The impact of invasive predators on native biodiversity is well documented, with an estimated 25 million native birds lost to predation every year[1].
New Zealand is a world leader in completely removing possums, rats and stoats – the three species generally understood to do the most damage[2] – 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.
What did we do?
We developed a modified technique in consultation with experts from the Department of Conservation, OSPRI and Landcare Research. The modified technique (sometimes referred as a ‘prescription’) is designed to eliminate all possums and rats from the treatment area - i.e. not just suppress them to low numbers.
Our own research and the advice of experts suggested that we would be more likely to achieve this goal if we could:
encourage possums and rats to treat the bait as a food source, and
increase the probability that the animals would encounter toxic bait on the ground.
The modified technique borrows heavily off the proven application methods from island eradications, to create a hybrid of the ‘standard’ aerial 1080 operation and island eradication best practice. Here’s how the techniques compare to one another:
The two drops of prefeed are designed to encourage the animals to identify the bait as a safe food source, increasing the likelihood that they will consume a lethal dose of the toxin when it is applied.
We also sought to increase the probability of animals encountering the toxic bait, by:
using an application rate that doubled the 'standard' amount of bait available on the ground for the target animals to find, and
sowing the baits in a way that ensured there were no gaps in the coverage, by applying the bait in overlapping swaths and having no exclusion zones.
How might this technique be used?
The Remove and Protect model for predator management that ZIP is developing seeks to completely remove invasive predators from large scale sites on the NZ mainland, and then protect those sites in perpetuity from predator population re-establishment (i.e. maintain them as predator free).
This modified technique could be the tool that makes a site predator-free in the first instance. If the site is able to be successfully defended from invading predators (via barriers, natural or ‘virtual’), the repeated use of aerial 1080 at landscape scale will not be required in that location again.
How have we trialled the modified technique?
ZIP has run two trials so far, testing this technique. You can read about how we trialled it in 2016-17 on Taranaki Mounga (Egmont National Park) in our 2016-17 Annual Report and again in 2017, at a site adjacent to the Arawhata River in South Westland, in our next Finding.
One final point
As mentioned above, this idea to test the ability of aerial 1080 to completely remove predators is not exclusively ZIP’s. Others, namely OSPRI and Manaaki Whenua-Landcare Research, have recently been conducting trials using similar (but not identical) techniques. Information on that work should be sought directly from those agencies.
[1] From: Innes, J., Kelly, D., Overton, J. M. & Gillies, C. 2010. Predation and other factors currently limiting New Zealand forest birds. New Zealand Journal of Ecology 34 (1):86-114
[2] From: Brown, K.; Elliott, G.; Innes, J.; Kemp, J. 2015: Ship rat, stoat and possum control on mainland New Zealand: an overview of techniques, successes and challenges. Department of Conservation, Wellington. 36p.