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 endangered Norfolk Island Green Parrot (Cyanoramphus cookii), endemic land snails, breeding sea birds and several species of endemic flora.

To combat the problem, a large-scale rodent baiting program was established by the Norfolk Island National Park (NINP) in 1993. 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.

“Rodent control has been happening on the island here for such a long time, but it had gotten to a state where we were getting a poor uptake of bait,” says NINP Natural Resource Manager Mel Wilson.

Partnering with Monash University PhD candidate Allie Nance, the duo 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. During the month-long trial, 19 rodents were removed from the population with most caught above-ground and with no evidence of impact on non-target species.

This project demonstrated that the use of above-ground control methods can greatly improve the effectiveness of invasive rodent control. NINP are 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,” says Allie, “resulting in significant improvements to the original rodent control system and ultimately improving the protection of key threatened species through evidence-based management action.”

By employing multiple control mechanisms, Mel and Allie have been able to concentrate control efforts to specific focal areas. To protect the critically endangered land snail (Advena campbellii), they have 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 have also strategically placed tree-mounted lethal traps around active nests of the endangered Green Parrot (Cyanoramphus cookii) to improve nesting outcomes.

“I can see us using this research to build on a whole new way of controlling the rodents here,” says Mel.

“It’ll be a mix of innovation, monitoring differently and being more responsive to where we’re seeing rodent activity go up and down.”

The Norfolk Island Threatened Species Recovery Plan (2010) is currently undergoing review, and the partnership has directly informed its rodent monitoring program and control targets for the next decade.

“Having our results feed into the review of the Threatened Species Recovery Plan was really exciting,” says Allie.

“Our work happened at the perfect time when we could push to implement a change. I think it’s going to make a big difference.”

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.

“We often feel like we’re in a very small bubble out here on Norfolk and we work away trying to be innovative even though we are such a long way from anywhere,” says Mel.

“There are a lot of issues with the work we do, like shipping and simply getting rat bait to the island. There are all these extraneous things that we have to deal with so winning an award like this gives a boost to the whole team because it’s recognition that we are doing some really amazing things out here.”

“We’re three to four years down the track from starting this project with Allie so this feels like a really nice culmination of all the hard work we put into it.”

Applied ecological research underpins the partnership between the Research Ecology Lab of Monash University and the Norfolk Island National Park (NINP) management team. Beginning in 2017, Monash and NINP have collaborated to extend the ecological understanding of threatening processes in island systems and use these new insights to deliver effective conservation management strategies.

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