Updated 1 April 2020
This Finding was originally published in November 2017, and has since been updated on the basis of peer review feedback. A scientific journal paper will be published soon, to supplement this Finding.
There is some anecdotal evidence to suggest that possum movement 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 movement 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 (as trialled at Bottle Rock Peninsula between 2015 and 2019).
Between August and October 2017, ZIP ran a trial in the Orongorongo Valley (Remutaka Forest Park), to determine whether possum movement into a controlled area was restricted by the presence of a small, braided river.
What we did
We became aware that a 1080 operation was being planned across both sides of the Orongorongo River in Remutaka Forest Park, in order to control possums (which pass on bovine tuberculosis). This operation presented an opportunity to measure how effectively a small braided river would restrict possum home range expansion from an uncontrolled area into a controlled area. We proposed to OSPRI and the Department of Conservation that a 250ha exclusion zone be established within the 1080 treatment zone as part of a trial to do this. The exclusion zone (an area without 1080 baiting) covered a 4km stretch of land on the true right side of the Orongorongo River. OSPRI and DOC agreed to our proposal, and so the 250ha zone was excluded from the aerial 1080 operation carried out on 30 July 2017.
Our trial was carried out over a period of nine weeks. During this time, we (i) used a non-toxic bio-marker called pyranine to mark possums within the exclusion zone, and (ii) captured possums using leg-hold traps on the ‘other’ (true left) side the river.
When consumed by animals, pyranine stains the stomach and intestinal tract, and appears as a vibrant green fluorescence (or ‘glow’) when viewed under ultra-violet (UV) light (see Figure 1, below).
Figure 1: Pyranine glow (under UV light) of the intestines of a possum
Each week, a zone in the lower half of the exclusion block (cross-hatched in Figure 2, below) was aerially sown with non-toxic cereal pellets dyed with pyranine, and the same pyranine bait was also deployed by hand along the true right edge of the Orongorongo River, to ensure that possums received continuous exposure to the biomarker before encountering the river.
We installed a network of automated reporting leghold traps at 20m spacing along the 4WD track on the true left side of the Orongorongo River, to intercept any possums that might expand their home range to include new areas across the river from the exclusion zone (or that had survived the aerial 1080 operation on the true left side of the river). This network was operated in accordance with the MPI Guidelines for good practice when remotely monitoring live capture traps for vertebrates.
Each possum caught in a leg-hold trap was examined externally and internally for the presence of pyranine using a UV black light.
What were the results?
None of the 44 possums we caught on the line of leg-hold traps on the true left side of the river were marked with pyranine. The majority of these animals were caught in small exclusion zones established around huts, suggesting that they survived the 1080 treatment.
Figure 2: Trial layout and results
This result suggests that the Orongorongo River was acting as an obstacle to possum movement (in particular, home range expansion) during the course of this trial. This helps to increase our confidence that during most times of the year (excluding sub-adult dispersal season) river boundaries, such as those that surround the Perth River Valley field site, are likely to prevent the majority of possums from reinvading sites under protection.
Acknowledgements
We’d like to thank staff from OSPRI and DOC for their support of this research project. Landcare Research (via Peter Berben) allowed our rangers to stay in the Landcare Research field base in the Valley – thank you. Phil Cowan (retired, from Landcare Research) helpfully answered our questions and provided us with information that supported our project. We would especially like to thank Roger Pope, the DOC caretaker at Catchpool Visitor Centre, for being the daily contact person for ZIP rangers working at the research site.
A note on pyranine bio-marker
During this project, field staff identified fluorescent green external markings (i.e. ‘glows’) on two possums caught on the leghold trapline. Strangely, neither possum showed any internal pyranine marking.
Consequently, we undertook another trial to assess the external effects of pyranine on possums exposed to high levels of the bio-marker compared with possums that had not been exposed to it, at our Lincoln facility.
We discovered that external fluorescence can be attributed to possum urine, or simply pale hairs around the whiskers and paws that appear to glow green under UV light – i.e. not the result of pyranine staining.
Our rangers also noted the presence of fungi in the Orongorongo Valley that glowed blue/green when viewed under a UV light, which suggests that naturally occurring environmental sources could mark possum fur.
As a result of this work, we concluded that pyranine can only be used as a reliable bio-marker for possums when it stains their internal organs.