Possum encounter and interaction rates Sam Brown and Bruce Warburton Landcare Research, Lincoln Funded by the Ministry of Science and Innovation
Possum encounter and
interaction rates
Sam Brown and Bruce Warburton
Landcare Research, Lincoln
Funded by the Ministry of Science and Innovation
Outline
• Background
• Current research trials
• Preliminary results
• Conclusions
Background theory…1
Probability of catching or
detecting an animal at the
centre of its home range on
one night = g0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1 3 5 7 9 11 13 15 17 19 21 23 25 27
Pro
b o
f ca
ptu
re
Distance from HR centre
Background theory…2
Capture or detection probability can be
partitioned into encounter and interaction
probabilities.
P(capture) = P(encounter) * P(interaction|encounter)
Background theory…3
• Steve Ball et al, 2005.
– Prob encounter = 0.12
– Prob interaction|encounter = 0.44
– g0 = 0.05
That is: if a trap is set at the centre of a possums
home range there is only a 5% chance of
catching it on any one night.
Methods
• 16 possums had active RFID tags
attached (10 of these had GPS
collars).
• Each trap site (11) had:
– RFID sensor
– Motion sensing camera (trail camera)
– Each trap was fixed open with the trigger
linked to a RFID tag and a LED to record
when a possum “triggered” a trap.
Detection System
0-12m
0-3m RFID
Scanner
and trap
Motion sensor
camera
Detection Zone: triggered by any
RFID tagged animal within 12m
Encounter Zone: PIR sensor triggered by
movement within 3m and if a RFID
tagged possum then recorded
Trail camera RFID detector & trap
Trap treatments
Standard
NPCA protocol
Hazed
(fenced)
Covered
Results…1
Probability of an encounter
given a detection at 12m
0.66 (79/119)
0.63 (95/151)
0.60 (73/121)
Results…2
Probability of a capture
given an encounter
0.21 (6/29)
0.34 (10/29)
0.40 (10/25)
Results…3
Probability of a capture given
one or more encounters/night
0.33 (6/18)
0.43 (10/23)
0.48 (10/21)
Number of nights to first
encounter
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Nu
mb
er
of
po
ssu
ms
Standard Hazed Covered
Number of nights between
first encounter & first capture
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6
Nu
mb
er
of
po
ssu
ms
Standard Hazed Covered
Conclusions…1
• Many possum visits to trap sites did not
result in a capture.
• Nights between first encounter and first
capture varied between 0 and 6.
• Hazing and covering traps increased the
probability of an interaction given an
encounter.
Conclusion…2
• Statistics still to be done and g0 generated
from GPS data.
• Future trials to look at density effect – i.e.
do encounter and interaction probabilities
change when possum densities are
reduced?
• Helen Nathan (PhD student – Auckland
Uni) starting similar trials on ship rats –
tracking tunnels, trap tunnels, bait stations.
Acknowledgements
• Jagath Ekanayake for technological wizardry
• Steve Hough for technical assistance
• Orari Gorge Station for access permission
• MO FRESH for animation