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Next-level possum control

The Australian common brushtail possum (Trichosurus vulpecula) has invaded most New Zealand landscapes and has established itself as a ravenous and problematic pest. Introduced in 1837 by settlers looking to start a trade in possum fur, the marsupials faced no natural predators and quickly got out of control. They have been linked to defoliation and native habitat die-back and have had a devasting impact on bird life. As potential carriers of bovine tuberculosis (TB), they are also a significant threat to New Zealand’s agricultural industries.

Several Manaaki Whenua researchers are working on possum-related projects to meet New Zealand’s Predator Free 2050 goal to rid the country of introduced predators and reduce populations to low levels to ensure TB is eradicated.

Genotyping of possums

On the West Coast outbreaks of TB have been recorded in cattle herds despite significant management of possums on-farm and regular possum control operations several kilometres into adjacent forest. In response, Manaaki Whenua was commissioned by OSPRI to assess the extent to which possums infected with TB might be migrating through control buffers.

Possums were surveyed (using chew cards) and captured (using traps or cyanide poison) for DNA genotyping on farmland near Harihari and in forest on both sides of the Wanganui River. We then analysed these tissue samples using genotyping by sequencing (GBS) methods and used these data to assess the inter-relatedness of all possible pairs of the sampled individuals.

We were able to ascertain the precise relatedness of all possums, and from this we were able to extrapolate that long-distance dispersal (over two kilometres) is extremely rare, and we found no evidence for dispersal over four kilometres. These population-level dispersal models would have been difficult to obtain with non-genetic methods.

We also found that the Wanganui River is a nearly complete barrier to possum migration, with the two populations on each side highly genetically diverged from each other. We did, however, identify two migrant possums that had crossed the river, and one ‘hybrid’, all caught within 350 m of the State Highway 6 bridge.

We found that possum densities in the farmland and buffer zones appeared to be too low for TB to persist. We explained the TB outbreaks as an unexpected spread from uncontrolled deep forest possums to farmland as a result of an unusually high prevalence of TB in valley floor possums upstream from the controlled areas. This has led to much more frequent transmission of TB between possums living in contiguous or overlapping home ranges, and a higher prevalence of TB in the few possums that have moved into the controlled area.

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Tracking urban possums

Predator Free Dunedin (PFD) has a big, hairy goal: eradicate possums in the city by 2050. However, it’s hard to do this until you know where the possums are and how trappable the population is. Currently there is a lack of understanding of habitat use and movement behaviour of adult possums in urban and peri-urban environments.

PFD has been working with Manaaki Whenua researchers Deb Wilson, Dave Latham, and Peter Sweetapple to capture, track, mark. and observe possums across the city. After using chew cards to confirm a possum presence and identify clusters of suitable residential properties inhabited by possums, researchers set up live-capture traps paired with motion-sensitive trail cameras in backyards. Two clusters were identified in the suburb of St Clair. Fifteen possums were captured, and eight of these were fitted with GPS collars and released. Unset lethal traps and motion-sensitive cameras were also set up in the possum home ranges revealed by the GPS data, to monitor individual possum behaviour at different types of trap. 

The last few survivors of an eradication attempt can be the most elusive and hardest to track down and kill, because they may be particularly wary of traps and other control devices. If left unmanaged, they go on to become responsible for an increase in possum numbers or reinvasions. Therefore, understanding variation in how animals behave around traps can guide management efforts to trap the final few.

The next steps for the project are to finish the GPS and camera monitoring of possums in the suburbs of Andersons Bay and Corstorphine, and then to establish several new clusters of properties with possums in other suburbs. These data will be added to the information already collected from the St Clair clusters and analysed to understand the optimal spacing for traps and monitoring equipment. The data will also be used to identify movement patterns and corridors that could provide potential routes for reinvasions.

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Motivating the public to support possum trapping programmes

Our researchers have also worked with Predator Free Dunedin (PFD) to understand what stops the public from trapping possums. PFD is working towards the goal of eradicating possums from the city by 2050, so knowing how the public views trapping is an important aspect of this.

Manaaki Whenua researcher Dr Geoff Kaine took an existing framework he had used successfully within farming communities to predict behaviour in response to policy initiatives and reworked it to fit the context of pest control. Drawing on an idea from social marketing called ‘involvement’ – a measure of motivation – the researchers conducted a large survey in Dunedin. The idea was to measure how motivated people were about the idea of reducing possum numbers, and about using traps as the way to do this.

The survey discovered that only 12% of people in Dunedin currently trap possums. “That means we need to focus on those among the other 88% that think trapping is a good idea but don’t trap, and try and identify why they don’t trap,” says Dr Kaine. 

Researcher Dr Nick Kirk led a follow-up project on how to encourage people to get involved with trapping programmes. “We looked at what was needed for people to acknowledge that getting involved in trapping was going to satisfy their personal goal of wanting to reduce possum numbers.” 

Some of the issues include people not perceiving possums as a problem, animal welfare concerns, and a lack of resourcing to set up and clear traps. 

“What we suggested to PFD was that they should develop an education and awareness campaign if they want to achieve their goal.  There is a need to reassure people that modern traps are safe for pets, and also to publish maps that show the distribution of possums, especially the areas that are overrun,” says Dr Kirk.

“Most importantly, because the research showed people are not strongly involved with the idea of using traps, we recommended that PFD needs to make it as easy as possible for residents to get involved in a trapping programme, for example, by having staff or volunteers who will help monitor and clear traps.” 

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