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