GREAT OPOHO POSSUM HUNT:
A CENSUS OF BRUSHTAIL POSSUMS LIVING IN SUBURBAN DUNEDIN.
abridged version of a major research project undertaken by
Room 2 at Opoho Primary School.
Room 2 (Year 4/5)
of Opoho Primary school set out to find out how many possums live
in a Dunedin suburb. The 28 children and their teacher, Mina Crooks,
worked with the father of one of the children, Dr Henrik Moller
of the University of Otago. In August 2001 we set 31 cage traps
for seven nights in a trapping zone covering about 32 hectares
to catch, tag and release possums. We estimated that there were
1.3 possums per hectare in Opoho. Our estimate matched our prediction
that there would be fewer possums living in town (5 possums per
hectare is the normal possum density in bush). We also did an
experiment to test a hypothesis that possums find their food by
smell. Finally, we estimated that the possums living in Opoho
eat enough leaves to fill four rubbish sacks every night!
Possums are pests in New Zealand because they kill native trees
and birds’ eggs and chicks. They also transmit tuberculosis to
cows and deer. New Zealand spends millions of dollars each year
to control possums and tuberculosis.
We noticed that there
were possums living in our neighbourhood... We decided to do a
census to find out:
• How many possums
live amongst us.
• What is the
‘population profile’ (number of males and females, young and old).
• How best to
• Where they moved
Although there have
been over 40 studies of possums in New Zealand, no one has yet
studied them in towns, so we were unsure what we would discover.
At first everyone in our class guessed how many possums we would
catch during a week-long study. We decided to set up a hypothesis
that might predict the number more accurately and test the idea
behind our prediction. We wanted to predict the density of possums
in town. In the bush there are usually about five possums per
hectare. We predicted that there would be fewer possums in town
because (1) food limits possum numbers and that less food is present
in towns because houses and roads take up the space; (2) people
may kill possums because they eat roses and vegetables in town;
(3) cars may run them over, and (4) light and noise may frighten
them and disrupt their feeding and movements.
Judging from an aerial
photograph of Opoho, at least half of the ground is covered by
buildings and road, so food would be reduced by at least half.
We hypothesised that possums would be less than half the density
in bush. All animals need to get adequate shelter and avoid predators.
This might reduce possum density in our town even further. So
we predicted that there would be about 1 possum per hectare in
our suburb. The main hypothesis (idea) behind this prediction
is that the density of possums is controlled by food availability
How can we catch most
Possums are mammals
and therefore have a strong sense of smell. We hypothesised that
possums find their food by smell. We decided to test this hypothesis
by doing an experiment. We put out three sorts of bait (apple
+ flour + aniseed, apple + flour + cinnamon, apple + flour). This
last bait type is called the control group. Our hypothesis was
that we should catch more possums in the traps baited with stronger
smells. We predicted that more possums would be trapped using
aniseed than cinnamon and fewer on control baits.
and Henrik read the school policy on working with animals and
Henrik got permission from the University Ethics Committee. Throughout
the study, we took great care to treat the animals humanely (even
though many of the parents and neighbours wanted them killed!).
Henrik did a pilot study by setting just five traps in the four
days before the class started work.
We set 31 cage traps
on Tuesday 21 August. We set the traps under trees and bushes,
in shady areas because wild animals like to stay close to where
they can hide. We also wanted to keep the possums warm and dry.
The three bait types
were set in sequence so that equal numbers of each sort were put
out. We checked the traps and released the possums early each
morning. Traps were checked every morning until Tuesday August
When we found a possum
in a cage, with Henrik’s help, we:
1. Got it
to go into a small ‘anaesthetising box’.
2. Blew ether
into the box to make the possum relax and go to sleep.
a numbered tag into its ear.
the possum in kilograms (kg) and gave it a name.
if it was a male or a female and whether it was a child, adolescent
- If it was a female
we checked its pouch to see if it had a ‘joey’ (pouch young).
If the joey did not have its eyes open yet, we just measured
its head length and left it attached to the mother’s teat inside
the pouch. If it had its eyes open it was safe to take it out
of the pouch to weigh it. We used callipers to measure the pouch
young’s head length to the nearest millimetre.
Where do possums move
We had one radio-transmitter
to attach to a possum’s neck. It allowed us to estimate the ‘home
range’ of the possum by tracking its movements from day to day.
We attached the radio-collar to ‘Pinky Poodle’ (Tag no, P9110),
a large adult male caught on the first day. We used a ‘hand-held
yagi aerial’ to track the possum. When the aerial was pointing
in the direction of Pinky Poodle, the receiver beeped. The closer
we got to the possum, the louder the beeping, so we were able
to narrow in on Pinky Poodle’s position.
the class trapped 19 possums (12 different ones) in the 7 days.
It was obvious that some traps caught a possum on most nights
while most caught none or hardly any. We showed this by calculating
the percentage of traps that caught 0, 1, 2, 3, etc possums. We
showed this as a table and as a bar graph.
Graphing let us see
that two traps were way out on their own and caught a possum virtually
every night. We also made a 3D pie graph that showed how often
each tagged possum was captured. Most possums were caught just
once, but five possums were captured twice and ‘Tony Hawke’ three
times. On the first night we caught five possums, but then the
number fell away until by 25 August we only caught one. We thought
about various hypotheses for why this drop off occurred and and
some ways to test them:
- Possums are becoming
scared of the traps.
- Possums are learning
not to go in the traps.
- Possums may avoid
traps that smell of possums that had been caught earlier.
- Possums may be
attracted to a new food source (spring buds)
- Possums are less
active because of the moonlight (most nocturnal animals stay
under cover when the moon is bright)
- Possums are less
active because of the rain.
- Possums may have
been too busy mating to come to the traps.
- The smell on the
baits may be fading.
We decided to test
Hypothesis 8 by re-baiting all the traps on Saturday 25 August.
We only caught one possum on Sunday, two on Monday and then it
increased to four on Tuesday. So our prediction was wrong and
we can conclude that Hypothesis 8 was therefore probably wrong
- no worry to a scientist because at least we can now throw away
one possible reason for what we observed.
What bait worked best?
We used a ‘doughnut’
graph to show which baits were most popular. Aniseed was the most
popular bait, so our original hypothesis that possums find their
food by smell was probably correct. We had expected there to be
more possums caught on the cinnamon than on the control bait,
but the reverse happened. We had one ‘super’ trap which was baited
with control bait. This might have affected the result. We would
need to sample again to check our results because chance can give
false leads. Scientists need to sample over and over to check
any patterns that turn up.
How many possums live
We had to stop trapping
after one week. Including the pilot study we caught 20 different
possums. About half the possums caught towards the end of the
trapping were tagged, so we reasoned that our 20 tagged possums
were about half of the possums living on the block and estimate
that 40 possums live in our ‘trapping zone’.
We know now roughly how many possums live in our study area. To
estimate the density we needed to calculate the size of the trapping
zone. We had to get a scale for the aerial photograph.We measured
the distance on the photograph that corresponded to 100m of ground
using a ‘trundle wheel’. 44mm on the map represented 100m in real
life. We marked the map off in 44 by 44mm squares so that each
square represented a hectare (100m by 100m).
We calculated the
area as 31.75 hectares. So our 40 possums lived in about 32 hectares.
Dividing 40/32 gave our estimated density as 1.3 possums per hectare.
This suggests that our hypotheses (that food, pest control and
disturbance reduces possum numbers in towns) may be right.
We calculated the percentage of possums that were male and female.
We caught mainly males. Ecologists have found that young males
tend to leave home and wander bigger distances. Females tend to
stay close to where they were born. The larger number of males
is a clue about one of the main hypotheses - that possums numbers
in town are reduced by cars and people killing them. We also calculated
the percentage that were children or adolescents or adults. Most
of our possums were adults.
Possum size: We calculated
the average weight of the males and females we caught by adding
each of their weights together and dividing by the number of items.
The average weight of the males was 3.111 kg, and females 3.075
We measured the head length of the pouch young. We counted back
from the date of capture to estimate the birthday for each of
our four pouch young. Two were born in April and two in June.
This pattern of autumn and winter births is the same as that found
by a local ecologist, Dr Murray Efford, just north of Dunedin.
We hypothesised that the possums may have their babies at the
best time for them to be warm and for their mothers to have plenty
of food to feed their young.
We each collected 50 leaves and weighed them. Altogether we had
1000 leaves and they weighed about 500 grams. Scientists estimate
that each possum eats around 500g of leaves a day. We packed our
leaves into a plastic bag, measured it and calculated its volume
at about 5 litres.
We measured the volume
of a large rubbish bag - about 45 litres. If 40 possums live on
our block, together they would eat about 4.5 garbage bags of leaves
per night. No wonder the possums cause a lot of damage to trees
What did we learn?
We learned that possums live in quite low density in our suburb
compared with the bush. This fitted our hypothesis that density
is reduced because there is little food compared to the bush,
and because people kill them. Our experiment with different baits
showed that aniseed is a good lure to attract possums to baits.
Cinnamon was not very successful. We also learned that science
can be a lot of fun and that maths can be useful.
This article was published
with generous sponsorship from Schneider Electric New Zealand