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General Discussion

PRACY. Am I right in assuming that you accept that the habitat was a good one with good condition animals?

GREEN. Yes, certainly the animals were in good condition for their mean weight was 3 kg or more, and many males weighing 4 kg were caught. The condition of animals varied with their position on the hill. Those in the middle region tended to be in poorer condition than those nearer the pasture edge. I would say the habitat is good at this stage compared with many other West Coast areas nearby.

PRACY. Would this be in relation to food availability?

GREEN. Yes.

PRACY. The reason I asked the question is that it is not uncommon for established possum populations to move distances of 1 km or more from forest to pasture margin, but I have never seen it with a colonising population.

GREEN. Clearly the food supply has changed a great deal from the peak population time. I am not suggesting it is as good as it used to be, but to the eye there is still a great amount of greenery which we normally class as palatable.

MORIARTY. Would you be prepared to guess how frequently a possum might journey down to the pasture and back again?

GREEN. We certainly have had some animals in the mid-hill zone caught on the pasture edge and then up in their forest area two nights later - some 1200 m away. Whether they do it nightly or whether they 'stopover' I cannot say until we have done radio-telemetry work. I would not be surprised if some do it nightly.

YOUNG. You have not opened up tracks on the slope which would facilitate possum movement?

GREEN. No, only in the alpine scrub zone did we actually cut a track. We have been well aware of the danger of influencing movements through cutting tracks. We have not really opened up the understorey to any extent.

WODZICKI. Many of your possums are coming to the grass, which is a renewable crop, and you actually find that they do not do much damage. What would be their range inside native forests? Do they do the damage they might have been expected to do had they not moved but had remained in their one area of native forest?

GREEN. It is a difficult argument because one would have to show that the numbers would be as high if there was no pasture. If there was no pasture then presumably it would be a more difficult habitat for them and numbers might be therefore lower. So I cannot follow that argument through to say that if they did not have the grass they would be wiping out all the forest.

B.D. BELL. In the Orongorongo study we have no evidence of such long movements in the adult animals. For 3½ years we trapped possums in two lowland forest areas on river terraces some 3 km apart and there was no evidence of lateral movement between areas. There is some evidence of dispersal by younger animals, of both sexes, up to distances of page 62 10.5 km. Within the forest study areas on reaching maturity the possums appear to settle into discrete home-ranges of a few hectares, perhaps making periodic excursions of only a hundred or so metres. We have little evidence of vertical movements from higher altitudes down to the river terraces, although only a little trapping has been carried out above our main study areas.

GREEN. This more sedentary pattern would suggest what? The drawing power of our pasture, or the food supply in the Orongorongo forest?

B.D. BELL. I think it likely that your pasture draws the possums down hill. In our area the Orongorongo river bed is relatively sparsely vegetated, and is possibly less of a draw being mostly visited by possums living in the immediate vicinity. However more extensive studies of movements would be worthwhile in the Orongorongos to clarify the situation.