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2002 Gwen Gawith: Re-defining research

2002 Opoho School: "The Opoho Possum Hunt"

2001 Gwen Gawith: "Infobliteracy" - Part 2.

2001 Gwen Gawith: Responding to "Into a Further World" by Elwyn Richardson.

2001 Carolyn Coil: Teachers make the world of information become knowledge.

2001 Gwen Gawith: Why Read?

2001 Gwen Gawith: "Infobliteracy" - Part 1.

2001 Linda Selby and Maureen Trebilcock: A call for teacher-librarians

2000 Gwen Gawith: Information Literacy: theory into practice - Part 2

2000 Gwen Gawith: Information literacy: problems and solutions.

2000 Alan Cooper: Information literacy; the past is not enough.

1999 Gwen Gawith: The origins of information literacy.

1998 Graham Prentice: Knowledge architects.

1998 Gwen Gawith: Getting a handle on information literacy.

1997 Gwen Gawith: NEMPing through information literacy.

1992 Gwen Gawith: learning for the future.

1987 Gwen Gawith: Information skills for an information age.

1986 Gwen Gawith Information skills for an information literate future.

1984 Gwen Gawith: Getting a handle on information skills.

 

information literacy:
definitions & discussion

THE GREAT OPOHO POSSUM HUNT:
A CENSUS OF BRUSHTAIL POSSUMS LIVING IN SUBURBAN DUNEDIN.

An 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!

Introduction: 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 catch them.

• Where they moved and slept.

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.

Hypotheses: 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 and predators.

How can we catch most possums?

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.

Methods: Mina 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 28.

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.

3. Attached a numbered tag into its ear.

4. Weighed the possum in kilograms (kg) and gave it a name.

5. Determined if it was a male or a female and whether it was a child, adolescent or adult.

  1. 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 and sleep?

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.

Results: Altogether 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:

  1. Possums are becoming scared of the traps.
  2. Possums are learning not to go in the traps.
  3. Possums may avoid traps that smell of possums that had been caught earlier.
  4. Possums may be attracted to a new food source (spring buds)
  5. Possums are less active because of the moonlight (most nocturnal animals stay under cover when the moon is bright)
  6. Possums are less active because of the rain.
  7. Possums may have been too busy mating to come to the traps.
  8. 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 in Opoho?

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’.

Possum density: 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.

Population profile: 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 kg.

Breeding season: 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.

Food consumption: 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 and plants!

Discussion:

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 Ltd.