Zero Invasive Predators Ltd (ZIP), with the support of the Department of Conservation (DOC) and Predator Free 2050 Limited, is currently undertaking a programme of research and development work in the Perth River valley on the West Coast of the South Island.
The purpose of the work is to test and refine an approach to completely remove possums from large areas and prevent them from re-establishing, and to develop this approach for ship rats and stoats. We call the approach ‘Remove and Protect’. This is the first time such an attempt has been made on the New Zealand mainland. The results are expected to help enable New Zealand to achieve predator-free status by 2050.
One component of the approach is the use of aerial 1080 to completely remove possums, using a method we developed in consultation with experts from the Department of Conservation (DOC), OSPRI and Manaaki Whenua - Landcare Research. We call this method ‘1080 to Zero’.
The DOC permission for ZIP to undertake an aerial 1080 operation in the Perth River research area acknowledged the risk to kea, and included a requirement that ZIP undertake a project to estimate how likely kea would be to consume bait containing 1080. This requirement was intended to assist DOC to assess and decide whether the application of toxic bait should proceed.
While kea have previously been monitored through aerial 1080 operations, as far as we know, this project is the first time that anyone has attempted to learn about the consumption of non-toxic baits by kea prior to toxic bait being sown.
This Finding reports the results of the project. The work that is underway to measure and mitigate the risks to kea since this project was completed are described in a companion Update.
ZIP’s automated reporting leghold traps have proven to be the key to a highly effective – and efficient – possum ‘virtual barrier’ at our Bottle Rock field site, stopping over 95% of invading possums. But what about the few individuals that do get through the barrier, and into the protected, possum free area..?
If you want to catch a possum, rat or stoat, you need the right lure.
Unfortunately, the lures we have found to be most attractive are fresh foods that typically don't last very long in the field.
To keep traps attractive and effective, these lures typically need to be refreshed by a ranger every 3-4 weeks.
This can be a time-consuming (and therefore expensive) task, particularly when working in difficult terrain, which significantly limits the size of an area that can be effectively trapped.
In an attempt to address this inefficiency (and make life easier for our hard-working rangers), we teamed up with the Christchurch-based mechanical and engineering company inFact in early 2016, to begin development of an automated food lure dispenser capable of holding and dispensing fresh food-based lures in the field for up to a year.
ZIP is developing a modified technique for the aerial application of 1080 to completely remove possums and rats from large mainland areas. If we are successful, and we also successfully develop techniques to prevent possum and rat invaders from re-establishing, then the large-scale repeated application of aerial 1080 may no longer be necessary to protect New Zealand’s biodiversity.
In 2017, we trialled this technique at a 2,300 hectare site in South Westland.
The impact of invasive predators on native biodiversity is well documented, with an estimated 25 million native birds lost to predation every year.
New Zealand is a world leader in completely removing possums, rats and stoats – the three species generally understood to do the most damage – from islands and fenced sanctuaries, through the aerial application of the toxin brodifacoum. However, aerially applied brodifacoum is not registered as a technique on the mainland, so we need to find another option as we push toward a predator free New Zealand by 2050.
Aerially applied 1080 is successfully used on the New Zealand mainland to ‘control’ possums and rats for conservation and disease management purposes, by suppressing populations to very low numbers. However, because standard 1080 operations do not remove all target individuals, and reinvasion back into those sites is not managed, predator populations recover and these operations must be repeated on a cyclical and ongoing basis (typically every 3-5 years) to sustain the benefits of the predator control.
This context led ZIP to conclude that it is a high priority to develop an aerially-based technique for the complete removal of possums and rats from a treatment area. We decided to test whether 1080 could form the foundation of that technique.
There is some anecdotal evidence to suggest that possum migration across landscapes is slowed by natural features, such as rivers and high mountain ranges. If we were able to confirm that these features really do halt or greatly slow down the migration of possums, then this new knowledge would broaden the range of sites where the Remove and Protect model of possum management could be applied across New Zealand. Natural barriers may also be cheaper to maintain than a virtual barrier.
Between August and October 2017, ZIP ran a trial in the Orongorongo Valley (Rimutaka Forest Park), to determine how effectively this relatively small river prevents possum migration.
The TUN200 trap box is our most effective tool for rats. Our usual spacing between traps along each of our virtual barrier defence lines is 10 metres. During 2015 and 2016, we ran trials to assess whether TUN200s could be placed at different intervals along the first defence line in our 'virtual barrier' at Bottle Rock, to enhance both the effectiveness and efficiency of the Remove and Protect system.
ZIP is always looking for ways to improve the effectiveness of the tools we use in the virtual barrier. With this in mind, a trial was run to determine whether a TUN200 trap box with a more ‘open’ entrance architecture might kill significantly more rats than a TUN200 with a standard 7.2 cm diameter entrance hole.
We were surprised by the result...
Studies have indicated that tree-mounted possum leghold traps may be most effective when paired with a sloping board that acts as a ramp from the ground to the trap.
During 2016, we ran a trial at Bottle Rock to assess the effectiveness of possum leghold traps with ramps in comparison with those without ramps.
When a leghold trap in our system catches a possum, like all possum trappers we are required to clear this trap within 12 hours of sunrise. Interestingly, our capture data shows that there is a one in three likelihood that the next possum to be caught on that line will be caught in the same trap, even though there are over 250 traps for that possum to choose from.
We now believe that this 'funnel' effect is likely to be the result of some form of ground-based scent trail left by possums.
When the virtual barrier was installed at Bottle Rock in November 2014, stoat defences were not included as we did not believe that the 400 ha peninsula was large enough in relation stoats' speed or 'typical' home range size (60 – 200 ha) to confirm the area was free of stoats at any point in time. Our suite of control tools and available monitoring tools for stoats at the time were also extremely limited.
However, the virtual barrier system caught a surprisingly large number of stoats (25) during the 2014 beech mast (lured with mice caught in mouse traps inside TUN200 boxes), and another 11 following the rat and possum removal. This gave us some confidence that it would be worthwhile to begin trialing stoat defences at Bottle Rock peninsula.
The leghold trap is by far our most effective tool for invading possums, intercepting approximately 60% of those that attempt to pass through each defence line.
When ZIP began operations at Bottle Rock in late 2014, our leghold traps were lured using a 90x90mm white corflute chew card baited with aniseed possum lure, refreshed every 6 weeks.
However, when we observed catch data over time, it became clear that refreshing the lures did little to improve trap effectiveness, and catch rates did not reduce between services.
We now believe that the visual ‘flag’ created by the white corflute square is acting as the attractant, and is itself sufficient to cause possums to investigate traps.
Following the ‘lonely’ rat releases on Bottle Rock between July and September 2015, we began to reconsider our approach to detecting invaders in the Remove and Protect system.
These releases demonstrated to us that the ‘footprint’ of an individual Ship rat in the landscape can be very small and therefore extremely difficult to detect using current methods.
We are now looking to determine whether the dispersal footprint of a first generation breeding event is both detectable and manageable using currently available tools.
To help us design our detection system at Bottle Rock, we wanted to learn more about what a ‘lonely’ rat does, and where it goes, when it enters an area with no other rats. In other words, we wanted to better understand the ‘footprint’ of a rat invasion in space and time, to help ensure early detection and removal.
Between 2011 and 2014 a trial was conducted as part of the Poutiri Ao ō Tāne project, a restoration project located at the Maungaharuru-Tutira catchment, 60 km north of Napier.
This trial sought to compare the efficacy of run-through tunnel traps and standard single-entry trap boxes for ship rats, stoats and ferrets.
Because our detection and removal devices are not designed to target mice, we need to be able to exclude them from these devices to ensure optimal performance of the system.
Here are just a few of the mouse-related challenges we’ve overcome since we began our operations.