research Ecology Lab led by Dr Clarke.

“Rodent control has been happening on the island here for such a long time, but it had gotten to a state where it wasn’t working anymore,” said NINP Natural Resource Manager Mel Wilson.

The research team discovered that the rodents were as active in the mid-storey and canopy of forests as they were on the ground. They also found that rodent density was twice as high in woody weed forests compared with native hardwood forests. These results demonstrated a misalignment between rodent activity and the exclusive use of ground-based baiting techniques.

Expanding on this work, the team conducted a study to determine the influence of bait-station design on bait uptake by invasive rodents. They found that baited tunnels above-ground resulted in a 60 per cent increase in bait uptake by rodents, whilst eliminating non-target uptake by feral chickens. The team also ran an experimental trial of tree-mounted lethal rodent traps deployed at heights up to 10 meters. During the month-long trial, 19 rodents were removed from the population with no evidence of impact on non-target species.

This project demonstrated that the above-ground use of standard and emerging control methods can greatly improve the effectiveness of invasive rodent control. NINP is now in the process of modifying the current ground-based rodent monitoring program.

“Our partnership has provided conservation managers with the information they need to modify a major management strategy,” said Allie.

“It has resulted in significant improvements to the original rodent control system and ultimately improving the protection of key threatened species through evidence-based management action,” she said.

By employing multiple control mechanisms, the team has been able to concentrate control efforts to specific focal areas.

To protect the Critically Endangered land snail (Advena campbellii), the team has intensified the use of above-ground bait tunnels and tree-mounted lethal traps in the single valley within the National Park where the species is found. The NINP team has also strategically placed tree-mounted lethal traps around active nests of the Endangered Green Parrot (Cyanoramphus cookii) to improve nesting outcomes.

Allie Nance and Mel Wilson will co-present a plenary on the team’s research at the Ecological Society of Australia annual conference in November.

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