Using plantings to manipulate possum behaviour

The New Zealand Biosecurity Institute (NZBI) held its annual national education and training seminar (NETS) in Whangarei, and the majority of the Boffa Miskell biosecurity team attended. The Whangarei office welcomed attendees from other offices, and the conference was an opportunity to showcase project work and thought-leadership.

Bec Simpson shared progress on a 2022 Boffa Miskell Research Programme project: Improved control of browsing animals with ‘Beacon and Corridor’ planting schemes, led by Brent Barrett.

The Avoca Valley lies on the Christchurch side of the Port Hills. Historically grazed, the valley is now the centre of an ambitious community-driven restoration project aiming to revegetate 38 hectares. Initially, this includes the planting of 45,000 plants and trees, with the vision of creating dry lowland podocarp forest. Some wetter areas will be planted with browsing vulnerable plants while others will initially use Kanuka as a nursery plant, allowing for grazing underneath.

Protecting the edible seedlings from pests until they are mature enough to withstand browsing pressures is key, and pest animal control will be crucial for their successful establishment. Possums move through landscapes in a predictive manner, favouring already-formed tracks (called pad trails) worn into the hillsides.

The innovative ‘beacon and corridor’ planting scheme has been devised to try and improve pest control outcomes by strategically designing planting patterns to guide and direct possums through the valley, away from vulnerable young plantings.

The 'beacon' plants emulate natural landscape features that possums typically use to navigate through landscapes and develop pad trails. These trails are often used by other predators, so the impacts of this planting research may extend beyond possums.

The ‘beacons’ are groups of plantings are obvious in the grassy valley and catch the attention of possums, drawing them down the valley and to the true left of the river, away from the new planting sites. There are currently eight beacons planted in the upper valley. Each beacon covers an area of 12m2 and is made up of a variety of native plants, as in the image below.

In the Avoca valley, 130 Tī kōuka, narrow leaf lacebark, ngaio, and mānatu/ribbonwood saplings were planted. The beacons were designed to align with the revegetation objectives of the valley. An obvious drawback of using trees as beacons is that they need time to grow. However, they are less obstructive than other geographic features that would act in the same way, like a large rock or pole. Trees also sequester carbon and fit in with the wider landscape context. 

The ‘corridor’ then comes into play. These guide the possums through the landscape past the sensitive restoration areas on true right of the river, and towards traps at the bottom of the valley, encouraging creation of a ‘possum highway’ that bypasses the new plantings. The corridors are also made up of natives, mainly kanuka and ngaio, shown below. The corridor plants are supplemented by another buffer on the far side of the river, where there are extensive riparian plantings. This is mainly harakeke, which acts as a visual barrier that blocks possums from noticing the new plantings on the far side of the river. 

This research project was developed with a thorough understanding of possum movement, dispersal, and behaviour in the context of the Port Hills. Several rounds of monitoring indicate that the possums are acting as predicted. This means the beacons are well-placed to catch their attention, leading to altered behaviour and redirecting them way from the plantings.

This planting system aims to optimise both outcomes, using plants simultaneously to manipulate pest behaviour and to restore the landscape. The plantings are permanent, providing an affordable, low-maintenance, long-term and eventually self-sufficient control technique.

By exploiting the natural behaviour of possums, they can be corralled and channelled through the landscape. Simultaneously, this creates opportunities that require less travel and effort from community trappers. This improves pest control efficiency and reduces the carbon footprint of the operation—a win for both the environment and community volunteers.

Results so far support the Research Project’s hypothesis that biosecurity can be incorporated into revegetation schemes from the outset, and prove useful right from the beginning, providing integrated biosecurity action.