possum encounter and interaction rates · encounter zone: pir sensor triggered by movement within...
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Possum encounter and
interaction rates
Sam Brown and Bruce Warburton
Landcare Research, Lincoln
Funded by the Ministry of Science and Innovation
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Outline
• Background
• Current research trials
• Preliminary results
• Conclusions
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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
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Background theory…2
Capture or detection probability can be
partitioned into encounter and interaction
probabilities.
P(capture) = P(encounter) * P(interaction|encounter)
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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.
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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.
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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
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Trail camera RFID detector & trap
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Trap treatments
Standard
NPCA protocol
Hazed
(fenced)
Covered
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Results…1
Probability of an encounter
given a detection at 12m
0.66 (79/119)
0.63 (95/151)
0.60 (73/121)
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Results…2
Probability of a capture
given an encounter
0.21 (6/29)
0.34 (10/29)
0.40 (10/25)
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Results…3
Probability of a capture given
one or more encounters/night
0.33 (6/18)
0.43 (10/23)
0.48 (10/21)
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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
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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
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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.
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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.
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Acknowledgements
• Jagath Ekanayake for technological wizardry
• Steve Hough for technical assistance
• Orari Gorge Station for access permission
• MO FRESH for animation
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