Our current possum defence system includes two lines of automated reporting leghold traps spanning the 2.5 km neck of Bottle Rock peninsula, with traps spaced at 10 m intervals along each line.
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.
As outlined below, we now believe that this 'funnel' effect is likely to be the result of some form of ground-based scent trail left by possums:
We have not observed any evidence of possums being caught in adjacent traps; rather, the funnel effect following a catch overwhelmingly appears to draw possums within an approximately 200 m wide approach zone into the specific trap where the previous possum was caught.
We have observed that this effect is very strong for 5 days, after which time it appears to decay, most likely removed by rain and other environmental factors, and is usually completely gone after 10 days.
The 200 m zone of influence we have identified for this effect is too great to be scent dispersed by air, particularly given our previous observations and trials by Morgan et al (1995) indicating that possums have a relatively weak sense of smell.
Possums have been observed by researchers (Brockie 1992) to leave a 'thin trail of urine' as they move about, which is likely to be some form of social signal to other possums.
It was suggested to us in 2015 by a researcher based in Tauranga that interdigital scent glands in the paws of possums may be one method by which possums mark their movements through the forest, and research supports the theory that possums may communicate with one another using scent glands (McLean et al 2012 and 2014).
Potential application of the funnel effect in trapping programmes
Given that the funnel effect drops off within a 10 day period, this effect is unlikely to be applicable to draw possums to less frequently serviced kill devices (with the exception of those that are self-clearing).
There is some potential to explore whether artificially-laid ground based scent trails could increase efficiency in leghold trapping operations, although at this stage that research is not a priority for ZIP, for two key reasons:
The high density of devices and use of visual luring (which our data indicates has a zone of influence of up to 10 m) in our system make the likelihood of both encounter and interaction very high for any possum approaching the virtual barrier.
The automated reporting system we are currently trialing at Bottle Rock enables remote inspection of our leghold traps, which means only those that have been sprung need to be visited by a ranger each day. The need to visit each trap in the system every 10 days to refresh the scent trail would greatly increase the labour required to maintain the system.
The potential utility of artificially-laid scent trails is likely to be greatest in a more 'traditional' trap network where traps are less densely spaced and daily manual inspection is required.
References:
D.R. Morgan, J. Innes, C.M. Frampton & A.D. Woodhouse (1995) Responses of captive and wild possums to lures used in poison baiting, New Zealand Journal of Zoology, 22:2, 123-129
Brockie, R (1992) A Living New Zealand Forest: A community of plants and animals, David Bateman Ltd.
S. McLean, N.W. Davies & N.L. Wiggins (2012) Scent Chemicals of the Brushtail Possum, Journal of Chemical Ecology, 38:1318-1339
S. McLean (2014) Scent glands of the common brushtail possum, New Zealand Journal of Zoology, 41:3, 193-202
A note about leghold traps:
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.
Under Animal Welfare legislation, all live capture traps must be inspected within 12 hours of sunrise, on every day the trap remains set. Utilising ZIP’s automated reporting system, traps at Bottle Rock can be cleared much more quickly than this as only those traps that have reported as sprung must be checked. Clearing the leghold traps is the first job of the day for the ZIP field team, and is usually completed before midday every day. Caught possums are dispatched quickly in line with national animal welfare standards for humaneness.
Kill traps for possums are also deployed by ZIP within the 'virtual barrier' system, as a multi-tool approach is necessary to target all invaders. In the long term, ZIP would like to be less reliant on live capture traps for possums and to this end is investigating an opportunity to develop a 'low interaction’ kill trap that can achieve similar effectiveness to that of the current leg hold trap.